Question

How to write an X86 assembly program that simply counts how many A’s, C’s, T’s, and G’s the input string contain.

Answer 1

; Initialization

; .ORIG x3000

AND R2, R2, #0 ; R2 is counter, initially 0

LD R3, PTR ; R3 is pointer to characters

GETC ; R0 gets character input

LDR R1, R3, #0 ; R1 gets first character

; Test character for end of file

; TEST ADD R4, R1, #-4 ; Test for EOT (ASCII x04)

BRz OUTPUT ; If done, prepare the output

; Test character for match. If a match, increment count.

NOT R1, R1

ADD R1, R1, R0 ; If match, R1 = xFFFF

NOT R1, R1 ; If match, R1 = x0000

BRnp GETCHAR ; If no match, do not increment

ADD R2, R2, #1

; Get next character from file.

GETCHAR ADD R3, R3, #1 ; Point to next character.

LDR R1, R3, #0 ; R1 gets next char to test

BRnzp TEST

;Output the count

OUTPUT LD R0, ASCII ; Load the ASCII template

ADD R0, R0, R2 ; Covert binary count to ASCII

OUT ; ASCII code in R0 is displayed

HALT ; Halt machine

; Storage for pointer and ASCII template

ASCII .FILL x0030

PTR .FILL x4000

.END