CSC103 Assembly Language Exercises and Solutions

--D. Thiebaut 12:08, 20 February 2011 (EST)

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LOD-C 1

Exercise 1

initialize the contents of several variables to 0

; Exercise 1
; D. Thiebaut
; Initialize the contents of several variables 
; to 55

start:
		lod-c	55		; put 0 in acc
		sto		var1	; store acc in var1
		sto		var2	; store acc in var2
		sto		var3	; store acc in var3
		hlt

@10					; the data section starts here
var1:	        data
var2:	        data
var3:	        data

LOD 10/STO 10

Exercise 2

increment a counter

; Exercise 2
; D. Thiebaut
; A program that sets a counter to 0
; and then increments it
;
start:	lod-c	0		        ; first initialize the counter to 0
		sto	counter

; increment counter
		lod	counter	; get counter in acc
		add-c	1		; add 1 to acc
		sto	counter	; store acc in counter

		hlt

@10						
counter:	data

Exercise 3

compute sum of 3 variables

; Exercise 3
; D. Thiebaut
; Compute the sum of 3 variables
; and stores it into a 4th variable called
; sum.

start:	lod		var1	; get var1 in acc
	add		var2	; add var2 to acc
	add		var3	; add var3 to acc
	sto		sum	; store acc into sum
	hlt

@10
var1:	12
var2:	5
var3:	3
sum:	0

LOD-I 10/STO-I 10

Exercise 4

create an index and use it to sum up the same 3 variables

; Exercise 4
; D. Thiebaut
; Compute the sum of 3 variables using an index
; and stores it into a 4th variable called
; sum.  

start:	lod-i 	index	        ; get var1 indirectly in acc
	sto	sum		; set sum to var1
	lod	index	        ; get index in acc
	add-c	1		; increment acc
	sto	index

	lod-i	index	        ; get var2 indirectly in acc
	add	sum	        ; add sum to acc
				; acc now contains var1 + var2
	sto	sum		; store acc in sum

	lod	index	        ; increment index
	add-c	1
	sto	index		

	lod-i	index	        ; get var3 indirectly in acc
	add	sum		; add var3 to sum (which contains var1+var2)
	sto	sum		; store acc back in sum
				; sum now contains var1+var2+var3

	hlt

@15
index: var1		       ; index points to var1
var1:	 12
var2:	 5
var3:	 3
sum:	 0

JMP instruction and labels

Exercise 5

create an infinite loop (incrementing a variable, for example)

; Exercise 5
; D. Thiebaut
; an infinite loop
start:	lod		index	; increment index
		add-c	1
		sto		index		

		jmp		start
		hlt

@15
index:	data		

Exercise 6

create an infinite loop that stores 55 in all the memory words the memory starting at 10

; Exercise 6
; D. Thiebaut
; an infinite loop
start:	lod-c	10		; store 10 in index
	sto	index

loop:	lod-c	55		; acc set to 55
	sto-i	index	        ; store 55 where index is pointing

	lod	index	        ; increment index
	add-c	1
	sto	index		

	jmp	loop	        ; loop forever
	hlt

@9
index:	data

Exercise 7

why does the program of Exercise 6 stop or misbehave?

The memory contains 1024 words of memory. So, each address can be expressed with 10 bits. Address 0 decimal is written as 0000000000 in binary. Address 1023 is written as 1111111111 in binary. So only 10 bits are used by the memory to figure out what address the processor wants to access. When the processor sends Address 1024, in binary it is written with 11 bits: 10000000000, but the memory will only look at the lower 10 bits, which are 0000000000, or memory word 0.

So the program, once it has reached the memory word at 1023 and stored 55 there, continues on with the words at address 0, 1, 2... Essentially the program overwrites itself from memory.

JMZ

Exercise 8

write a loop that loops 10 times (use a counter)

; Exercise 8
; D. Thiebaut
; A program that loops 10 times
;
start:	lod-c	10		; store 10 in counter
	sto	counter

loop:	lod	counter	; decrement counter
	sub-c	1
	sto	counter	; store new value in counter

	jmz	done		; if value in acc is 0, jump to done
	jmp	loop		; otherwise, go back to loop label
done:   hlt			; we're done. Stop!

@10
counter:	data

Exercise 9

write a loop that stores 55 in the memory locations between 15 and 30

; Exercise 8
; D. Thiebaut
; A program that loops 10 times
;
start:	
loop:	lod-c	55		; store 55 indirecty where
	sto-i	index		; index is pointing
	lod	index		; get index
	add-c	1		; increment its value
	sto	index		; store value back in index
	lod	counter	; load counter
	sub-c	1		; decrement it
	sto	counter	; store new value in counter

	jmz	done		; if value in acc is 0, jump to done
	jmp	loop		; otherwise, go back to loop label
done:  hlt			; we're done. Stop!

@13
counter:16		; between 15 and 30, there are 16 words of memory
index:	15
var1:	data