THE
HISTORY
OF FORTRAN
I,
II,
AND
III
John Backus
IBM R e s e a r c h L a b o r a t o r y
San Jose, C a l i f o r n i a
I.
Early background
1.1
Attitudes
in the 1950's.
A f t e r M a y 1954 the A-2 c o m p i l e r a c q u i r e d
a " p s e u d o c o d e " w h i c h was s i m i l a r to the order c o d e s for m a n y f l o a t i n g p o i n t i n t e r p r e t ive s y s t e m s t h a t w e r e a l r e a d y in o p e r a t i o n
in 1953:
e.g., the Los A l a m o s systems, D U A L
and S H A C O [Bouricius 1953; S c h l e s i n g e r 1953],
the M I T "Summer S e s s i o n C o m p u t e r " [Adams and
L a n i n g 1954], a s y s t e m for the I L L I A C des i g n e d by D. J. W h e e l e r [Muller 1954], and
the S P E E D C O D I N G s y s t e m for the IBM 701
[Backus 1954].
and e n v i r o n m e n t .
about automatic
programming
B e f o r e 1954 a l m o s t all p r o g r a m m i n g was
d o n e in m a c h i n e l a n g u a g e or a s s e m b l y language.
Programmers rightly regarded their
w o r k as a c o m p l e x , c r e a t i v e art t h a t req u i r e d h u m a n i n v e n t i v e n e s s to p r o d u c e an
efficient program.
M u c h of their e f f o r t
was d e v o t e d to o v e r c o m i n g the d i f f i c u l t i e s
c r e a t e d by the c o m p u t e r s of t h a t era: the
lack of i n d e x r e g i s t e r s , the lack of b u i l t in f l o a t i n g p o i n t o p e r a t i o n s , r e s t r i c t e d
i n s t r u c t i o n sets (which m i g h t h a v e A N D but
not OR, for e x a m p l e ) , and p r i m i t i v e i n p u t output arrangements.
G i v e n the n a t u r e of
c o m p u t e r s , the s e r v i c e s w h i c h " a u t o m a t i c
p r o g r a m m i n g " p e r f o r m e d for the p r o g r a m m e r
w e r e c o n c e r n e d w i t h o v e r c o m i n g the m a c h i n e ' s
shortcomings.
Thus the p r i m a r y c o n c e r n of
some " a u t o m a t i c p r o g r a m m i n g " s y s t e m s was to
a l l o w the use of s y m b o l i c a d d r e s s e s and
d e c i m a l n u m b e r s (e.g., the M I D A C I n p u t
T r a n s l a t i o n P r o g r a m [Brown and C a r r 1954]).
The L a n i n g and z i e r l e r s y s t e m was q u i t e
a d i f f e r e n t story: it was the w o r l d ' s f i r s t
o p e r a t i n g a l g e b r a i c c o m p i l e r , a r a t h e r eleg a n t but s i m p l e one.
K n u t h and P a r d o [1977]
a s s i g n this h o n o r to A l i c k G l e n n i e ' s A U T O CODE, but I, for one, am u n a b l e to r e c o g n i z e
the s a m p l e A U T O C O D E p r o g r a m they g i v e as
" a l g e b r a i c " , e s p e c i a l l y w h e n it is c o m p a r e d
to the c o r r e s p o n d i n g L a n i n g and Z i e r l e r
program.
All of the e a r l y " a u t o m a t i c p r o g r a m m i n g "
s y s t e m s w e r e c o s t l y to use, s i n c e they slowed the m a c h i n e d o w n by a f a c t o r of five or
ten.
The m o s t c o m m o n r e a s o n for the slowd o w n was t h a t t h e s e s y s t e m s w e r e s p e n d i n g
m o s t of t h e i r time in f l o a t i n g p o i n t subroutines.
S i m u l a t e d i n d e x i n g and o t h e r
" h o u s e k e e p i n g " o p e r a t i o n s c o u l d be d o n e w i t h
s i m p l e i n e f f i c i e n t t e c h n i q u e s , since, s l o w
as they were, t h e y t o o k far less time t h a n
the f l o a t i n g p o i n t work.
But m o s t of the l a r g e r "automatic. p r o g r a m m i n g " s y s t e m s (with the e x c e p t i o n of
L a n i n g and Z i e r l e r ' s a l g e b r a i c s y s t e m [Laning and Z i e r l e r 1954] and the A-2 c o m p i l e r
[ R e m i n g t o n R a n d 1953; M o s e r 1954]) s i m p l y
p r o v i d e d a s y n t h e t i c " c o m p u t e r " w i t h an order c o d e d i f f e r e n t f r o m that of the real
machine.
This s y n t h e t i c c o m p u t e r u s u a l l y
had f l o a t i n g p o i n t i n s t r u c t i o n s and i n d e x
r e g i s t e r s and had i m p r o v e d i n p u t - o u t p u t commands; it was t h e r e f o r e m u c h e a s i e r to prog r a m t h a n its real c o u n t e r p a r t .
Experience with slow "automatic programm i n g " systems, plus t h e i r own e x p e r i e n c e
w i t h the p r o b l e m s of o r g a n i z i n g loops
and a d d r e s s m o d i f i c a t i o n , had c o n v i n c e d
p r o g r a m m e r s t h a t e f f i c i e n t p r o g r a m m i n g was
s o m e t h i n g that c o u l d not be a u t o m a t e d .
Ano t h e r r e a s o n that " a u t o m a t i c p r o g r a m m i n g "
was not t a k e n s e r i o u s l y by the c o m p u t i n g
c o m m u n i t y came f r o m the e n e r g e t i c p u b l i c
r e l a t i o n s e f f o r t s of some v i s i o n a r i e s to
s p r e a d the w o r d t h a t t h e i r " a u t o m a t i c p r o g r a m m i n g " s y s t e m s had a l m o s t h u m a n a b i l i t i e s
to u n d e r s t a n d the l a n g u a g e and n e e d s of the
user; w h e r e a s c l o s e r i n s p e c t i o n of t h e s e
same s y s t e m s w o u l d o f t e n r e v e a l a c o m p l e x ,
e x c e p t i o n - r i d d e n p e r f o r m e r of c l e r i c a l tasks
w h i c h was b o t h d i f f i c u l t to use and i n e f f i cient.
W h a t e v e r the r e a s o n s , it is d i f f i c u l t to c o n v e y to a r e a d e r in the late sev-
The A-2 c o m p i l e r a l s o c a m e to be a synt h e t i c c o m p u t e r s o m e t i m e a f t e r e a r l y 1954.
But in e a r l y 1954 its i n p u t had a m u c h
c r u d e r form; i n s t e a d of " p s e u d o - i n s t r u c tions" its i n p u t was t h e n a c o m p l e x s e q u e n c e
of " c o m p i l i n g i n s t r u c t i o n s " t h a t c o u l d take
a v a r i e t y of forms r a n g i n g f r o m m a c h i n e c o d e
i t s e l f to l e n g t h y g r o u p s of w o r d s c o n s t i t u t i n g r a t h e r c l u m s y c a l l i n g s e q u e n c e s for
the d e s i r e d f l o a t i n g p o i n t s u b r o u t i n e , to
" a b b r e v i a t e d form" i n s t r u c t i o n s that w e r e
c o n v e r t e d by a " T r a n s l a t o r " into o r d i n a r y
" c o m p i l i n g i n s t r u c t i o n s " [Moser 1954].
© 1978 Association for Computing Machinery, Inc.
1 65
ACM SlGPLAN Notices, VoI. 13, No. 8, August 1978
e n t i e s the s t r e n g t h of the s k e p t i c i s m a b o u t
" a u t o m a t i c p r o g r a m m i n g " in g e n e r a l and a b o u t
its a b i l i t y to p r o d u c e e f f i c i e n t p r o g r a m s
in p a r t i c u l a r , as it e x i s t e d in 1954.
s t a n d p o i n t of its i n p u t " p r o g r a m s " it p r o v i d e d f e w e r c o n v e n i e n c e s t h a n m o s t of the
then c u r r e n t i n t e r p r e t i v e s y s t e m s m e n t i o n ed e a r l i e r ; it l a t e r a d o p t e d a " p s e u d o code" as i n p u t w h i c h was s i m i l a r to the
i n p u t c o d e s of t h e s e i n t e r p r e t i v e systems.
(In the a b o v e d i s c u s s i o n of a t t i t u d e s
a b o u t " a u t o m a t i c p r o g r a m m i n g " in 1954 I h a v e
m e n t i o n e d o n l y t h o s e a c t u a l s y s t e m s of w h i c h
m y c o l l e a g u e s and I w e r e a w a r e at the time.
For a c o m p r e h e n s i v e t r e a t m e n t of e a r l y p r o g r a i n i n g s y s t e m s and l a n g u a g e s I r e c o m m e n d
the a r t i c l e by K n u t h and P a r d o [1977] and
S a m m e t [1969].)
1.2
The e c o n o m i c s
The L a n i n g and Z i e r l e r s y s t e m a c c e p t e d
as i n p u t an e l e g a n t b u t r a t h e r s i m p l e a l g e braic language.
It p e r m i t t e d s i n g l e - l e t t e r
variables (identifiers) which could have a
s i n g l e c o n s t a n t or v a r i a b l e s u b s c r i p t .
The
r e p e r t o i r e of f u n c t i o n s one c o u l d use w e r e
d e n o t e d by "F" w i t h an i n t e g e r s u p e r s c r i p t
to i n d i c a t e the " c a t a l o g n u m b e r " of the desired function.
Algebraic expressions were
c o m p i l e d into c l o s e d s u b r o u t i n e s and p l a c e d
on a m a g n e t i c d r u m for s u b s e q u e n t use.
The
s y s t e m was o r i g i n a l l y d e s i g n e d for the
W h i r l w i n d c o m p u t e r w h e n it had 1,024 s t o r age cells, w i t h the r e s u l t t h a t it c a u s e d
a s l o w d o w n in e x e c u t i o n s p e e d by a f a c t o r
of a b o u t ten [Adams and L a n i n g 1954].
of p r o g r a m m i n g .
A n o t h e r f a c t o r w h i c h i n f l u e n c e d the dev e l o p m e n t of F O R T R A N w a s the e c o n o m i c s of
p r o g r a m m i n g in 1954.
The c o s t of p r o g r a m m e r s a s s o c i a t e d w i t h a c o m p u t e r c e n t e r was
u s u a l l y at l e a s t as g r e a t as the c o s t of the
c o m p u t e r itself.
(This f a c t f o l l o w s f r o m
the a v e r a g e s a l a r y - p l u s - o v e r h e a d and n u m b e r
of p r o g r a m m e r s at e a c h c e n t e r and f r o m the
computer rental figures.)
In a d d i t i o n , f r o m
one q u a r t e r to one h a l f of the c o m p u t e r ' s
t i m e was s p e n t in d e b u g g i n g .
Thus p ~ o g r a m m i n g and d e b u g g i n g a c c o u n t e d for as m u c h as
t h r e e q u a r t e r s of the c o s t of o p e r a t i n g a
c o m p u t e r ; and o b v i o u s l y , as c o m p u t e r s g o t
c h e a p e r , this s i t u a t i o n w o u l d get worse.
The e f f e c t of the L a n i n g and Z i e r l e r
s y s t e m on the d e v e l o p m e n t of F O R T R A N is a
q u e s t i o n w h i c h has b e e n m u d d l e d by m a n y
m i s s t a t e m e n t s on my part.
For m a n y y e a r s
I b e l i e v e d t h a t we had g o t t e n the idea for
u s i n g a l g e b r a i c n o t a t i o n in F O R T R A N f r o m
s e e i n g a d e m o n s t r a t i o n of the L a n i n g and
Z i e r l e r s y s t e m at MIT.
In p r e p a r i n g a paper [Backus 1976] for the I n t e r n a t i o n a l
R e s e a r c h C o n f e r e n c e on the H i s t o r y of Comp u t i n g at Los A l a m o s (June 10-15, 1976),
I r e v i e w e d the m a t t e r w i t h I r v i n g Z i l l e r
and o b t a i n e d a c o p y of a 1954 l e t t e r [Backus
1954a] (which Dr. L a n i n g k i n d l y s e n t to me).
As a r e s u l t the f a c t s of the m a t t e r h a v e
b e c o m e clear.
The l e t t e r in q u e s t i o n is
one I sent to Dr. L a n i n g a s k i n g for a
d e m o n s t r a t i o n of his system.
It m a k e s c l e a r
t h a t we had l e a r n e d of his w o r k at the
O f f i c e of N a v a l R e s e a r c h S y m p o s i u m on A u t o m a t i c P r o g r a m m i n g for D i g i t a l C o m p u t e r s ,
M a y 13-14, 1954, and t h a t the d e m o n s t r a t i o n
t o o k p l a c e on J u n e 2, 1954.
The l e t t e r
a l s o m a k e s c l e a r t h a t the F O R T R A N p r o j e c t
was w e l l u n d e r w a y w h e n the l e t t e r was s e n t
(May 21, 1954) and i n c l u d e d H a r l a n H e r r i c k ,
R o b e r t A. N e l s o n , and I r v i n g Z i l l e r as w e l l
as m y s e l f .
F u r t h e r m o r e , an a r t i c l e in the
p r o c e e d i n g s of t h a t same O N R S y m p o s i u m by
H e r r i c k and m y s e l f [Backus and H e r r i c k 1954]
shows c l e a r l y t h a t we w e r e a l r e a d y c o n s i d e r i n g i n p u t e x p r e s s i o n s like "Zaij •b jk "
T h i s e c o n o m i c f a c t o r was one of the p r i m e
m o t i v a t i o n s w h i c h led m e to p r o p o s e the F O R T R A N p r o j e c t in a l e t t e r to m y boss, C u t h b e r t Hurd, in late 1953 (the e x a c t d a t e is
n o t k n o w n but o t h e r f a c t s s u g g e s t D e c e m b e r
1953 as a l i k e l y date).
