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.

    Exercise

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

    1
    R0
    x00000
    R1
    x00000
    R2
    x00000
    R3
    x00000
    R4
    x00000
    R5
    x00000
    R6
    x00000
    R7
    x00000
    PCx3000
    CC
    NZP
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