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Branching - out of order program flow

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• Normal and out of order program flow
  • Recall (from this webpage: click here):

    The in-order or normal program flow is when: After executing the current instruction in the program, the CPU fetches and executes the instruction that physically follows the current instruction in the program An out-of-order program flow is when: After executing an instruction in the program, the CPU fetches and executes a instruction that is not the physically next instruction in the program

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• The branch assembler instruction
  • Syntax:

    b label // Branch to label or bal label // Branch always to label

    Effect:

    Jump to the instruction marked by the label label I.e.: after executing the b label instruction, the next instruction that will be executed is the instruction at the location marked by the label label

    Example: consider the following program

    .text main: mov r1, #1111 // Instructions executed in sequence mov r2, #1111 // "normal" program flow mov r3, #1111 b there // Branch to location marked by "there" mov r4, #2222 // These instructions are SKIPPED over ! mov r5, #2222 mov r6, #2222 there: mov r4, #4444 // Out of order program flow mov r5, #4444 mov r6, #4444

    The program flow will execute these instructions:

    mov r1, #1111 // Instructions executed in sequence mov r2, #1111 // "normal" program flow mov r3, #1111 mov r4, #4444 // Out of order program flow mov r5, #4444 mov r6, #4444

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/6-if/branch.s

    How to run the program:

    To compile:   as255 branch          To run: use EGTAPI