I b e l i e v e t h a t the
e c o n o m i c n e e d for a s y s t e m like F O R T R A N was
one r e a s o n w h y IBM and my s u c c e s s i v e bosses,
Hurd, C h a r l e s D e C a r l o , and J o h n M c P h e r s o n ,
p r o v i d e d for our c o n s t a n t l y e x p a n d i n g n e e d s
o v e r the n e x t five y e a r s w i t h o u t e v e r asking us to p r o j e c t or j u s t i f y t h o s e n e e d s in
a f o r m a l budget.
1.3
Programming
systems
in 1954.
It is d i f f i c u l t for a p r o g r a m m e r of tod a y to c o m p r e h e n d w h a t " a u t o m a t i c p r o g r a m m i n g " m e a n t to p r o g r a m m e r s in 1954.
To
m a n y it t h e n m e a n t s i m p l y p r o v i d i n g m n e m o n ic o p e r a t i o n c o d e s and s y m b o l i c a d d r e s s e s ,
to o t h e r s it m e a n t the s i m p l e ' p r o c e s s of
o b t a i n i n g s u b r o u t i n e s f r o m a l i b r a r y and
i n s e r t i n g the a d d r e s s e s of o p e r a n d s i n t o
each subroutine.
Most "automatic programming" systems were either assembly programs,
or s u b r o u t i n e - f i x i n g p r o g r a m s , or, m o s t
p o p u l a r l y , i n t e r p r e t i v e s y s t e m s to p r o v i d e
f l o a t i n g p o i n t and i n d e x i n g o p e r a t i o n s .
My f r i e n d s and I w e r e a w a r e of a n u m b e r of
a s s e m b l y p r o g r a m s and i n t e r p r e t i v e systems,
some of w h i c h h a v e b e e n m e n t i o n e d above;
b e s i d e s t h e s e t h e r e w e r e p r i m a r i l y two
o t h e r s y s t e m s of s i g n i f i c a n c e : the A-2
c o m p i l e r [ R e m i n g t o n R a n d 1953; M o s e r 1954]
and the L a n i n g and Z i e r l e r [1954] a l g e b r a i c
c o m p i l e r at MIT.
As n o t e d above, the A-2
c o m p i l e r was at t h a t time l a r g e l y a subr o u t i n e - f i x e r (its o t h e r p r i n c i p a l t a s k was
to p r o v i d e for " o v e r l a y s " ) ; but f r o m the
and "X÷Y".
We w e n t on to r a i s e the q u e s tion "...can a machine translate a sufficiently rich mathematical language into a
s u f f i c i e n t l y e c o n o m i c a l p r o g r a m at a suff i c i e n t l y low c o s t to m a k e the w h o l e a f f a i r
feasible?"
T h e s e and o t h e r r e m a r k s in o u r p a p e r
p r e s e n t e d at the S y m p o s i u m in M a y 1954 m a k e
it c l e a r t h a t we w e r e a l r e a d y c o n s i d e r i n g
algebraic input considerably more sophist i c a t e d t h a n t h a t of L a n i n g and Z i e r l e r ' s
s y s t e m w h e n we f i r s t h e a r d of t h e i r p i o n e e r ing work.
Thus, a l t h o u g h L a n i n g and Z i e r l e r
had a l r e a d y p r o d u c e d the w o r l d ' s f i r s t al-
166
g e b r a i c c o m p i l e r , our b a s i c ideas for F O R T R A N had b e e n d e v e l o p e d i n d e p e n d e n t l y ; thus
it is d i f f i c u l t to k n o w what, if any, new
ideas we got f r o m s e e i n g the d e m o n s t r a t i o n
of t h e i r system.
e x i s t e d to p r o v i d e ; second, it i n c r e a s e d the
p r o b l e m of g e n e r a t i n g e f f i c i e n t p r o g r a m s by
an o r d e r of m a g n i t u d e by s p e e d i n g up f l o a t ing p o i n t o p e r a t i o n s by a f a c t o r of ten and
t h e r e b y l e a v i n g i n e f f i c i e n c i e s n o w h e r e to
hide.
In v i e w of the w i d e s p r e a d s k e p t i c i s m
a b o u t the p o s s i b i l i t y of p r o d u c i n g e f f i c i e n t
p r o g r a m s w i t h an a u t o m a t i c p r o g r a m m i n g system and the f a c t that i n e f f i c i e n c i e s c o u l d
no l o n g e r be hidden, we w e r e c o n v i n c e d t h a t
the k i n d of s y s t e m we had in m i n d w o u l d be
w i d e l y u s e d o n l y if we c o u l d d e m o n s t r a t e
that it w o u l d p r o d u c e p r o g r a m s a l m o s t as
e f f i c i e n t as h a n d c o d e d o n e s and do so on
v i r t u a l l y e v e r y job.
Q u a s i - f o o t n o t e : In r e s p o n s e
to s u g g e s t i o n s of the P r o g r a m C o m m i t t e e
let me try to d e a l e x p l i c i t l y w i t h
the q u e s t i o n of w h a t w o r k m i g h t h a v e inf l u e n c e d our e a r l y ideas for F O R T R A N , alt h o u g h it is m o s t l y a m a t t e r of l i s t i n g
w o r k of w h i c h we w e r e then u n a w a r e .
I have
a l r e a d y d i s c u s s e d the w o r k of L a n i n g and
Z i e r l e r and the A-2 c o m p i l e r .
The w o r k of
H e i n z R u t i s h a u s e r [1952] is d i s c u s s e d l a t e r
on.
L i k e m o s t of the w o r l d (except p e r h a p s
R u t i s h a u s e r and C o r r a d o B 6 h m - - w h o was the
f i r s t to d e s c r i b e a c o m p i l e r in its own
l a n g u a g e [B6hm 195~]) we w e r e e n t i r e l y una w a r e of the w o r k of K o n r a d Zuse [1959;
1972].
Zuse's " P l a n k a l k ~ l " , w h i c h he comp l e t e d in 1945, was, in some ways, a m o r e
e l e g a n t and a d v a n c e d p r o g r a m m i n g l a n g u a g e
t h a n t h o s e t h a t a p p e a r e d ten and f i f t e e n
y e a r s later.
It was our b e l i e f that if F O R T R A N , d u r ing its f i r s t m o n t h s , w e r e to t r a n s l a t e any
r e a s o n a b l e " s c i e n t i f i c " s o u r c e p r o g r a m into
an o b j e c t p r o g r a m o n l y half as fast as its
h a n d c o d e d c o u n t e r p a r t , then a c c e p t a n c e of
our s y s t e m w o u l d be in s e r i o u s danger.
This b e l i e f c a u s e d us to r e g a r d the d e s i g n
of the t r a n s l a t o r as the r e a l c h a l l e n g e ,
not the s i m p l e t a s k of d e s i g n i n g the language.
Our b e l i e f in the s i m p l i c i t y of
l a n g u a g e d e s i g n was p a r t l y c o n f i r m e d by the
relative ease with which similar languages
had b e e n i n d e p e n d e n t l y d e v e l o p e d by R u t i s h a u s e r [1952], L a n i n g and Z i e r l e r [1954],
and o u r s e l v e s ; w h e r e a s we w e r e a l o n e in
s e e k i n g to p r o d u c e r e a l l y e f f i c i e n t o b j e c t
programs.
We w e r e a l s o u n a w a r e of the w o r k of
M a u c h l y et al. ("Short Code", 1950) , B u r k s
( " I n t e r m e d i a t e PL", 1950) , B 6 h m (1951) ,
G l e n n i e ("AUTOCODE", 1952) as d i s c u s s e d in
K n u t h and P a r d o [1977].
We w e r e a w a r e of
but not i n f l u e n c e d by the a u t o m a t i c p r o g r a m ming efforts which simulated a synthetic
c o m p u t e r (e.g., M I T "Summer S e s s i o n Computer", SHACO, DUAL, S P E E D C O D I N G , and the
I L L I A C system), s i n c e t h e i r l a n g u a g e s and
s y s t e m s w e r e so d i f f e r e n t f r o m t h o s e of
FORTRAN.
Nor w e r e we i n f l u e n c e d by a l g e braic systems which were designed after
our " P r e l i m i n a r y R e p o r t " [1954] but w h i c h
b e g a n o p e r a t i o n b e f o r e F O R T R A N (e.g.,
B A C A I C [Grems and P o r t e r 1956], IT [Perlis, S m i t h and V a n Z o e r e n 1957], M A T H M A T I C [Ash et al. 1957]).
Although PACT
I [Baker 1956] was not an a l g e b r a i c compiler, it d e s e r v e s m e n t i o n as a s i g n i f i c a n t d e v e l o p m e n t d e s i g n e d a f t e r the F O R T R A N l a n g u a g e but in o p e r a t i o n b e f o r e
F O R T R A N , w h i c h a l s o did not i n f l u e n c e our
work.
(End of q u a s i - f o o t n o t e . )
To this d a y I b e l i e v e t h a t our e m p h a s i s
on o b j e c t p r o g r a m e f f i c i e n c y r a t h e r t h a n on
l a n g u a g e d e s i g n was b a s i c a l l y correct.
I
b e l i e v e t h a t had we f a i l e d to p r o d u c e eff i c i e n t p r o g r a m s , the w i d e s p r e a d use of
l a n g u a g e s like F O R T R A N w o u l d h a v e b e e n seriously delayed.
In fact, I b e l i e v e t h a t
we are in a similar, but u n r e c o g n i z e d , situ a t i o n today: in s p i t e of all the fuss t h a t
has b e e n m a d e o v e r m y r i a d l a n g u a g e d e t a i l s ,
c u r r e n t c o n v e n t i o n a l l a n g u a g e s are still
v e r y w e a k p r o g r a m m i n g aids, and far m o r e
p o w e r f u l l a n g u a g e s w o u l d be in use t o d a y
if a n y o n e had f o u n d a w a y to m a k e t h e m run
with adequate efficiency.
In o t h e r words,
the n e x t r e v o l u t i o n in p r o g r a m m i n g w i l l
take p l a c e o n l y w h e n b o t h of the f o l l o w i n g
r e q u i r e m e n t s h a v e b e e n met: (a) a new k i n d
of p r o g r a m m i n g l a n g u a g e , far m o r e p o w e r f u l
t h a n those of today, has b e e n d e v e l o p e d
and (b) a t e c h n i q u e has b e e n f o u n d for exe c u t i n g its p r o g r a m s at not m u c h g r e a t e r
c o s t than t h a t of t o d a y ' s p r o g r a m s .
Our O N R S y m p o s i u m a r t i c l e [Backus and
H e r r i c k 195~] a l s o m a k e s c l e a r t h a t the
F O R T R A N g r o u p was a l r e a d y a w a r e t h a t it
f a c e d a n e w k i n d of p r o b l e m in a u t o m a t i c
programming.
The v i a b i l i t y of m o s t c o m p i l ers and i n t e r p r e t e r s p r i o r to F O R T R A N had
r e s t e d on the f a c t that m o s t s o u r c e l a n g u a g e
o p e r a t i o n s w e r e not m a c h i n e o p e r a t i o n s .
Thus e v e n l a r g e i n e f f i c i e n c i e s in p e r f o r m ing
b o t h l o o p i n g / t e s t i n g o p e r a t i o n s and
c o m p u t i n g a d d r e s s e s w e r e m a s k e d by m o s t ope r a t i n g time b e i n g s p e n t in f l o a t i n g p o i n t
subroutines.
B u t the a d v e n t of the 70~ w i t h
b u i l t in. f l o a t i n g p o i n t and i n d e x i n g r a d i c a l l y a l t e r e d the s i t u a t i o n .
The 70~ p r e s e n t e d a d o u b l e c h a l l e n g e to t h o s e w h o
w a n t e d to s i m p l i f y p r o g r a m m i n g ; f i r s t it rem o v e d the r a i s o n d ' E t r e of e a r l i e r s y s t e m s
by p r o v i d i n g in h a r d w a r e the o p e r a t i o n s t h e y
B e c a u s e of our 1954 v i e w t h a t s u c c e s s in
p r o d u c i n g e f f i c i e n t p r o g r a m s was m o r e imp o r t a n t than the d e s i g n of the F O R T R A N language, I c o n s i d e r the h i s t o r y of the comp i l e r c o n s t r u c t i o n and the w o r k of its inv e n t o r s an i n t e g r a l p a r t of the h i s t o r y of
the F O R T R A N l a n g u a g e ; t h e r e f o r e a later
s e c t i o n d e a l s w i t h that subject.
2. The e a r l y
s t a g e s of the F O R T R A N p r o j e c t .
After Cuthbert Hurd approved my proposal
to d e v e l o p a p r a c t i c a l a u t o m a t i c p r o g r a m m i n g s y s t e m for the 704 in D e c e m b e r 1953 or
167
J a n u a r y 1954, I r v i n g Z i l l e r w a s a s s i g n e d to
the p r o j e c t .
We s t a r t e d w o r k in one of the
m a n y s m a l l o f f i c e s the p r o j e c t w a s to occ u p y in the v i c i n i t y of IBM h e a d q u a r t e r s
at 590 M a d i s o n A v e n u e in N e w York; the
f i r s t of t h e s e was in the Jay T h o r p e B u i l d ing on F i f t h A v e n u e .
By M a y 1954 we h a d
b e e n j o i n e d by H a r l a n H e r r i c k and then by
a n e w e m p l o y e e w h o h a d b e e n h i r e d to do
t e c h n i c a l typing, R o b e r t A. N e l s o n (with
Ziller, he soon b e g a n d e s i g n i n g o n e of the
m o s t s o p h i s t i c a t e d s e c t i o n s of the c o m p i l e r ;
he is n o w an IBM F e l l o w ) .
