Branch Instruction

The LC-3 provides a single instruction used to change program flow based on the result of a previous instruction

Normal program flow is to execute each instruction in-order until the last instruction is complete. While this good enough to solve simple problems, more complex problems require code to be skipped or repeated

Common Flow Control Constructs

If - Based on a logical test (if(x > 12)), execute or skip a block is instructions

If/Else - Based on a logical test, execute or skip a block is instructions

For() - Based on a logical test, repeat a block is instructions

Logical Conditional Test

Condition Code (CC) Register

The LC-3 sets the Condition Code (cc) Register as part of the execution of the following instructions:

• ADD
• AND
• NOT
• LD

This register can be used by future instructions that might decide to change the program's flow (like an If/Else -or- Loop behavior)

The CC will be in one of three possible states:

N - The last operation resulted in a Negative Value

Z - The last operation resulted in a Zero Value

P - The last operation resulted in a Positive Value

Other instructions do not change the CC, so it will holding last value until another of the above 4 instructions is executed

.ORIG x3000
  AND R0, R0, #0; CC set to Z
  ADD R1, R0, #2; CC set to P
  LD R2, MyData;  CC set to N
Done HALT

;End of Program

;Data Declarations-------------
    MyData .FILL #-2
.END

Labels

If needed, a label is places before the instruction. It is, in effect, labeling the location os that instruction for latter use. They are any text the programmer chooses, as long as it is not a keyword or numeric vale

Labels in assembly source code are the method of denoting a particular location in the program that may be branched to from some other location in the program

Labels are human-readable text, named to help the programmer understand what the label is used for. During the assembly progress, the LC-3 Edit will convert labels to a PCOffset values needed for the BR instruction

.ORIG x3000
  MyLabel AND R0, R0, #0; Instruction associated with MyLabel
  MyOtherLabel 
        ADD R1, R0, #2; Instruction associated with MyOtherLabel
  Done HALT

;End of Program

;Data Declarations-------------
.END

Whitespace in Labels

Labels can be on the same line -or- a separate line, before the instruction. This is a matter of style. The extra whitespace of the sends option makes the code a bit more readable for humans. The assembler, however does not care, and removed the separation

Line 2 has an added label on the same line as the instruction

Line 3 has an added label on a line before the instruction

BR

Branches use both Labels and the CC to cause an executing program to change from the normal program flow

The BR instruction includes references used to compare to the CC as the logical test

BRn label - Branch (change program flow to) to label if CC is Negative

BRp label - Branch (change program flow to) to label if CC is Positive

BRz label - Branch (change program flow to) to label if CC is Zero

References can be combined to test two (2) or all three (3) references

BRnz label - Branch (change program flow to) to label if CC is Negative or Zero

BRzp label - Branch (change program flow to) to label if CC is Zero or Positive

BRnzp label - Branch (change program flow to) to label if CC is Negative, Positive, or Zero

Order is Important

When using multiple references with BR they must be in the nzp order. If the order is different an assembler error will occur

.ORIG x3000
  LD R0, MyData
  BRz JumpHereIfZero
  BRp JumpHereIfPositive
  BRn JumpHereIfNegative

  JumpHereIfZero
    NOP;        Do something if R0 contains Zero
    BRnzp Done; Skip everything else and halt
    
  JumpHereIfPositive
    NOP;        Do something if R0 is Positive
    BRnzp Done; Skip everything else and halt
    
  JumpHereIfNegative
    NOP;        Do something if R0 is Negative
       ;        No need to 'BRnzp Done' here since 
       ;           normal program flow ends the program
  Done HALT

;End of Program

;Data Declarations-------------
  MyData .FILL xF00D
.END

• Lines 3 - 5 are reacting to the CC set by line 2
• Program flow will branch to line 7, 11, or 15 based on the CC
• After JumpHereIfZero or JumpHereIfPositive execute their block of instructions, there is am unconditional branch (jump regardless of the CC value) (Lines 9 & 13) to the Done label. This prevents the normal program flow from executing other instruction blocks
• As noted in the comment in lines 17 & 18, an unconditional branch is not needed here because normal program flow causes the program to fall through to Done.