Control Flow

Loops

Implementing while loops and for loops with branches.

Loops in Assembly

There's no for or while in assembly. We build loops with:

A counter in a register

A branch instruction that jumps back to the top

Counting Loop Pattern

Here's the classic "count down to zero" loop:

.ORIG x3000
AND R0, R0, #0    ; R0 = sum = 0
AND R1, R1, #0
ADD R1, R1, #5    ; R1 = counter = 5

LOOP
ADD R1, R1, #0    ; Set CC for R1
BRz DONE          ; If counter == 0, exit loop
ADD R0, R0, R1    ; sum += counter
ADD R1, R1, #-1   ; counter--
BR LOOP           ; Go back to top

DONE HALT          ; R0 = 5+4+3+2+1 = 15
.END

Iterating Over an Array

Combine a pointer register with a counter to walk through data:

.ORIG x3000
; Sum 5 numbers starting at ARRAY
AND R0, R0, #0      ; R0 = sum
LEA R1, ARRAY       ; R1 = pointer to current element
AND R2, R2, #0
ADD R2, R2, #5      ; R2 = count

LOOP
ADD R2, R2, #0      ; Check counter
BRz DONE
LDR R3, R1, #0      ; R3 = current element
ADD R0, R0, R3      ; sum += element
ADD R1, R1, #1      ; pointer++
ADD R2, R2, #-1     ; count--
BR LOOP

DONE HALT

ARRAY .FILL #3
      .FILL #7
      .FILL #1
      .FILL #4
      .FILL #5
.END

The pattern ADD R2, R2, #0 is a common idiom — it doesn't change R2 but sets the condition codes so we can branch on its value.

Exercise

Write a program that counts down from 9 to 1, printing each digit. Output should be "987654321".

.ORIG x3000
AND R1, R1, #0
ADD R1, R1, #9    ; counter = 9
 
LOOP
; Check if counter is 0
 
 
; Convert counter to ASCII digit (add 48)
; Put in R0 and print with OUT
 
 
; Decrement counter
 
 
; Loop back
 
 
DONE HALT
.END

.ORIG x3000
AND R1, R1, #0
ADD R1, R1, #9    ; counter = 9

LOOP
; Check if counter is 0


; Convert counter to ASCII digit (add 48)
; Put in R0 and print with OUT


; Decrement counter


; Loop back


DONE HALT
.END

x00000

PCx3000

NZP

← Branching (BR)JMP, JSR & RET →