By a b o u t M a y we
h a d m o v e d to the 19th f l o o r of the a n n e x of
590 M a d i s o n A v e n u e , n e x t to the e l e v a t o r
m a c h i n e r y ; the g r o u n d f l o o r of this b u i l d ing h o u s e d the 701 i n s t a l l a t i o n on w h i c h
c u s t o m e r s t e s t e d t h e i r p r o g r a m s b e f o r e the
a r r i v a l of t h e i r o w n m a c h i n e s .
It w a s h e r e
t h a t m o s t of the F O R T R A N l a n g u a g e w a s designed, m o s t l y by H e r r i c k , Z i l l e r and m y self, e x c e p t t h a t m o s t of the i n p u t - o u t p u t
l a n g u a g e and f a c i l i t i e s w e r e d e s i g n e d by
Roy Nutt, an e m p l o y e e of U n i t e d A i r c r a f t
Corp. w h o w a s soon to b e c o m e a m e m b e r of
the F O R T R A N p r o j e c t .
puter, n o t to m e n t i o n t h o s e of o t h e r m a n u facturers.
(After all, t h e r e w e r e v e r y few
c o m p u t e r s a r o u n d then.)
B u t we did e x p e c t
our s y s t e m to h a v e a big impact, in the
s e n s e t h a t it w o u l d m a k e p r o g r a m m i n g for
the 704 v e r y m u c h faster, c h e a p e r , m o r e reliable.
W e a l s o e x p e c t e d that, if we w e r e
s u c c e s s f u l in m e e t i n g our g o a l s , o t h e r
g r o u p s and m a n u f a c t u r e r s w o u l d f o l l o w our
e x a m p l e in r e d u c i n g the c o s t of p r o g r a m m i n g
by p r o v i d i n g s i m i l a r s y s t e m s w i t h d i f f e r e n t
but similar languages
[Preliminary Report
1954].
By the fall of 1954 w e had b e c o m e the
" P r o g r a m m i n g R e s e a r c h G r o u p " and I h a d bec o m e its " m a n a g e r " .
By N o v e m b e r of t h a t
y e a r we h a d p r o d u c e d a paper: " P r e l i m i n a r y
R e p o r t , S p e c i f i c a t i o n s for the I B M M a t h e m a t ical F O R m u l a T R A N s l a t i n g S y s t e m , F O R T R A N "
[ P r e l i m i n a r y R e p o r t 1954] d a t e d N o v e m b e r 10.
In its i n t r o d u c t i o n w e n o t e d t h a t " s y s t e m s
w h i c h h a v e s o u g h t to r e d u c e the job of c o d ing and d e b u g g i n g p r o b l e m s h a v e o f f e r e d the
c h o i c e of e a s y c o d i n g and s l o w e x e c u t i o n or
l a b o r i o u s c o d i n g and f a s t e x e c u t i o n . "
On
the b a s i s m o r e of f a i t h t h a n of k n o w l e d g e ,
we s u g g e s t e d t h a t p r o g r a m s "will be e x e c u t e d
in a b o u t the same time t h a t w o u l d be req u i r e d h a d the p r o b l e m b e e n l a b o r i o u s l y
hand coded."
In w h a t t u r n e d o u t to be a
t r u e s t a t e m e n t , we said t h a t " F O R T R A N m a y
a p p l y c o m p l e x , l e n g t h y t e c h n i q u e s in c o d i n g
a p r o b l e m w h i c h the h u m a n c o d e r w o u l d h a v e
n e i t h e r the t i m e nor i n c l i n a t i o n to d e r i v e
or a p p l y . "
A f t e r we h a d f i n i s h e d d e s i g n i n g m o s t of
the l a n g u a g e we h e a r d a b o u t R u t i s h a u s e r ' s
p r o p o s a l s for a s i m i l a r l a n g u a g e [Rutish a u s e r 1952].
It w a s c h a r a c t e r i s t i c of the
u n s c h o l a r l y a t t i t u d e of m o s t p r o g r a m m e r s
then, and of o u r s e l v e s in p a r t i c u l a r , t h a t
we d i d n o t b o t h e r to c a r e f u l l y r e v i e w the
s k e t c h y t r a n s l a t i o n of his p r o p o s a l s t h a t
we f i n a l l y o b t a i n e d , s i n c e f r o m t h e i r symb o l i c c o n t e n t they d i d n o t a p p e a r to add
a n y t h i n g n e w to our p r o p o s e d l a n g u a g e .
R u t i s h a u s e r ' s l a n g u a g e h a d a for s t a t e m e n t
and o n e - d i m e n s i o n a l a r r a y s , b u t no IF, GOTO,
n o r I/O s t a t e m e n t s .
Subscript variables
c o u l d n o t be u s e d as o r d i n a r y v a r i a b l e s and
operator precedence was ignored.
His 1952
a r t i c l e d e s c r i b e d two c o m p i l e r s for this
l a n g u a g e (for m o r e d e t a i l s see [Knuth and
P a r d o 1977]).
As far as we w e r e aware, w e s i m p l y m a d e
up the l a n g u a g e as we w e n t along.
We did
not r e g a r d l a n g u a g e d e s i g n as a d i f f i c u l t
p r o b l e m , m e r e l y a s i m p l e p r e l u d e to the
real problem: designing a compiler which
could produce efficient programs.
Of
c o u r s e o n e of our g o a l s was to d e s i g n a
l a n g u a g e w h i c h w o u l d m a k e it p o s s i b l e for
e n g i n e e r s and s c i e n t i s t s to w r i t e p r o g r a m s
t h e m s e l v e s for the 704.
W e a l s o w a n t e d to
e l i m i n a t e a lot of the b o o k k e e p i n g and detailed, repetitive planning which hand coding i n v o l v e d .
V e r y e a r l y in our w o r k w e
h a d in m i n d the n o t i o n s of a s s i g n m e n t s t a t e m e n t s , s u b s c r i p t e d v a r i a b l e s , and the DO
s t a t e m e n t (which I b e l i e v e w a s p r o p o s e d by
Herrick).
We felt that these provided a
g o o d b a s i s for a c h i e v i n g o u r g o a l s for the
l a n g u a g e , and w h a t e v e r else w a s n e e d e d em e r g e d as w e t r i e d to b u i l d a w a y of p r o g r a m m i n g o n t h e s e b a s i c ideas.
The l a n g u a g e d e s c r i b e d in the " P r e l i m i n a r y R e p o r t " h a d v a r i a b l e s of o n e or two
c h a r a c t e r s in length, f u n c t i o n n a m e s of
t h r e e or m o r e c h a r a c t e r s , r e c u r s i v e l y defined "expressions",
subscripted variables
w i t h up to t h r e e s u b s c r i p t s ,
"arithmetic
formulas"
(which t u r n o u t to be a s s i g n m e n t
s t a t e m e n t s ) , and " D O - f o r m u l a s " .
T h e s e latter f o r m u l a s c o u l d s p e c i f y b o t h the f i r s t
and l a s t s t a t e m e n t s to be c o n t r o l l e d , thus
p e r m i t t i n g a DO to c o n t r o l a d i s t a n t seq u e n c e of s t a t e m e n t s , as w e l l as s p e c i f y i n g
a t h i r d s t a t e m e n t to w h i c h c o n t r o l w o u l d
p a s s f o l l o w i n g the end of the i t e r a t i o n .
If o n l y o n e s t a t e m e n t was s p e c i f i e d , the
"range" of the DO w a s the s e q u e n c e of s t a t e m e n t s f o l l o w i n g the DO d o w n to the s p e c i f i e d
statement.
E x p r e s s i o n s in " a r i t h m e t i c f o r m u l a s "
c o u l d be "mixed": i n v o l v e b o t h " f i x e d p o i n t "
(integer) a n d " f l o a t i n g p o i n t " q u a n t i t i e s .
The a r i t h m e t i c u s e d (all i n t e g e r or all
f l o a t i n g point) to e v a l u a t e a m i x e d e x p r e s sion w a s d e t e r m i n e d by the t y p e of the
v a r i a b l e on the l e f t of the "=" sign.
"IFf o r m u l a s " e m p l o y e d an e q u a l i t y or i n e q u a l ity sign ("=" or ">" or ">=") b e t w e e n two
(restricted) e x p r e s s i o n s , f o l l o w e d by two
s t a t e m e n t n u m b e r s , o n e for the "true" case,
the o t h e r for the "false" case.
A " R e l a b e l f o r m u l a " w a s d e s i g n e d to m a k e
it e a s y to r o t a t e , say, the i n d i c e s of the
r o w s of a m a t r i x so t h a t the same c o m p u t a -
W e c e r t a i n l y h a d no idea t h a t l a n g u a g e s
a l m o s t i d e n t i c a l to the o n e w e w e r e w o r k i n g
on w o u l d be u s e d for m o r e t h a n one IBM com-
168
a p r o g r a m "by i n d e p e n d e n t l y r e c r e a t i n g the
s p e c i f i c a t i o n s for a p r o b l e m f r o m its F O R T R A N f o r m u l a t i o n [!]"
It says n o t h i n g
a b o u t the s y s t e m c a t c h i n g s y n t a c t i c errors,
but n o t e s t h a t an e r r o r - f i n d i n g p r o g r a m can
be w r i t t e n a f t e r some e x p e r i e n c e w i t h e r r o r s
has b e e n a c c u m u l a t e d .
tion w o u l d apply, a f t e r r e l a b e l l i n g , e v e n
t h o u g h a n e w r o w had b e e n r e a d in and the
n e x t c o m p u t a t i o n was n o w to take p l a c e on
a d i f f e r e n t , r o t a t e d set of rows.
Thus,
for e x a m p l e , if b is a 4 by 4 m a t r i x , a f t e r
R E L A B E L b(3,1), a r e f e r e n c e to b(1,j) has
the same m e a n i n g as b(3,j) b e f o r e r e l a b e l ling; b(2,j) a f t e r = b(4,j) before; b(3,j)
a f t e r = b(1,j) before; and b(4,j) a f t e r =
b(2,j) b e f o r e r e l a b e l l i n g .
U n f o r t u n a t e l y we w e r e h o p e l e s s l y o p t i m i s t i c in 1954 a b o u t the p r o b l e m s of d e b u g g i n g F O R T R A N p r o g r a m s (thus we find on p a g e
2 of the Report: " S i n c e F O R T R A N s h o u l d v i r t u a l l y e l i m i n a t e c o d i n g and d e b u g g i n g . . .
[!]") and h e n c e s y n t a c t i c e r r o r c h e c k i n g
f a c i l i t i e s in the f i r s t d i s t r i b u t i o n of
F O R T R A N I w e r e weak.
Better facilities
w e r e a d d e d not long a f t e r d i s t r i b u t i o n and
f a i r l y g o o d s y n t a c t i c c h e c k i n g was p r o v i d e d
in F O R T R A N II.
The i n p u t - o u t p u t s t a t e m e n t s p r o v i d e d inc l u d e d the b a s i c n o t i o n of s p e c i f y i n g the
s e q u e n c e in w h i c h d a t a was to be r e a d in or
out, but did not i n c l u d e any "Format" s t a t e ments.
The R e p o r t also lists four k i n d s of
" s p e c i f i c a t i o n s e n t e n c e s " : (I) " d i m e n s i o n
s e n t e n c e s " for g i v i n g the d i m e n s i o n s of arrays, (2) " e q u i v a l e n c e s e n t e n c e s " for ass i g n i n g the same s t o r a g e l o c a t i o n s to v a r iables, (3) " f r e q u e n c y s e n t e n c e s " for ind i c a t i n g e s t i m a t e d r e l a t i v e f r e q u e n c y of
b r a n c h p a t h s or loops to h e l p the c o m p i l e r
o p t i m i z e the o b j e c t p r o g r a m , and (4) "rela t i v e c o n s t a n t s e n t e n c e s " to i n d i c a t e subs c r i p t v a r i a b l e s w h i c h are e x p e c t e d to
change their values very infrequently.
The F O R T R A N l a n g u a g e d e s c r i b e d in the
Programmer's Reference Manual dated October
15, 1956 [IBM 1956] d i f f e r e d in a few res p e c t s f r o m t h a t of the P r e l i m i n a r y Report,
but, c o n s i d e r i n g our i g n o r a n c e in 1954 of
the p r o b l e m s we w o u l d later e n c o u n t e r in
p r o d u c i n g the c o m p i l e r , there w e r e r e m a r k a b l y few d e l e t i o n s (the R e l a b e l and R e l a tive C o n s t a n t s t a t e m e n t s ) , a few r e t r e a t s ,
some f o r t u n a t e , some not ( s i m p l i f i c a t i o n of
DO s t a t e m e n t s , d r o p p i n g i n e q u a l i t i e s f r o m
IF s t a t e m e n t s - - f o r lack of a ">" symbol,
and p r o h i b i t i n g m o s t "mixed" e x p r e s s i o n s
and s u b s c r i p t e d s u b s c r i p t s ) , and the r e c t i f i c a t i o n of a few o m i s s i o n s ( a d d i t i o n of
FORMAT, C O N T I N U E , c o m p u t e d and a s s i g n e d GOTO s t a t e m e n t s , i n c r e a s i n g the l e n g t h of v a r i a b l e s to up to six c h a r a c t e r s , and g e n e r a l
i m p r o v e m e n t of i n p u t - o u t p u t s t a t e m e n t s ) .
T o w a r d the end of the R e p o r t (pp. 26-27)
t h e r e is a s e c t i o n " F u t u r e a d d i t i o n s to the
F O R T R A N system".
Its f i r s t i t e m is: "a
v a r i e t y of new i n p u t - o u t p u t f o r m u l a s w h i c h
w o u l d e n a b l e the p r o g r a m m e r to s p e c i f y v a r ious f o r m a t s for cards, p r i n t i n g , i n p u t
tapes and o u t p u t tapes"
It is b e l i e v e d
t h a t this i t e m is a r e s u l t of our e a r l y
c o n s u l t a t i o n s w i t h Roy Nutt.
