Control Flow

Branching (BR)

Conditional and unconditional branches using condition codes.

Condition Codes

Every instruction that writes to a register sets the condition codes:

N = 1 if the result is negative

Z = 1 if the result is zero

P = 1 if the result is positive

Exactly one of N, Z, P is set at any time. These instructions set CC: ADD, AND, NOT, LD, LDI, LDR, LEA.

The BR Instruction

BR[n][z][p] LABEL — Branch to LABEL if the specified condition codes are set.

You specify which flags to check by including n, z, and/or p:

BRn NEGATIVE    ; Branch if result was negative
BRz ZERO        ; Branch if result was zero
BRp POSITIVE    ; Branch if result was positive
BRnz NOT_POS    ; Branch if negative OR zero
BRnp NOT_ZERO   ; Branch if negative OR positive (not zero)
BRzp NOT_NEG    ; Branch if zero OR positive (not negative)
BRnzp ALWAYS    ; Branch always (unconditional)
BR ALWAYS       ; Same as BRnzp — always branches

Example: If-Else

Here's how to implement an if-else in LC-3:

.ORIG x3000
LD R0, X
ADD R0, R0, #0     ; Set CC based on R0 (important!)
BRn IS_NEG         ; if R0 < 0, goto IS_NEG
LEA R0, POS_MSG    ; else: print "Positive"
PUTS
BR DONE            ; skip the negative branch
IS_NEG
LEA R0, NEG_MSG    ; print "Negative"
PUTS
DONE
HALT
X .FILL #5
POS_MSG .STRINGZ "Positive"
NEG_MSG .STRINGZ "Negative"
.END

If you load a value with LD or ADD R0, R0, #0, the CC is already set. But after operations like ST, the CC is not updated. Always make sure CC reflects what you want to test before branching.

Choosing the Right Branch

The trick is mapping your high-level condition to a CC check:

High-Level Condition

Setup

Branch

if (x == 0)	`ADD R0, R0, #0`	`BRz`
if (x != 0)	`ADD R0, R0, #0`	`BRnp`
if (x > 0)	`ADD R0, R0, #0`	`BRp`
if (x < 0)	`ADD R0, R0, #0`	`BRn`
if (x >= 0)	`ADD R0, R0, #0`	`BRzp`
if (a == b)	subtract a - b	`BRz`
if (a != b)	subtract a - b	`BRnp`
if (a > b)	subtract a - b	`BRp`
always jump	(no setup needed)	`BRnzp` or `BR`

The pattern is always the same: get the value you care about into the condition codes (by doing an operation that writes to a register), then branch.

Exercise

Write a program that loads a number from memory. If it's zero, print "Z". If positive, print "P". If negative, print "N".

main.asm

.ORIG x3000
LD R1, NUM
; Hint: LD already sets CC
 
; Check if zero
 
 
; Check if positive
 
 
; Must be negative
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #3
OUT
BR DONE
 
IS_POS
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #5
OUT
BR DONE
 
IS_ZERO
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
OUT
 
DONE HALT
 
NUM .FILL #5
.END

.ORIG x3000
LD R1, NUM
; Hint: LD already sets CC

; Check if zero


; Check if positive


; Must be negative
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #3
OUT
BR DONE

IS_POS
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #5
OUT
BR DONE

IS_ZERO
AND R0, R0, #0
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #15
OUT

DONE HALT

NUM .FILL #5
.END

x00000

PCx3000

NZP

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