3D1-Microprocessor Systems 1
Lecture 5: Introduction to Assembly Language Programming - Examples
In this lecture, we will write programs to return the average of an array of word integers.
Learning Outcomes:
On completion of this lecture, you will be able to:
 Write simple sequential statements in 68k assembly language;
 Implement conditional and loop statements.
5.1
Write an assembly language program to find the (integer) average value
of ten unsigned word integers.
The numbers are stored in an array starting at $2000.The result should be placed in
$1FFE.
You'll need the unsigned divide instruction - DIVU.
The destination must be a data register. The source can be anything sensible. This
instruction will divide the longword destination by the word source, replacing the longword
destination with two words. The least significant word is the integer result, and the most
significant word is the integer remainder.
*label
RSLT
A
B
C
D
E
F
G
H
I
J
instructions
operands
comments
ORG
DS.W
$1FFE
1
result location
reserve 1 longword in memory
ORG
MOVEQ.L
ADD.W
ADD.L
ADD.L
ADD.L
ADD.L
ADD.L
ADD.L
ADD.L
ADD.L
ADD.L
$2000
#0, D0
A, D0
B, D0
C, D0
D, D0
E, D0
F, D0
G, D0
H, D0
I, D0
J, D0
origin of program
initialise D0 to zero
*
*
*
*
*
* accumulate the sum into D0
*
*
*
*
DIVU
#10, DO
get the average
MOVE.W
D0,RSLT
save result
TRAP
#0
return control of the CPU to the Monitor (OS)
ORG
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
$1100
1
2
3
4
5
6
7
8
9
10
origin of data
*
*
*
*
* example of data entry
*
*
*
*
*
END
5-1
3D1-Microprocessor Systems 1
5.2
How would you calculate the average of an array of, say, a 1000 unsigned
word integers?
*label
opcode
operands
comments
N
START
EQU
EQU
1000
$2000
set array size
the array starts at address $2000
RSLT
ORG
DS.W
$1FFE
1
result location
reserves 1 word in memory for the result
ORG
MOVEQ
MOVE.L
MOVE.L
MOVE.W
ADD.L
DBRA
DIVU
MOVE.W
$1000
#0,D1
#START,A0
#N-1,D0
(A0)+,D2
D2,D1
D0,SUM
#N,D1
D1,RSLT
returns the integer average of an array of N integers
clear accumulator
A0 points at the start of the array
init. number counter: loop to be run from 999 to 0
make local copy of a number & point at the next one
accumulate the longworld sum to avoid overflow and errors
repeat the loop as long [D0] is positive
div D1 by N
store integer part in memory
TRAP
#0
returns control of the CPU to the Monitor (OS)
SUM
END
5.3
Conclusion
To calculate the sum of a large number of values in a practical way, we need to get the
same sequence of instructions executed multiple times -- we need to control the flow of
program execution.
It’s no use executing exactly the same operations over and over.
For example, we have to be able to get a different element (actually the next element)
each time. Here we have managed to vary the address an instruction uses while the
program is executing.
REFERENCES

Dr. Mike Brady, Microprocessor Systems 1, dept of Computer Science, Trinity College
Dublin: http://www.tcd.ie/Engineering/Courses/BAI/JS_Subjects/3D1/

Look on the Web at http://www.mee.tcd.ie/~assambc/3D1
5-2