This s e c t i o n
goes on to list o t h e r p r o p o s e d f a c i l i t i e s
to be added: c o m p l e x and d o u b l e p r e c i s i o n
arithmetic, matrix arithmetic, sorting,
solving simultaneous equations, differential
e q u a t i o n s , and l i n e a r p r o g r a m m i n g p r o b l e m s .
It a l s o d e s c r i b e s f u n c t i o n d e f i n i t i o n capa b i l i t i e s s i m i l a r to those w h i c h later app e a r e d in F O R T R A N II; f a c i l i t i e s for numerical integration; a summation operator;
and t a b l e l o o k u p f a c i l i t i e s .
The final s e c t i o n of the R e p o r t (pp 2829) d i s c u s s e s p r o g r a m m i n g t e c h n i q u e s to use
to h e l p the s y s t e m p r o d u c e e f f i c i e n t p r o grams.
It d i s c u s s e s h o w to use p a r e n t h e s e s
to h e l p the s y s t e m i d e n t i f y i d e n t i c a l sube x p r e s s i o n s w i t h i n an e x p r e s s i o n and t h e r e by e l i m i n a t e t h e i r d u p l i c a t e c a l c u l a t i o n .
T h e s e p a r e n t h e s e s had to be s u p p l i e d o n l y
w h e n a r e c u r r i n g s u b e x p r e s s i o n o c c u r r e d as
p a r t of a t e r m (e.g., if a~b o c c u r r e d in
s e v e r a l places, it w o u l d be b e t t e r to w r i t e
the t e r m a ~ b ~ c as (a~b)~c to a v o i d d u p l i c a t e
c a l c u l a t i o n ) ; o t h e r w i s e the s y s t e m w o u l d
i d e n t i f y d u p l i c a t e s w i t h o u t any a s s i s t a n c e .
It a l s o o b s e r v e s t h a t the s y s t e m w o u l d not
p r o d u c e o p t i m a l c o d e for loops c o n s t r u c t e d
w i t h o u t DO s t a t e m e n t s .
This final s e c t i o n of the R e p o r t a l s o
n o t e s that "no s p e c i a l p r o v i s i o n s h a v e b e e n
i n c l u d e d in the F O R T R A N s y s t e m for l o c a t i n g
e r r o r s in f o r m u l a s " .
It s u g g e s t s c h e c k i n g
169
S i n c e our e n t i r e a t t i t u d e a b o u t l a n g u a g e
d e s i g n had a l w a y s b e e n a v e r y c a s u a l one,
the c h a n g e s w h i c h we felt to be d e s i r a b l e
d u r i n g the c o u r s e of w r i t i n g the c o m p i l e r
were made equally casually.
We n e v e r f e l t
t h a t any of t h e m i n v o l v e d a real s a c r i f i c e
in c o n v e n i e n c e or p o w e r (with the p o s s i b l e
e x c e p t i o n of the R e l a b e l s t a t e m e n t , w h o s e
p u r p o s e was to c o o r d i n a t e i n p u t - o u t p u t w i t h
c o m p u t a t i o n s on arrays, but this was one
f a c i l i t y w h i c h we felt w o u l d h a v e b e e n
r e a l l y d i f f i c u l t to i m p l e m e n t ) .
I believe
the s i m p l i f i c a t i o n of the o r i g i n a l DO s t a t e m e n t r e s u l t e d f r o m the r e a l i z a t i o n t h a t
(a) it w o u l d be h a r d to d e s c r i b e p r e c i s e l y ,
(b) it was a w k w a r d to c o m p i l e , and (c) it
p r o v i d e d l i t t l e p o w e r b e y o n d t h a t of the
final v e r s i o n .
In our n a i v e u n a w a r e n e s s of l a n g u a g e
d e s i g n p r o b l e m s - - o f c o u r s e we k n e w n o t h i n g
of m a n y i s s u e s w h i c h w e r e later t h o u g h t to
be i m p o r t a n t , e.g., b l o c k s t r u c t u r e , cond i t i o n a l e x p r e s s i o n s , type d e c l a r a t i o n s - it s e e m e d to us t h a t o n c e one had the not i o n s of the a s s i g n m e n t s t a t e m e n t , the subs c r i p t e d v a r i a b l e , and the DO s t a t e m e n t in
h a n d (and t h e s e w e r e a m o n g our e a r l i e s t ideas), then the r e m a i n i n g p r o b l e m s of lang u a g e d e s i g n w e r e t r i v i a l : e i t h e r t h e i r solu t i o n was t h r u s t u p o n one by the n e e d to
p r o v i d e some m a c h i n e f a c i l i t y such as r e a d -
ing input, or by some p r o g r a m m i n g t a s k w h i c h
c o u l d n o t be d o n e w i t h e x i s t i n g s t r u c t u r e s
(e.g., s k i p p i n g to the end of a DO l o o p
w i t h o u t s k i p p i n g the i n d e x i n g i n s t r u c t i o n s
there: this g a v e rise to the C O N T I N U E s t a t e ment).
Washington
(D.C.), A l b u q u e r q u e , P i t t s b u r g h ,
Los A n g e l e s , and o n e or two o t h e r p l a c e s
were not very helpful.
O n e t r i p to g i v e o u r talk, p r o b a b l y in
J a n u a r y 1955, h a d an e x c e l l e n t p a y o f f . This
talk, at U n i t e d A i r c r a f t Corp., r e s u l t e d in
an a g r e e m e n t b e t w e e n o u r g r o u p and W a l t e r
R a m s h a w at U n i t e d A i r c r a f t t h a t Roy N u t t
w o u l d b e c o m e a r e g u l a r p a r t of o u r e f f o r t
( a l t h o u g h r e m a i n i n g an e m p l o y e e of U n i t e d
A i r c r a f t ) to c o n t r i b u t e his e x p e r t i s e on
i n p u t - o u t p u t and a s s e m b l y r o u t i n e s .
With a
few b r e a k s d u e to his i n v o l v e m e n t in w r i t i n g
v a r i o u s S H A R E p r o g r a m s , he w o u l d t h e n c e f o r t h
c o m e to N e w Y o r k two or t h r e e t i m e s a w e e k
u n t i l e a r l y 1957.
O n e m u c h - c r i t i c i z e d d e s i g n c h o i c e in
F O R T R A N c o n c e r n s the u s e of spaces: b l a n k s
w e r e i g n o r e d , e v e n b l a n k s in the m i d d l e of
an i d e n t i f i e r .
Roy Nutt reminds me that
t h a t c h o i c e w a s p a r t l y in r e c o g n i t i o n of a
p r o b l e m w i d e l y k n o w n in SHARE, the 704 users' a s s o c i a t i o n .
There was a common prob l e m w i t h k e y p u n c h e r s n o t r e c o g n i z i n g or
p r o p e r l y c o u n t i n g b l a n k s in h a n d w r i t t e n
data, a n d this c a u s e d m a n y e r r o r s .
We also
r e g a r d e d i g n o r i n g b l a n k s as a d e v i c e to ena b l e p r o g r a m m e r s to a r r a n g e t h e i r p r o g r a m s
in a m o r e r e a d a b l e f o r m w i t h o u t a l t e r i n g
t h e i r m e a n i n g or i n t r o d u c i n g c o m p l e x r u l e s
for f o r m a t t i n g s t a t e m e n t s .
It is d i f f i c u l t to a s s e s s the i n f l u e n c e
the e a r l y w o r k of the F O R T R A N g r o u p h a d on
other projects.
C e r t a i n l y the d i s c u s s i o n
of L a n i n g and Z i e r l e r ' s a l g e b r a i c c o m p i l e r
at the O N R S y m p o s i u m in M a y 1954 w o u l d h a v e
b e e n m o r e l i k e l y to p e r s u a d e s o m e o n e to und e r t a k e a s i m i l a r l i n e of e f f o r t t h a n w o u l d
the b r i e f d i s c u s s i o n of the m e r i t s of u s i n g
"a f a i r l y n a t u r a l m a t h e m a t i c a l
language"
t h a t a p p e a r e d t h e r e in the p a p e r by H e r r i c k
and m y s e l f [Backus and H e r r i c k 1954].
But
it w a s o u r i m p r e s s i o n t h a t our d i s c u s s i o n s
w i t h v a r i o u s g r o u p s a f t e r t h a t time, t h e i r
a c c e s s to our P r e l i m i n a r y R e p o r t , and t h e i r
a w a r e n e s s of the e x t e n t and s e r i o u s n e s s of
our e f f o r t s , t h a t t h e s e f a c t o r s e i t h e r g a v e
the i n i t i a l s t i m u l u s to some o t h e r p r o j e c t s
or at l e a s t c a u s e d t h e m to be m o r e a c t i v e
than they might have been otherwise.
It
w a s o u r i m p r e s s i o n , for e x a m p l e , t h a t the
"IT" p r o j e c t [Perlis, S m i t h and V a n Z o e r e n
1957] at P u r d u e and l a t e r at C a r n e g i e - M e l l o n
b e g a n s h o r t l y a f t e r the d i s t r i b u t i o n of our
P r e l i m i n a r y R e p o r t , as d i d the " M A T H - M A T I C "
p r o j e c t [Ash et al. 1957] at S p e r r y Rand.
A n o t h e r d e b a t a b l e d e s i g n c h o i c e w a s to
rule out "mixed" m o d e e x p r e s s i o n s i n v o l v i n g
b o t h i n t e g e r and f l o a t i n g p o i n t q u a n t i t i e s .
A l t h o u g h our P r e l i m i n a r y R e p o r t had i n c l u d e d
such e x p r e s s i o n s , a n d r u l e s for e v a l u a t i n g
them, we f e l t t h a t if c o d e for t y p e c o n v e r sion w e r e to be g e n e r a t e d , the u s e r s h o u l d
be a w a r e of that, and the b e s t w a y to i n s u r e
t h a t he w a s a w a r e w a s to ask h i m to s p e c i f y
them.
I b e l i e v e we w e r e a l s o d o u b t f u l of
the u s e f u l n e s s of the r u l e s in o u r R e p o r t
for e v a l u a t i n g m i x e d e x p r e s s i o n s .
In a n y
case, the m o s t c o m m o n sort of " m i x t u r e s "
was a l l o w e d : i n t e g e r e x p o n e n t s and f u n c t i o n a r g u m e n t s w e r e a l l o w e d in a f l o a t i n g
point expression.
In late 1954 and e a r l y 1955, a f t e r comp l e t i n g the P r e l i m i n a r y R e p o r t , H a r l a n H e r rick, I r v i n g Z i l l e r and I g a v e p e r h a p s five
or six t a l k s a b o u t our p l a n s for F O R T R A N to
v a r i o u s g r o u p s of I B M c u s t o m e r s w h o h a d ord e r e d a 704 (the 704 h a d b e e n a n n o u n c e d
a b o u t M a y 1954).
A t t h e s e t a l k s we c o v e r e d
the m a t e r i a l in the R e p o r t and d i s c u s s e d o u r
p l a n s for the c o m p i l e r (which w a s to be comp l e t e d w i t h i n a b o u t six m o n t h s [!] ; this
w a s to r e m a i n the i n t e r v a l - t o - c o m p l e t i o n
u n t i l it a c t u a l l y was c o m p l e t e d o v e r two
y e a r s later, in A p r i l 1957).
In a d d i t i o n
to i n f o r m i n g c u s t o m e r s a b o u t our p l a n s , ano t h e r p u r p o s e of t h e s e t a l k s was to a s s e m b l e
a list of t h e i r o b j e c t i o n s and f u r t h e r requirements.
In this we w e r e d i s a p p o i n t e d ;
o u r l i s t e n e r s w e r e m o s t l y s k e p t i c a l ; I bel i e v e t h e y h a d h e a r d too m a n y g l o w i n g d e s c r i p t i o n s of w h a t t u r n e d o u t to be c l u m s y
s y s t e m s to take us s e r i o u s l y .
In t h o s e d a y s
o n e w a s a c c u s t o m e d to f i n d i n g lots of p e c u l iar b u t s i g n i f i c a n t r e s t r i c t i o n s in a s y s t e m
w h e n it f i n a l l y a r r i v e d t h a t h a d n o t b e e n
m e n t i o n e d in its o r i g i n a l d e s c r i p t i o n .
Most
of all, our c l a i m s t h a t w e w o u l d p r o d u c e efficient object programs were disbelieved.
Whatever thereasons,
w e r e c e i v e d a l m o s t no
s u g g e s t i o n s or f e e d b a c k ; o u r l i s t e n e r s h a d
d o n e a l m o s t no t h i n k i n g a b o u t the p r o b l e m s
w e f a c e d and h a d a l m o s t no s u g g e s t i o n s or
criticisms.
T h u s we f e l t t h a t our t r ip s to
It is n o t c l e a r w h a t i n f l u e n c e , if any,
our Los A n g e l e s t a l k and e a r l i e r c o n t a c t s
w i t h m e m b e r s of t h e i r g r o u p had on the P A C T
I e f f o r t [Baker 1956], w h i c h I b e l i e v e w a s
a l r e a d y in its f o r m a t i v e s t a g e s w h e n we g o t
to Los A n g e l e s .
It is clear, w h a t e v e r inf l u e n c e the s p e c i f i c a t i o n s for F O R T R A N m a y
h a v e had on o t h e r p r o j e c t s in 1 9 5 4 - 5 5 - 5 6 ,
t h a t our p l a n s w e r e w e l l a d v a n c e d a n d q u i t e
f i r m by the end of 1954 and b e f o r e w e h a d
c o n t a c t or k n o w l e d g e of t h o s e o t h e r p r o jects.
Our s p e c i f i c a t i o n s w e r e n o t a f f e c t e d
by t h e m in a n y s i g n i f i c a n t w a y as far as I
am aware, e v e n t h o u g h some w e r e o p e r a t i n g
b e f o r e F O R T R A N w a s (since t h e y w e r e p r i m a r i l y i n t e r e s t e d in p r o v i d i n g an i n p u t lang u a g e r a t h e r t h a n in o p t i m i z a t i o n , t h e i r
t a s k w a s c o n s i d e r a b l y s i m p l e r t h a n ours).
