3D1-Microprocessor Systems 1
Lecture 16: Subroutines and Stack – Examples and Review Questions
In this lecture we will work on two programs using the stack to pass parameters
between a subroutine and its caller. Your attention is drawn to the precautions you
should take when using the stack and the system stack pointer for passing
parameters. You will also work on writing ‘well behaved’ subroutines.
Learning Outcomes:
On completion of this lecture, you will be able
 Use the stack to pass parameters to and from a subroutine;
 Take adequate precautions when using the stack,
 Write a “well behaved” subroutine is.
16.1
Review Questions

What is special about A7, What is it used for and what special precaution
should you take when using it?

Describe how the 68000 uses the stack in subroutines. What advantages and
drawbacks are there to the 68000’s way of calling and returning from
subroutines?

Explain what a “well behaved” subroutine is.

Explain what happens when a subroutine is called, i.e. how the JSR/BSR and
RTS instructions operate.

What is a subroutine parameter and how would you pass parameters to and
from a subroutine?

When you are writing a subroutine, what precaution should you take to
ensure that the subroutine does not damage the caller’s environment?
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3D1-Microprocessor Systems 1
16.2 Example 1:
Write a complete program to count all the vowels in a NUL terminated character
string using a well behaved subroutine. Define a suitable character string and also a
longword result space.
RSLT
ORG
DS.L
ORG
vowels DC.B
char
DC.B
ORG
LEA
$1FFC
1
$2000
'aeiouAEIOU'
'Nul Terminated Character String',$0
$4000
char,A1
Load the character’s Effective Address
into A1
MOVE.L #0,RSLT
Clear accummulator
loop1
MOVE.B
pointer
BEQ
LEA
A0
MOVE.L
MOVE.L
BSR
BRA
(A1)+,D1
make local copy of char and incr. char
exit
vowels,A0
exit if is the NUL char has been moved
Load the vowels’ Effective Address into
#DoStuff,-(A7)
#9,-(A7)
Loop2
loop1
Push address of loop subroutine to stack
Push counter initial value to stack
call the search subroutine
repeat until end of string
exit
#0
return control to Monitor
(A0)+,D1
test if the current char a vowel and
return
if not vowel, return to calling point
TRAP
DoStuff CMP.B
incr. pointer
BNE
incr
MOVE.L RSLT,D0
ADDQ.L #1,D0
MOVE.L D0,RSLT
return RTS
Loop2
loop
*
*increment vowel counter
*
return to calling point
MOVEM.L D0-D7/A0-A6,-(A7)
save register on the stack
LEA
(A7),A1
MOVE.L 64(A1),D2
MOVE.L 68(A1),A2
use A1 as local stack pointer
get counter initial value from the stack
get address of loop subroutine
JSR
DBRA
call loop subroutine
repeat until counter down to -1
(A2)
D2,loop
MOVEM.L (A7)+,D0-D7/A0-A6
restore register
RTS
return to calling point
END
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3D1-Microprocessor Systems 1
16.3
Example 2:
Write a complete, well-behaved subroutine to translate a sequence of ASCII
characters, meant to represent an unsigned decimal number, to an unsigned binary
longword.
op1
num
main
ORG
DS.W
ORG
DC.B
$2000
1
$2100
'651 ',$0
ORG
LEA
PEA
BSR
TRAP
$4000
num,A0
op1
load string’s address into A0
Push the reference of op1 o stack
GetNum
Call subroutine GetNum
#0
return control of CPU to Monitor
ORG
$3000
D0-D2/A0-A2,-(A7) save working registers on stack
GetNum MOVEM.L
loop
exit
Reserve 1 longword for result
Define input value
CLR
LEA
MOVE.B
CMP.B
BMI
CMP.B
BPL
SUB.B
MOVE.L
LSL.L
LSL.L
ADD.L
ADD.L
BRA
LEA
MOVEA.L
MOVE.L
D1
(A7),A1
(A0)+,D0
#$30,D0
exit
#$3A,D0
exit
#$30,D0
D1,D2
#3,D1
#1,D2
D2,D1
D0,D1
loop
28(A1),A2
(A2),A2
D1,(A2)
clear accumulator D1
MOVEM.L
RTS
(A7)+,D0-D2/A0-A2 Restore working registers
return to caller
copy addr top of stack in A1 (local stack pointer)
copy ascii char in D0
*
*
*if char not a digit then exit
*
else subtract $30 from ascii code
save D1 in D2
multiply contents of D1 by 8
multiply contents of D2 by 2
[D1]+[D2] (product of by 10 mult)
add [D0] and [D1]
goto loop
Get pointer to address of op1
Get actual address of op1
Modify op1 in the calling routine
END
16.4
Conclusion
You now know how to use the stack to pass parameters to and from subroutine. You
should also be aware of the precautions to take when writing ‘well behaved’ and
‘ecological’ subroutines.
REFERENCES

Dr. Mike Brady, Microprocessor Systems 1, dept of Computer Science, Trinity
College Dublin: http://www.tcd.ie/Engineering/Courses/BAI/JS_Subjects/3D1/
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3D1-Microprocessor Systems 1

Look on the Web. Look at http://www.mee.tcd.ie/~assambc/3D1
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