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.

    Loop Templates

    Most loops in LC-3 follow one of these templates:

    Count-down loop (when you know how many iterations): `` AND counter, counter, #0 ADD counter, counter, #N LOOP ADD counter, counter, #0 BRz DONE ; ... body ... ADD counter, counter, #-1 BR LOOP DONE ` Pointer-walk loop (for strings and arrays): ` LEA ptr, DATA ; or LD ptr, ADDR LOOP LDR char, ptr, #0 BRz DONE ; null terminator = stop ; ... process char ... ADD ptr, ptr, #1 BR LOOP DONE ` Sentinel loop (stop when a condition is met): ` LOOP ; ... do something ... ; ... compute a condition ... BRz DONE ; or BRn, BRp depending on your stop condition BR LOOP DONE ``

    Recognize which template fits your problem, then fill in the body.

    Exercise

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

    main.asm
    1
    R0
    x00000
    R1
    x00000
    R2
    x00000
    R3
    x00000
    R4
    x00000
    R5
    x00000
    R6
    x00000
    R7
    x00000
    PCx3000
    CC
    NZP
    ← Branching (BR)JMP, JSR & RET →