3.
The
construction
of the
compiler.
T h e F O R T R A N c o m p i l e r (or " t r a n s l a t o r " as
w e c a l l e d it then) w a s b e g u n in e a r l y 1955,
a l t h o u g h a lot of w o r k on v a r i o u s s c h e m e s
w h i c h w o u l d be u s e d in it h a d b e e n d o n e in
1954; e.g., H e r r i c k had d o n e a lot of t r i a l
p r o g r a m m i n g to t e s t o u t o u r l a n g u a g e and w e
h a d w o r k e d o u t the b a s i c sort of m a c h i n e
170
p r o g r a m s w h i c h w e w a n t e d the c o m p i l e r to
g e n e r a t e for v a r i o u s s o u r c e l a n g u a g e
p h r a s e s ; Z i l l e r and I h a d w o r k e d o u t a b a s i c
s c h e m e for t r a n s l a t i n g a r i t h m e t i c e x p r e s sions.
the a d d r e s s of the p r e c e d i n g r e f e r e n c e to
the a r r a y A w h e n e v e r I and J a r e c h a n g i n g
linearly.
To e m p l o y this far m o r e e f f i c i e n t
m e t h o d s e c t i o n 2 h a d to d e t e r m i n e w h e n the
s u r r o u n d i n g p r o g r a m was c h a n g i n g I and J
linearly.
B u t the real w o r k on the c o m p i l e r g o t
u n d e r w a y in our t h i r d l o c a t i o n o n the f i f t h
f l o o r of 15 E a s t 56th Street.
By the m i d d l e
of F e b r u a r y t h r e e s e p a r a t e e f f o r t s w e r e underway.
The f i r s t two of t h e s e c o n c e r n e d
s e c t i o n s I and 2 of the c o m p i l e r , and the
t h i r d c o n c e r n e d the input, o u t p u t and ass e m b l y p r o g r a m s w e w e r e g o i n g to n e e d (see
below). We b e l i e v e d t h e n t h a t t h e s e e f f o r t s
w o u l d p r o d u c e m o s t of the c o m p i l e r .
Thus one p r o b l e m w a s t h a t of d i s t i n g u i s h ing b e t w e e n , on the o n e hand, r e f e r e n c e s to
an a r r a y e l e m e n t w h i c h the t r a n s l a t o r m i g h t
t r e a t by i n c r e m e n t i n g the a d 4 r e s s u s e d for
a p r e v i o u s r e f e r e n c e , and t h o s e a r r a y r e f e r e n c e s , o n the o t h e r hand, w h i c h w o u l d req u i r e an a d d r e s s c a l c u l a t i o n s t a r t i n g f r o m
s c r a t c h w i t h the c u r r e n t v a l u e s of the subscripts.
(The e n t i r e p r o j e c t w a s c a r r i e d on by a
l o o s e c o o p e r a t i o n b e t w e e n a u t o n o m o u s , sepa r a t e g r o u p s of one, two, or t h r e e people;
each g r o u p was r e s p o n s i b l e for a " s e c t i o n "
of the c o m p i l e r ; e a c h g r o u p g r a d u a l l y d e v e l o p e d and a g r e e d u p o n its own i n p u t and output s p e c i f i c a t i o n s w i t h the g r o u p s for
neighboring sections; each group invented
and p r o g r a m m e d the n e c e s s a r y t e c h n i q u e s for
d o i n g its a s s i g n e d job.)
S e c t i o n I was to r e a d the e n t i r e s o u r c e
p r o g r a m , c o m p i l e w h a t i n s t r u c t i o n s it could,
and fi]e all the r e s t of the i n f o r m a t i o n
from the s o u r c e p r o g r a m in a p p r o p r i a t e
tables'.
Thus the c o m p i l e r w a s "one pass"
in the s e n s e that it "saw" the s o u r c e prog r a m o n l y once.
Herrick was responsible
for c r e a t i n g m o s t of the tables, P e t e r
S h e r i d a n (who had r e c e n t l y j o i n e d us) comp i l e d all the a r i t h m e t i c e x p r e s s i o n s , and
Roy N u t t c o m p i l e d a n d / o r f i l e d the I/O
statements.
H e r r i c k , S h e r i d a n and N u t t g o t
some h e l p l a t e r on f r o m R. J. B e e b e r and H.
Stern, but they w e r e the a r c h i t e c t s of section I and w r o t e m o s t of its code.
Sheridan
d e v i s e d and i m p l e m e n t e d a n u m b e r of o p t i m i z ing t r a n s f o r m a t i o n s on e x p r e s s i o n s
[Sheridan
1959] w h i c h s o m e t i m e s r a d i c a l l y a l t e r e d t h e m
(of c o u r s e w i t h o u t c h a n g i n g t h e i r m e a n i n g ) .
Nutt transformed
the I/O "lists of q u a n t i t i e s " into n e s t s of DO s t a t e m e n t s w h i c h
w e r e then t r e a t e d by the r e g u l a r m e c h a n i s m s
of the c o m p i l e r .
The r e s t of the I/O i n f o r m a t i o n he f i l e d for l a t e r t r e a t m e n t in section 6, the a s s e m b l e r s e c t i o n .
(For f u r t h e r
d e t a i l s a b o u t h o w the v a r i o u s s e c t i o n s of
the c o m p i l e r w o r k e d see [Backus et al.
1957] .)
U s i n g the i n f o r m a t i o n t h a t w a s f i l e d
in s e c t i o n I, s e c t i o n 2 f a c e d a c o m p l e t e l y
n e w k i n d of p r o b l e m ; it was r e q u i r e d to ana l y z e the e n t i r e s t r u c t u r e of the p r o g r a m
i n o r d e r to g e n e r a t e o p t i m a l c o d e f r o m DO
s t a t e m e n t s and r e f e r e n c e s to s u b s c r i p t e d
variables.
The s i m p l e s t w a y to e f f e c t a
r e f e r e n c e to A(I,J) is to e v a l u a t e an exp r e s s i o n i n v o l v i n g the a d d r e s s of A ( I , 1 ) ,
I, and K×J, w h e r e K is the l e n g t h of a c o l umn (when A is s t o r e d c o l u m n - w i s e ) . B u t this
calculation,
w i t h its m u l t i p l i c a t i o n ,
is
m u c h less e f f i c i e n t than the w a y m o s t h a n d
c o d e d p r o g r a m s e f f e c t a r e f e r e n c e to A ( I , J ) ,
n a m e l y , by a d d i n g an a p p r o p r i a t e c o n s t a n t to
171
It was d e c i d e d that it was n o t p r a c t i c a l
to t r a c k d o w n and i d e n t i f y l i n e a r c h a n g e s
in s u b s c r i p t s r e s u l t i n g from a s s i g n m e n t
statements.
Thus the sole c r i t e r i o n for
l i n e a r c h a n g e s , and h e n c e for e f f i c i e n t
h a n d l i n g of a r r a y r e f e r e n c e s , w a s to be
that the s u b s c r i p t s i n v o l v e d w e r e b e i n g
c o n t r o l l e d by DO s t a t e m e n t s .
D e s p i t e this
s i m p l i f y i n g a s s u m p t i o n , the n u m b e r of c a s e s
that s e c t i o n 2 had to a n a l y z e in o r d e r to
p r o d u c e o p t i m a l or n e a r - o p t i m a l c o d e was
v e r y large.
(The n u m b e r of such c a s e s inc r e a s e d e x p o n e n t i a l l y w i t h the n u m b e r of
s u b s c r i p t s ; this w a s a p r i m e f a c t o r in o u r
d e c i s i o n to l i m i t t h e m to three;
the fact
t h a t the 704 had o n l y t h r e e i n d e x r e g i s t e r s
was n o t a factor.)
It is b e y o n d the s c o p e of this p a p e r to
go into the d e t a i l s of the a n a l y s i s w h i c h
s e c t i o n 2 c a r r i e d out.
It w i l l s u f f i c e to
say t h a t it p r o d u c e d code of s u c h e f f i c i e n cy that its o u t p u t w o u l d s t a r t l e the p r o g r a m m e r s w h o s t u d i e d it.
It m o v e d c o d e o u t
of l o o p s w h e r e t h a t was p o s s i b l e ; it t o o k
a d v a n t a g e of the d i f f e r e n c e s b e t w e e n r o w w i s e and c o l u m n - w i s e scans; it took n o t e of
s p e c i a l c a s e s to o p t i m i z e e v e n the e x i t s
from loops.
The d e g r e e of o p t i m i z a t i o n
p e r f o r m e d by s e c t i o n 2 in its t r e a t m e n t of
i n d e x i n g , a r r a y r e f e r e n c e s , and l o o p s was
not equalled again until optimizing compilers b e g a n to a p p e a r in the m i d d l e and l a t e
sixties.
The a r c h i t e c t u r e and all the t e c h n i q u e s
e m p l o y e d in s e c t i o n 2 w e r e i n v e n t e d by Robert A. N e l s o n and I r v i n g Ziller.
They planned and p r o g r a m m e d the e n t i r e section. O r i g i n a l l y it was t h e i r i n t e n t i o n to p r o d u c e
the c o m p l e t e c o d e for t h e i r area, i n c l u d i n g
the c h o i c e of the i n d e x r e g i s t e r s to be
u s e d (the 704 had t h r e e i n d e x r e g i s t e r s ) .
W h e n t h e y s t a r t e d l o o k i n g at t h a t p r o b l e m
it r a p i d l y b e c a m e c l e a r t h a t it w a s n o t going to be e a s y to t r e a t it o p t i m a l l y .
At
that p o i n t I p r o p o s e d t h a t they s h o u l d p r o d u c e a p r o g r a m for a 704 w i t h an u n l i m i t e d
n u m b e r of i n d e x r e g i s t e r s , and t h a t l a t e r
s e c t i o n s w o u l d a n a l y z e the f r e q u e n c y of exe c u t i o n of v a r i o u s p a r t s of the p r o g r a m
(by a M o n t e C a r l o s i m u l a t i o n of its e x e c u tion) and t h e n m a k e i n d e x r e g i s t e r a s s i g n m e n t s so as to m i n i m i z e the t r a n s f e r s of
i t e m s b e t w e e n the s t o r e and the i n d e x r e g -
isters.
fully
ging.
This p r o p o s a l g a v e r i s e to two n e w sect ion s of the c o m p i l e r w h i c h we h a d n o t ant i c i p a t e d , s e c t i o n s 4 and 5 ( s e c t i o n 3 w a s
a d d e d s t i l l l a t e r to c o n v e r t the o u t p u t of
s e c t i o n s I and 2 to the f o r m r e q u i r e d for
s e c t i o n s 4, 5, and 6).
In the fall of 1955
L o i s M i t c h e l l H a i b t j o i n e d our g r o u p to
p l a n and p r o g r a m s e c t i o n 4, w h i c h w a s to
a n a l y z e the f l o w of a p r o g r a m p r o d u c e d by
s e c t i o n s I and 2, d i v i d e it into "basic
blocks"
(which c o n t a i n e d no b r a n c h i n g ) , do
a M o n t e C a r l o (statistical) a n a l y s i s of the
e x p e c t e d f r e q u e n c y of e x e c u t i o n of b a s i c
b l o c k s - - b y s i m u l a t i n g the b e h a v i o r of the
p r o g r a m and k e e p i n g c o u n t s of the use of
e a c h b l o c k - - u s i n g i n f o r m a t i o n f r o m DO s t a t e m e n t s and F R E Q U E N C Y s t a t e m e n t s , a n d c o l l e c t
i n f o r m a t i o n a b o u t i n d e x r e g i s t e r u s a g e (for
m o r e d e t a i l s see [Backus et al. 1957; C o c k e
and S c h w a r t z 1970 p.511]) . S e c t i o n 5
w o u l d t h e n do the a c t u a l t r a n s f o r m a t i o n of
the p r o g r a m f r o m o n e h a v i n g an u n l i m i t e d
n u m b e r of i n d e x r e g i s t e r s to o n e h a v i n g o n l y
three.
A g a i n , the s e c t i o n w a s e n t i r e l y
p l a n n e d and p r o g r a m m e d by Haibt.
and
took
charge
of
its
final
debug-
T h e f i n a l s e c t i o n of the c o m p i l e r , sect i o n 6, a s s e m b l e d the f i n a l p r o g r a m into
a r e l o c a t a b l e b i n a r y p r o g r a m (it c o u l d also
p r o d u c e a s y m b o l i c p r o g r a m in SAP, the
S H A R E A s s e m b l y P r o g r a m for the 704).
It
p r o d u c e d a s t o r a g e m a p of the p r o g r a m and
d a t a t h a t w a s a c o m p a c t s u m m a r y of the F O R TRAN output.
Of c o u r s e it a l s o o b t a i n e d
the n e c e s s a r y l i b r a r y p r o g r a m s for i n c l u s i o n
in the o b j e c t p r o g r a m , i n c l u d i n g t h o s e req u i r e d to i n t e r p r e t F O R M A T s t a t e m e n t s and
perform input-output operations.
Taking
a d v a n t a g e of the s p e c i a l f e a t u r e s of the
p r o g r a m s it a s s e m b l e d , this a s s e m b l e r w a s
a b o u t ten t i m e s f a s t e r t h a n SAP.
It w a s
d e s i g n e d and p r o g r a m m e d by Roy N u t t , w h o
a l s o w r o t e all the I/O p r o g r a m s and the rel o c a t i n g b i n a r y l o a d e r for l o a d i n g o b j e c t
programs.
By the s u m m e r of 1956 l a r g e p a r t s of the
system were working.
S e c t i o n s I, 2, and 3
c o u l d p r o d u c e w o r k a b l e c o d e p r o v i d e d no
more than three index registers were needed.
A n u m b e r of t e s t p r o g r a m s w e r e c o m p i l e d and
run at this time.
N u t t t o o k p a r t of the
s y s t e m to U n i t e d A i r c r a f t
( s e c t i o n s I, 2,
and 3 and the p a r t of s e c t i o n 6 w h i c h p r o d u c e d SAP o u t p u t ) .
T h i s p a r t of the s y s t e m
w a s p r o d u c t i v e t h e r e f r o m the s u m m e r of
1956 u n t i l the c o m p l e t e s y s t e m w a s a v a i l a b l e
in e a r l y 1957.
S e c t i o n 5 w a s p l a n n e d and p r o g r a m m e d by
S h e l d o n Best, w h o w a s l o a n e d to our g r o u p
by a g r e e m e n t w i t h his e m p l o y e r , C h a r l e s W.
A d a m s , at the D i g i t a l C o m p u t e r L a b o r a t o r y
at MIT; d u r i n g his s t a y w i t h us B e s t w a s a
t e m p o r a r y IBM e m p l o y e e .
S t a r t i n g in the
e a r l y f a l l of 1955, he d e s i g n e d w h a t t u r n e d
o u t to be, a l o n g w i t h s e c t i o n 2, o n e of the
m o s t i n t r i c a t e and c o m p l e x s e c t i o n s of the
c o m p i l e r , o n e w h i c h h a d p e r h a p s m o r e inf l u e n c e on the m e t h o d s u s e d in l a t e r comp i l e r s t h a n any o t h e r p a r t of the F O R T R A N
compiler.
(For a d i s c u s s i o n of his t ec h n i q u e s see [Cocke and S c h w a r t z 1970 pp. 510515].)
It is i m p o s s i b l e to d e s c r i b e his
r e g i s t e r a l l o c a t i o n m e t h o d here; it s u f f i c e s
to say t h a t it w a s to b e c o m e the b a s i s for
m u c h s u b s e q u e n t w o r k and p r o d u c e d code w h i c h
w a s v e r y d i f f i c u l t to i m p r o v e .
F r o m late s p r i n g of 1956 to e a r l y 1957
the p a c e of d e b u g g i n g w a s i n t e n s e ; o f t e n w e
w o u l d r e n t r o o m s in the L a n g d o n H o t e l (which
d i s a p p e a r e d long ago) on 56th S t r e e t ,
s l e e p t h e r e a l i t t l e d u r i n g the d a y and t h e n
s t a y up all n i g h t to g e t as m u c h u s e of the
c o m p u t e r (in the h e a d q u a r t e r s a n n e x on 57th
Street) as p o s s i b l e .
It w a s an e x c i t i n g p e r i o d ; w h e n l a t e r on
w e b e g a n to g e t f r a g m e n t s of c o m p i l e d p r o g r a m s out of the s y s t e m , w e w e r e o f t e n ast o n i s h e d at the s u r p r i s i n g t r a n s f o r m a t i o n s
in the i n d e x i n g o p e r a t i o n s and in the arr a n g e m e n t of the c o m p u t a t i o n w h i c h the comp i l e r m a d e , c h a n g e s w h i c h m a d e the o b j e c t
p r o g r a m e f f i c i e n t b u t w h i c h we w o u l d n o t
h a v e t h o u g h t to m a k e as p r o g r a m m e r s o u r s e l v e s (even t h o u g h , of c o u r s e , N e l s o n or
Z i l l e r c o u l d f i g u r e o u t h o w the i n d e x i n g
w o r k e d , S h e r i d a n c o u l d e x p l a i n h o w an exp r e s s i o n had b e e n o p t i m i z e d b e y o n d r e c o g n i t i o n , and G o l d b e r g or S a y r e c o u l d tell us
h o w s e c t i o n 5 h a d g e n e r a t e d a d d i t i o n a l indexing operations).
T r a n s f e r s of c o n t r o l
a p p e a r e d w h i c h c o r r e s p o n d e d to no s o u r c e
s t a t e m e n t , e x p r e s s i o n s w e r e r a d i c a l l y rea r r a n g e d , and the same DO s t a t e m e n t m i g h t
p r o d u c e no i n s t r u c t i o n s in the o b j e c t p r o g r a m in o n e c o n t e x t , and in a n o t h e r it
w o u l d p r o d u c e m a n y i n s t r u c t i o n s in d i f f e r ent p l a c e s in the p r o g r a m .
A l t h o u g h I b e l i e v e that no p r o v a b l y
o p t i m a l r e g i s t e r a l l o c a t i o n a l g o r i t h m is
k n o w n for g e n e r a l p r o g r a m s w i t h loops, etc.,
e m p i r i c a l l y B e s t ' s 1 9 5 5 - 5 6 p r o c e d u r e app e a r e d to be o p t i m a l .
For s t r a i g h t - l i n e
c o d e B e s t ' s r e p l a c e m e n t p o l i c y w a s the same
as t h a t u s e d in B e l a d y ' s M I N a l g o r i t h m ,
w h i c h B e l a d y p r o v e d to be o p t i m a l [Belady
1965].
Although Best did not publish a
f o r m a l p r o o f , he h a d c o n v i n c i n g a r g u m e n t s
for the o p t i m a l i t y of his p o l i c y in 1955.
L a t e in 1955 it w a s r e c o g n i z e d t h a t y e t
a n o t h e r s e c t i o n , s e c t i o n 3, w a s n e e d e d .
T h i s s e c t i o n m e r g e d the o u t p u t s of the prec e d i n g s e c t i o n s into a s i n g l e u n i f o r m 704
p r o g r a m w h i c h c o u l d r e f e r to any n u m b e r of
index registers.
It w a s p l a n n e d and p r o g r a m m e d by R i c h a r d G o l d b e r g , a m a t h e m a t i c i a n
w h o j o i n e d us in N o v e m b e r 1955.
Also, late
in 1956, a f t e r B e s t h a d r e t u r n e d to M I T a n d
d u r i n g the d e b u g g i n g of the system, s e c t i o n
5 was t a k e n o v e r by G o l d b e r g and D a v i d
S a y r e (see b e l o w ) , w h o d i a g r a m m e d it c a r e -
By the s u m m e r of 1956 w h a t a p p e a r e d to
be the i m m i n e n t c o m p l e t i o n of the p r o j e c t
s t a r t e d us w o r r y i n g (for p e r h a p s the f i r s t
172
time) a b o u t d o c u m e n t a t i o n .
D a v i d Sayre, a
c r y s t a l l o g r a p h e r w h o had j o i n e d us in the
s p r i n g (he h a d e a r l i e r c o n s u l t e d w i t h B e s t
on the d e s i g n of s e c t i o n 5 and had l a t e r b e g u n s e r v i n g as s e c o n d - i n - c o m m a n d of w h a t w a s
n o w the '~Programming R e s e a r c h D e p a r t m e n t " )
took up the task of w r i t i n g the P r o g r a m m e r ' s
R e f e r e n c e M a n u a l [IBM 1956].
It a p p e a r e d
in a g l o s s y cover, h a n d s o m e l y p r i n t e d , w i t h
the d a t e O c t o b e r 15, 1956.
It s t o o d for
some time as a u n i q u e e x a m p l e of a m a n u a l
for a p r o g r a m m i n g l a n g u a g e (perhaps it s t i l l
does): it h a d w i d e m a r g i n s , y e t was o n l y 51
p a g e s long. Its d e s c r i p t i o n of the F O R T R A N
l a n g u a g e , e x c l u s i v e of i n p u t - o u t p u t s t a t e m e n t s , w a s 21 pages; the I/O d e s c r i p t i o n
o c c u p i e d a n o t h e r 11 pages; the r e s t of it
was e x a m p l e s and d e t a i l s a b o u t a r i t h m e t i c ,
t a b l e s , etc..
It g a v e an e l e g a n t r e c u r s i v e
d e f i n i t i o n of e x p r e s s i o n s
(as g i v e n b y S h e r idan), and c o n c i s e , c l e a r d e s c r i p t i o n s , w i t h
e x a m p l e s , of e a c h s t a t e m e n t type, of w h i c h
t h e r e w e r e 32, m o s t l y m a c h i n e d e p e n d e n t items like S E N S E LIGHT, IF D I V I D E CHECK,
PUNCH, R E A D DRUM, and so on.
(For e x a m p l e s
of its s t y l e see figs. I, 2, and 3.)
time at n i g h t on t h e i r 704 to h e l p us in
our m a d r u s h to d i s t r i b u t e the system.
Up
to this p o i n t t h e r e h a d b e e n r e l a t i v e l y
l i t t l e i n t e r e s t f r o m 704 i n s t a b l a t i o n s
(with
the e x c e p t i o n of R a m s h a w ' s U n i t e d A i r c r a f t
shop, H a r r y C a n t r e l l ' s GE i n s t a l l a t i o n in
S c h e n e c t a d y , and S i d n e y F e r n b a c h ' s L i v e r m o r e o p e r a t i o n ) , b u t n o w t h a t the full system was b e g i n n i n g to g e n e r a t e o b j e c t p r o grams, i n t e r e s t p i c k e d up in a n u m b e r of
places.
S o m e t i m e in e a r l y A p r i l 1957 we felt the
s y s t e m w a s s u f f i c i e n t l y b u g - f r e e to d i s t r i b ute to all 704 i n s t a l l a t i o n s .
S a y r e and
Grace Mitchell
(see below) s t a r t e d to p u n c h
out the b i n a r y d e c k s of the system, e a c h of
a b o u t 2,000 cards, w i t h the i n t e n t i o n to
m a k e 30 or 40 d e c k s for d i s t r i b u t i o n .
This
p r o c e s s was so e r r o r - p r o n e t h a t they h a d to
g i v e up a f t e r s p e n d i n g an e n t i r e n i g h t in
p r o d u c i n g o n l y one or two decks.
( A p p a r e n t l y one of t h o s e d e c k s was sent,
w i t h o u t any i d e n t i f i c a t i o n or d i r e c t i o n s ,
to the W e s t i n g h o u s e B e t t i s i n s t a l l a t i o n ,
w h e r e a p u z z l e d g r o u p h e a d e d by H e r b e r t S.
B r i g h t , s u s p e c t i n g t h a t it m i g h t be the
l o n g - a w a i t e d F O R T R A N deck, p r o c e e d e d , ent i r e l y by g u e s s w o r k , to g e t it to c o m p i l e
a test p r o g r a m - - a f t e r a d i a g n o s t i c p r i n t out noting
that a
comma
was
missing
in a s p e c i f i c s t a t e m e n t !
This p r o g r a m then
p r i n t e d 28 p a g e s of c o r r e c t r e s u l t s - - w i t h a
few F O R M A T errors.
The date: A p r i l 20,
1957.
A n a m u s i n g a c c o u n t of this i n c i d e n t
by B r i g h t is in C o m p u t e r s and A u t o m a t i o n
[Bright 1971].)
One f e a t u r e of F O R T R A N I is m i s s i n g f r o m
the P r o g r a m m e r ' s R e f e r e n c e M a n u a l , n o t f r o m
an o v e r s i g h t of S a y r e ' s , b u t b e c a u s e it w a s
a d d e d to the s y s t e m a f t e r the m a n u a l was
w r i t t e n and b e f o r e the s y s t e m w a s d i s t r i b uted.
This f e a t u r e was the a b i l i t y to define a f u n c t i o n by a " f u n c t i o n s t a t e m e n t " .
T h e s e s t a t e m e n t s h a d to p r e c e d e the r e s t of
the p r o g r a m .
They looked like assignment
s t a t e m e n t s , w i t h the d e f i n e d f u n c t i o n and
d u m m y a r g u m e n t s on the left and an e x p r e s sion i n v o l v i n g t h o s e a r g u m e n t s on the right.
T h e y are d e s c r i b e d in the a d d e n d a to the
P r o g r a m m e r ' s R e f e r e n c e M a n u a l [Addenda 1957]
w h i c h we sent on F e b r u a r y 8, 1957 to J o h n
G r e e n s t a d t , w h o was in c h a r g e of I B M ' s facility
for d i s t r i b u t i n g i n f o r m a t i o n to
SHARE.
T h e y are a l s o d e s c r i b e d in all subs e q u e n t m a t e r i a l o n F O R T R A N I.
A f t e r f a i l i n g to p r o d u c e b i n a r y decks,
S a y r e d e v i s e d and p r o g r a m m e d the s i m p l e
e d i t o r and l o a d e r that m a d e it p o s s i b l e to
d i s t r i b u t e and u p d a t e the s y s t e m f r o m m a g n e t i c tapes; this a r r a n g e m e n t s e r v e d as the
m e c h a n i s m for c r e a t i n g n e w s y s t e m t a p e s
from a m a s t e r tape and the b i n a r y c o r r e c t i o n
c a r d s w h i c h our g r o u p w o u l d g e n e r a t e in
l a r g e n u m b e r s d u r i n g the long f i e l d d e b u g g i n g and m a i n t e n a n c e p e r i o d w h i c h f o l l o w e d
distribution.
The n e x t d o c u m e n t a t i o n t a s k we set ours e l v e s w a s to w r i t e a p a p e r d e s c r i b i n g the
F O R T R A N l a n g u a g e and the t r a n s l a t o r p r o g r a m .
The r e s u l t was a p a p e r e n t i t l e d "The F O R T R A N a u t o m a t i c c o d i n g s y s t e m " [Backus et al.
1957] w h i c h we p r e s e n t e d at the W e s t e r n
J o i n t C o m p u t e r C o n f e r e n c e in Los A n g e l e s in
F e b r u a r y 1957.
I h a v e m e n t i o n e d all of the
t h i r t e e n a u t h o r s of t h a t p a p e r in the prec e d i n g n a r r a t i v e e x c e p t one: R o b e r t A.
Hughes.
He was e m p l o y e d by the L i v e r m o r e
R a d i a t i o n L a b o r a t o r y ; by a r r a n g e m e n t w i t h
S i d n e y F e r n b a c h , he v i s i t e d us for two or
t h r e e m o n t h s in the s u m m e r of 1956 to h e l p
us d o c u m e n t the system.
(The a u t h o r s of
t h a t p a p e r were: J. W. B a c k u s , R. J. B e e b e r ,
S. Best, R. G o l d b e r g , L. M. Haibt, H. L.
H e r r i c k , R. A. H u g h e s , R. A. N e l s o n , R.
Nutt, D. Sayre, P. B. S h e r i d a n , H. Stern,
I. Ziller.)
W i t h the d i s t r i b u t i o n of the s y s t e m
tapes went a "Preliminary Operator's Manual" [ O p e r a t o r ' s M a n u a l 1957] d a t e d A p r i l 8,
1957.
It d e s c r i b e s h o w to use the tape editor and h o w to m a i n t a i n the l i b r a r y of
functions.
F i v e p a g e s of such g e n e r a l ins t r u c t i o n s are f o l l o w e d by 32 p a g e s of err o r stops; m a n y of t h e s e say " s o u r c e
p r o g r a m error, g e t off m a c h i n e , c o r r e c t form u l a in q u e s t i o n and r e s t a r t p r o b l e m " and
then, for e x a m p l e (stop 3624) " n o n - z e r o
l e v e l r e d u c t i o n due to i n s u f f i c i e n t or red u n d a n t p a r e n t h e s e s in a r i t h m e t i c or IFtype f o r m u l a " .
S h o r t l y a f t e r the d i s t r i b u t i o n of the s y s t e m we d i s t r i b u t e d - - o n e
c o p y per i n s t a l l a t i o n - - w h a t was f o n d l y
k n o w n as the "Tome", the c o m p l e t e s y m b o l i c
l i s t i n g of the e n t i r e c o m p i l e r plus o t h e r
s y s t e m and d i a g n o s t i c i n f o r m a t i o n , an 11"
by 15" v o l u m e a b o u t four or five i n c h e s
thick.
At a b o u t the time of the W e s t e r n J o i n t
C o m p u t e r C o n f e r e n c e w e s p e n t some t i m e in
Los A n g e l e s s t i l l f r a n t i c a l l y d e b u g g i n g the
system.
N o r t h A m e r i c a n A v i a t i o n g a v e us
173
Subscripts.
GENERAL
FORM
EXAMPLES
Let v represent any fixed point variable
and c (or c') any-unsigned fixed point
constant. Then a subscript is
an expression of one of the forms:
I
3
MU+2
MU-2
5*J
5"J+2
5"J-2
V
C
V+C or V--C
c*v
c* V + C ' or c*v--c'
T h e symbol • denotes multiplication. T h e variable v must not itself b e subscripted.
Subscripted Variables.
GENERAL
FORM
EXAMPLES
A fixed or floating point variable
A(I)
followed, by parentheses enclosing 1, 2, or 3
subscripts separated by commas.
K(3)
BEIA(5*.I-2, K + 2,L)
For each wlriable that appears in subscripted form the size of the array (i.e. the
maxinuun wdues which its subscripts can attain) must be stated in a D I M E N SION statement (see Chapter 6) preceding the first appearance of the variable.
The m i n i m u m value which a subscript may assume in the object p r o g r a m is + 1.
A rrangement o / A rrays in Storage.
A 2-dimensional array A will, in the object program, be stored sequentially in
the order A1,1, A2.1, • . . . . .
Am,l, A],z, A2,2, • . . . . .
Am,2, • . . . . . . . .
Am,,. Thus
it is stored "columnwise", with the first of its subscripts varying most rapidly,
and the last varying least rapidly. The same is true of 3-dimensional arrays.
l-dimensional arrays are of course simply stored sequentially. All arrays are
stored backwards in storage; i.e. the above sequence is in the order of decreasing absolute location.
II
Figure
I:
O r i g i n a l F O R T R A N Manual,
174
Page
11
A n y such routine will be compiled into the object program as a closed subroutine. In the section on Writing Subroutines for the Master Tape in Chapter 7
are given the specifications which any such routine must meet.
Expressions
A n expression is any sequence of constants, w~riables (subscripted or not subscripted), and functions, separated by operation symbols, commas, and parentheses so as to form a meaningful mattmmatical expression.
However, one special restriction does exist. A FORTRAN expression may
be either a fixed or a lloating point expression, but it must not be a mixed
expression. This does not mean that a floating point quantity can not appear
in a fixed point expression, or vice versa, but rather that a quantity of one
mode can appear in an expression of the other mode only in certain ways.
Brielty, a floating point quantity can appear in a fixed point expression only
as an a r g u m e n t of a function; a fixed point quantity can appear in a floating
point expression only as an argument of a function, or as a subscript, or as
an exponent.
Formal Rules /or Forming Expressions.
By repeated use of the following
rules, all permissible expressions may be derived.
1. Any fixed point (floating point) constant, variable, or subscripted variable
is an expression of the same mode. Thus 3 and I are fixed point expressions,
and AI.I'HA and A(I,J,K) are tloating point exprcssi~ms.
2.
If SOMEF is some function of n wLriahles, and if E, F . . . . . .
H are a set
of n expressions of the correct modes for SOMEF, then SONIEF (E, F,
. . . . , H) is an expression of the same mode as SOMEF.
3.
If E is an expression, and if its lirst character is not -t or --, then t- E and
--E are expressions of the same mode as E. Thus - A is an expression, but
-k-A is not.
4.
If E is an expression, then (E) is an expression of the same mode as E.
Thus (A), ( ( A ) ) , ( ( ( A ) ) ) , . c t c . are expressions.
5.
If E and F are expressions of the same mode, and if the first character of
F is not + o r - - , then
E+F
E-F
E* F
[/F
are expressions of the same mode. Thus A - - + B and A / 4 B are not expressions. The characters + , - , *, and / denote addition, subtraction, multiplication, and division.
14
Figure 2: Original FORTRAN Manual, Page 14
175
STOP
GENERAL FORM
EXAMPLES
"STOP" or "STOP n" where n is an
STOP
unsigned octal fixed point constant.
STOP 77777
This statement causes the m a c h i n e to H A L T in such a way that pressing the
S T A R T button has no effect. Therefore, in c o n t r a s t to the P A U S E , it is used
where a g e t - o i l - t h e - m a c h i n e stop, r a t h e r than a t e m p o r a r y stop, is desired. T h e
octal n u m b e r n is d i s p l a y e d on the 704 console in the a d d r e s s field of the
storage register. ( I f n is not stated it is taken to be 0.)
DO
GENERAL FORM
EXAMPLES
"DO n i = m,, m2" or "DO n i = m,, m2, m3"
DO 301 = 1,10
where n is a statement number, i is a
DO301 = 1, M, 3
non-subscripted fixed point variable, and
m,, m2, ma are each either an unsigned fixed point
constant or a non-subscripted fixed point variable.
If ma is not stated it is taken to be 1.
T h e D O s t a t e m e n t is a c o m m a n d to " D O the statements which follow, to and
including the s t a t e m e n t with s t a t e m e n t n u m b e r n, repeatedly, the first time with
i = m~ a n d with i increased by mz for each succeeding time; after they have
been done with i equal to the highest of this sequence of values which does not
e x c e e d m., let control reach the s t a t e m e n t following the s t a t e m e n t with statem c n t n u m b e r n".
T h e range of a D O is the set of statements which will be e x e c u t e d rep e a t e d l y ; it is the sequence of consecutive statements i m m e d i a t e l y following
the D O , to a n d including the s t a t e m e n t n u m b e r e d n.
T h e index of a D O is the fixed p o i n t variable i, which is c o n t r o l l e d by the
D O in such a way that its value begins at m l a n d is i n c r e a s e d each time by
ma until it is a b o u t to exceed m > T h r o u g h o u t the range it is available for c o m putation, either as an o r d i n a r y fixed p o i n t variable o r as the variable of a
subscript. D u r i n g the last execution of the range, the D O is said to be satisfied.
Suppose, for e x a m p l e , that c o n t r o l has r e a c h e d s t a t e m e n t
10 of the
program
10
DO 11 I =
11
A(I) =
1, 10
I*N(I)
12
20
Figure
3: Original
FORTRAN Manual,
176
Page
20
d i a g n o s t i c s y s t e m and d e s c r i b e s the n e w
"subroutine definition" and END statements,
plus some o t h e r s w h i c h w e r e n o t i m p l e m e n t e d .
It d e s c r i b e s h o w s y m b o l i c i n f o r m a t i o n is
r e t a i n e d in the r e l o c a t a b l e b i n a r y f o r m of
a s u b r o u t i n e so t h a t the " b i n a r y s y m b o l i c
s u b r o u t i n e [BSS] l o a d e r " can i m p l e m e n t refe r e n c e s to s e p a r a t e l y c o m p i l e d s u b r o u t i n e s .
It d e s c r i b e s n e w p r o l o g u e s for t h e s e subr o u t i n e s and p o i n t s o u t that m i x t u r e s of
F O R T R A N - c o d e d and a s s e m b l y - c o d e d r e l o c a t a b l e b i n a r y p r o g r a m s c o u l d be l o a d e d and
run t o g e t h e r .
It d o e s n o t d i s c u s s the F U N C T I O N s t a t e m e n t t h a t w a s a l s o a v a i l a b l e in
F O R T R A N II.
F O R T R A N II w a s d e s i g n e d m o s t l y
by N e l s o n , Ziller, and m y s e l f .
Mitchell
p r o g r a m m e d the m a j o r i t y of n e w c o d e for
F O R T R A N II (with the m o s t u n u s u a l f e a t u r e
that she d e l i v e r e d it a h e a d of s c h e d u l e ) .
She w a s a i d e d in this b y B e r n y c e B r a d y and
L e R o y May.
S h e r i d a n p l a n n e d and m a d e the
n e c e s s a r y c h a n g e s in his p a r t of s e c t i o n I;
N u t t did the same for s e c t i o n 6.
FORTRAN
II w a s d i s t r i b u t e d in the s p r i n g of 1958.
The p r o p r i e t o r s of the six s e c t i o n s w e r e
k e p t b u s y t r a c k i n g d o w n b u g s e l i c i t e d by
our c u s t o m e r s ' u s e of F O R T R A N u n t i l the late
s u m m e r of 1957.
Hal S t e r n s e r v e d as the coo r d i n a t o r of the f i e l d d e b u g g i n g and m a i n t e n a n c e effort; he r e c e i v e d a s t r e a m of
t e l e g r a m s , m a i l and p h o n e c a l l s f r o m all
o v e r the c o u n t r y and d i s t r i b u t e d the inc o m i n g p r o b l e m s to the a p p r o p r i a t e m e m b e r s
of our g r o u p to t r a c k d o w n the e r r o r s and
g e n e r a t e c o r r e c t i o n cards, w h i c h he then
d i s t r i b u t e d to e v e r y i n s t a l l a t i o n .
In the s p r i n g of 1957 G r a c e E. M i t c h e l l
j o i n e d o u r g r o u p to w r i t e the P r o g r a m m e r ' s
P r i m e r [IBM 1957]
for F O R T R A N .
The P r i m e r
w a s d i v i d e d into t h r e e s e c t i o n s ; e a c h d e s c r i b e d s u c c e s s i v e l y l a r g e r s u b s e t s of the
l a n g u a g e a c c o m p a n i e d by m a n y e x a m p l e p r o grams.
The f i r s t e d i t i o n of the P r i m e r was
i s s u e d in the late fall or w i n t e r of 1957;
a s l i g h t l y r e v i s e d e d i t i o n a p p e a r e d in
March
1958.
M i t c h e l l p l a n n e d and w r o t e the
6 4 - p a g e P r i m e r w i t h some c o n s u l t a t i o n w i t h
the r e s t of the group; she l a t e r
programmed
m o s t of the e x t e n s i v e c h a n g e s in the s y s t e m
w h i c h r e s u l t e d in F O R T R A N II (see below).
5.
A r e p o r t on F O R T R A N
u s a g e in N o v e m b e r
1958 [Backus 1958] says that "a s u r v e y in
A p r i l [1958] of t w e n t y - s i x 704 i n s t a l l a t i o n s
i n d i c a t e s t h a t o v e r half of t h e m use F O R T R A N
[I] for m o r e than h a l f of t h e i r p r o b l e m s .
M a n y use it for 80~ or m o r e of t h e i r w o r k . . .
and a l m o s t all use it for some of t h e i r
work."
By the fall of 1958 t h e r e w e r e some
60 i n s t a l l a t i o n s w i t h a b o u t 66 704s,
and "... m o r e t h a n h a l f the m a c h i n e i n s t r u c tions for t h e s e m a c h i n e s are b e i n g p r o d u c e d
by F O R T R A N .
SHARE recently designated FORT R A N as the s e c o n d o f f i c i a l m e d i u m for
t r a n s m i t t a l of p r o g r a m s [SAP w a s the first]
4.
."
FORTRAN
III
W h i l e F O R T R A N II w a s b e i n g d e v e l o p e d ,
Z i l l e r w a s d e s i g n i n g an e v e n m o r e a d v a n c e d
s y s t e m t h a t he c a l l e d F O R T R A N III.
It all o w e d o n e to w r i t e i n t e r m i x e d s y m b o l i c ins t r u c t i o n s and F O R T R A N s t a t e m e n t s .
The symb o l i c (704) i n s t r u c t i o n s c o u l d h a v e F O R T R A N
v a r i a b l e s (with or w i t h o u t s u b s c r i p t s ) as
"addresses".
In a d d i t i o n to this m a c h i n e
d e p e n d e n t f e a t u r e (which a s s u r e d the d e m i s e
of F O R T R A N III a l o n g w i t h t h a t of the 704),
it c o n t a i n e d e a r l y v e r s i o n s of a n u m b e r of
i m p r o v e m e n t s t h a t w e r e l a t e r to a p p e a r in
F O R T R A N IV.
It had " B o o l e a n " e x p r e s s i o n s ,
f u n c t i o n and s u b r o u t i n e n a m e s c o u l d be
p a s s e d as a r g u m e n t s , and it had f a c i l i t i e s
for h a n d l i n g a l p h a n u m e r i c data, i n c l u d i n g
a n e w F O ~ 4 A T c o d e "A" s i m i l a r to c o d e s "I"
and "E".
This s y s t e m w a s p l a n n e d and p r o g r a m m e d by Z i l l e r w i t h some h e l p f r o m N e l s o n
and Nutt.
Z i l l e r m a i n t a i n e d it and m a d e it
a v a i l a b l e to a b o u t 20 (mostly IBM) i n s t a l lations.
It w a s n e v e r d i s t r i b u t e d g e n e r a l ly.
It was a c c o m p a n i e d by a b r i e f d e s c r i p tive d o c u m e n t [ A d d i t i o n s to F O R T R A N II
1958].
It b e c a m e a v a i l a b l e on this l i m i t e d
s c a l e in the w i n t e r of 1 9 5 8 - 5 9 and was in
o p e r a t i o n u n t i l the e a r l y s i x t i e s , in p a r t
on the 709 u s i n g the c o m p a t i b i l i t y f e a t u r e
(which m a d e the 709 o r d e r code the same as
that of the 704).
The P r i m e r had an i m p o r t a n t i n f l u e n c e on
the s u b s e q u e n t g r o w t h in the use of the system.
I b e l i e v e it was the o n l y a v a i l a b l e
s i m p l i f i e d i n s t r u c t i o n m a n u a l (other t h a n
r e f e r e n c e m a n u a l s ) u n t i l the l a t e r a p p e a r ance of b o o k s such as [ M c C r a c k e n 1961],
[ O r g a n i c k 1963] and m a n y others.
.,
FORTRAN
II
D u r i n g the f i e l d d e b u g g i n g p e r i o d some
s h o r t c o m i n g s of the s y s t e m d e s i g n , w h i c h we
h a d b e e n a w a r e of e a r l i e r b u t h a d no time
to deal with, w e r e c o n s t a n t l y c o m i n g to o u r
attention.
In the e a r l y fall of 1957 we
s t a r t e d to p l a n w a y s of c o r r e c t i n g t h e s e
shortcomings; a document dated September
25, 1957 [Proposed S p e c i f i c a t i o n s
1957]
c h a r a c t e r i z e s t h e m as (a) a n e e d for b e t t e r
d i a g n o s t i c s , c l e a r e r c o m m e n t s a b o u t the
n a t u r e of s o u r c e p r o g r a m errors, and (b)
the n e e d for s u b r o u t i n e d e f i n i t i o n c a p a b i l ities.
"(Although o n e F O R T R A N I d i a g n o s t i c
w o u l d p i n p o i n t , in a p r i n t o u t , a m i s s i n g
c o m m a in a p a r t i c u l a r s t a t e m e n t , o t h e r s
c o u l d be v e r y c r y p t i c . )
This d o c u m e n t is
t i t l e d " P r o p o s e d S p e c i f i c a t i o n s for F O R T R A N
II for the 704"; it s k e t c h e s a m o r e g e n e r a l
6.
FORTRAN
after
1958;
comments.
By the end of 1958 or e a r l y 1959 the
FORTRAN
g r o u p (the P r o g r a m m i n g R e s e a r c h
D e p a r t m e n t ) , w h i l e s t i l l h e l p i n g w i t h an
occasional debugging problem with FORTRAN
II, was p r i m a r i l y o c c u p i e d w i t h o t h e r research.
A n o t h e r IBM d e p a r t m e n t had long
s i n c e t a k e n r e s p o n s i b i l i t y for the F O R T R A N
s y s t e m and was r e v i s i n g it in the c o u r s e of
p r o d u c i n g a t r a n s l a t o r for the 709 w h i c h
u s e d the same p r o c e d u r e s as the 704 F O R T R A N
II t r a n s l a t o r .
S i n c e m y f r i e n d s and I no
l o n g e r h a d r e s p o n s i b i l i t y for F O R T R A N and
177
w e r e b u s y t h i n k i n g a b o u t o t h e r t h i n g s by
the end of 1958, t h a t s e e m s like a g o o d
p o i n t to b r e a k off this a c c o u n t .
There
r e m a i n o n l y a f e w c o m m e n t s to be m a d e a b o u t
the s u b s e q u e n t d e v e l o p m e n t of F O R T R A N .
The m o s t o b v i o u s d e f e c t in F O R T R A N II
for m a n y of its u s e r s w a s the t i m e s p e n t in
compiling.
E v e n t h o u g h the f a c i l i t i e s of
F O R T R A N II p e r m i t t e d s e p a r a t e c o m p i l a t i o n
of s u b r o u t i n e s and h e n c e e l i m i n a t e d the
n e e d to r e c o m p i l e an e n t i r e p r o g r a m at e a c h
s t e p in d e b u g g i n g it, n e v e r t h e l e s s c o m p i l e
t i m e s w e r e long and, d u r i n g d e b u g g i n g , the
c o n s i d e r a b l e time s p e n t in o p t i m i z i n g w a s
wasted.
I r e p e a t e d l y s u g g e s t e d to t h o s e
w h o w e r e in c h a r g e of F O R T R A N t h a t t h e y
should now develop a fast compiler and/or
i n t e r p r e t e r w i t h o u t any o p t i m i z i n g a t all
for use d u r i n g d e b u g g i n g and for s h o r t - r u n
jobs.
U n f o r t u n a t e l y the d e v e l o p e r s of
F O R T R A N IV t h o u g h t t h e y c o u l d h a v e the b e s t
of b o t h w o r l d s in a s i n g l e c o m p i l e r , o n e
w h i c h was b o t h f a s t and p r o d u c e d o p t i m i z e d
code.
I w a s u n s u c c e s s f u l in c o n v i n c i n g t h e m
t h a t two c o m p i l e r s w o u l d h a v e b e e n far b e t ter t h a n the c o m p r o m i s e w h i c h b e c a m e the
o r i g i n a l F O R T R A N IV c o m p i l e r .
The latter
w a s not n e a r l y as f a s t as l a t e r c o m p i l e r s
like W A T F O R [Cress, D i r k s e n a n d G r a h a m 1970]
nor did it p r o d u c e as g o o d c o d e as F O R T R A N
II.
(For m o r e d i s c u s s i o n of l a t e r d e v e l o p m e n t s w i t h F O R T R A N see [Backus and H e i s i n g
196~] .)
M y o w n o p i n i o n as to the e f f e c t of F O R T R A N on l a t e r l a n g u a g e s and the c o l l e c t i v e
i m p a c t of such l a n g u a g e s on p r o g r a m m i n g g e n e r a l l y is n o t a p o p u l a r o p i n i o n .
That
v i e w p o i n t is the s u b j e c t of a long p a p e r
[Backus 1978] w h i c h s h o u l d a p p e a r soon in
the C o m m u n i c a t i o n s of the ACM.
I n o w reg a r d all c o n v e n t i o n a l l a n g u a g e s (e.g., the
F O R T R A N s , the A L G O L s , t h e i r s u c c e s s o r s and
derivatives)
as i n c r e a s i n g l y c o m p l e x e l a b o r a t i o n s of the s t y l e of p r o g r a m m i n g d i c t a t e d by the v o n N e u m a n n c o m p u t e r .
These
"von N e u m a n n l a n g u a g e s " c r e a t e e n o r m o u s ,
u n n e c e s s a r y i n t e l l e c t u a l r o a d b l o c k s in
t h i n k i n g a b o u t p r o g r a m s and in c r e a t i n g the
h i g h e r l e v e l c o m b i n i n g f o r m s r e q u i r e d in a
really powerful programming methodology.
V o n N e u m a n n l a n g u a g e s c o n s t a n t l y k e e p our
n o s e s p r e s s e d in the d i r t of a d d r e s s comp u t a t i o n and the s e p a r a t e c o m p u t a t i o n of
s i n g l e w o r d s , w h e r e a s w e s h o u l d be f o c u s i n g
o n the f o r m and c o n t e n t of the o v e r a l l res u l t we are t r y i n g to p r o d u c e .
We have
c o m e to r e g a r d the DO, FOR, W H I L E s t a t e m e n t s
and the like as p o w e r f u l tools, w h e r e a s
t h e y are in f a c t w e a k p a l l i a t i v e s t h a t are
n e c e s s a r y to m a k e the p r i m i t i v e v o n N e u m a n n
s t y l e of p r o g r a m m i n g v i a b l e at all.
s t a r t i n g w i t h a small, e l e g a n t f r a m e w o r k - b u t m u s t be b u i l t into the f r a m e w o r k of the
l a n g u a g e at the outset.
T h e G a r g a n t u a n size
of r e c e n t v o n N e u m a n n l a n g u a g e s is e l o q u e n t
p r o o f of t h e i r i n a b i l i t y to d e f i n e n e w constructs:
for no o n e w o u l d b u i l d in so m a n y
c o m p l e x f e a t u r e s if t h e y c o u l d be d e f i n e d
and w o u l d fit i n t o the e x i s t i n g f r a m e w o r k
l a t e r on.
The w o r l d of e x p r e s s i o n s has s o m e e l e g a n t
and u s e f u l m a t h e m a t i c a l p r o p e r t i e s w h e r e a s
the w o r l d of s t a t e m e n t s is a d i s o r d e r l y one,
without useful mathemetical properties.
S t r u c t u r e d p r o g r a m m i n g c a n be v i e w e d as a
m o d e s t e f f o r t to i n t r o d u c e a s m a l l a m o u n t
of o r d e r into the c h a o t i c w o r l d of s t a t e ments.
The
work
of
Hoare
[1969],
Dijkstra
[1976]
and o t h e r s to a x i o m a t i z e the p r o p e r t i e s of the s t a t e m e n t w o r l d
can be v i e w e d as a v a l i a n t and e f f e c t i v e
e f f o r t to be p r e c i s e a b o u t t h o s e p r o p e r t i e s ,
u n g a i n l y as t h e y m a y be.
This is n o t the p l a c e for m e to e l a b o r a t e
any f u r t h e r m y v i e w s a b o u t v o n N e u m a n n languages.
M y p o i n t is this: w h i l e it w a s
p e r h a p s n a t u r a l and i n e v i t a b l e t h a t lang u a g e s like F O R T R A N and its s u c c e s s o r s
s h o u l d h a v e d e v e l o p e d o u t of the c o n c e p t of
the v o n N e u m a n n c o m p u t e r as t h e y did, the
fact that such languages have dominated our
t h i n k i n g for t w e n t y y e a r s is u n f o r t u n a t e .
It is u n f o r t u n a t e b e c a u s e t h e i r l o n g - s t a n d ing f a m i l i a r i t y w i l l m a k e it h a r d for us to
u n d e r s t a n d and a d o p t n e w p r o g r a m m i n g s t y l e s
w h i c h one d a y w i l l o f f e r far g r e a t e r i n t e l l e c t u a l and c o m p u t a t i o n a l power.
Acknowledgments
My g r e a t e s t d e b t in c o n n e c t i o n w i t h this
p a p e r is to m y o l d f r i e n d s and c o l l e a g u e s
w h o s e c r e a t i v i t y , h a r d w o r k and i n v e n t i o n
made FORTRAN possible.
It is a p l e a s u r e to
a c k n o w l e d g e m y g r a t i t u d e to t h e m for t h e i r
c o n t r i b u t i o n s to the p r o j e c t , for m a k i n g
our w o r k t o g e t h e r so long ago s u c h a c o n g e n i a l and m e m o r a b l e e x p e r i e n c e , and, m o r e
r e c e n t l y , for p r o v i d i n g m e w i t h a g r e a t
a m o u n t of i n f o r m a t i o n and h e l p f u l m a t e r i a l
in p r e p a r i n g this p a p e r a n d for t h e i r c a r e ful r e v i e w s of an e a r l i e r draft.
F o r all
this I t h a n k all t h o s e w h o w e r e a s s o c i a t e d
w i t h the F O R T R A N p r o j e c t b u t w h o are too
n u m e r o u s to l i s t here.
In p a r t i c u l a r I
w a n t to t h a n k t h o s e w h o w e r e the p r i n c i p a l
m o v e r s in m a k i n g F O R T R A N a r e a l i t y : S h e l d o n
Best, R i c h a r d G o l d b e r g , L o i s H a i b t , H a r l a n
H e r r i c k , G r a c e M i t c h e l l , R o b e r t N e l s o n , Roy
Nutt, D a v i d Sayre, P e t e r S h e r i d a n , and
I r v i n g Ziller.
I a l s o w i s h to t h a n k B e r n a r d G a l l e r ,
J. A. N. Lee, a n d H e n r y T r o p p for t h e i r amiable, e x t e n s i v e and i n v a l u a b l e s u g g e s t i o n s
for i m p r o v i n g the f i r s t d r a f t of this paper.
I am g r a t e f u l too for all the w o r k of the
p r o g r a m c o m m i t t e e in p r e p a r i n g h e l p f u l q u e s t i o n s t h a t s u g g e s t e d a n u m b e r of t o p i c s in
the paper.
By s p l i t t i n g p r o g r a m m i n g into a w o r l d of
e x p r e s s i o n s on the o n e h a n d and a w o r l d of
s t a t e m e n t s on the o t h e r , v o n N e u m a n n lang u a g e s p r e v e n t the e f f e c t i v e u s e of h i g h e r
leve l c o m b i n i n g forms; the l a c k of the latter m a k e s the d e f i n i t i o n a l c a p a b i l i t i e s of
y o n N e u m a n n l a n g u a g e s so w e a k t h a t m o s t of
t h e i r i m p o r t a n t f e a t u r e s c a n n o t be d e f i n e d - -
178
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P r o p o s e d S p e c i f i c a t i o n s for F O R T R A N II
for the 704.
(Unpublished m e m o r a n d u m ,
P r o g r a m m i n g R e s e a r c h Dept. IBM.)
180