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Calling a subroutine --- the bl (branch and link) instruction

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• General discussion: Assembler Instructions to implement subroutine (method) calling
  • Modern computer typically provides 2 assembler instructions that user can use to implement methods:

    An assembler instruction that is used by the caller to invoke a method An assembler instruction that is used by the callee (= the called method) to return to its caller

    We will now discuss the first of these 2 instruction for ARM.

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• ARM assembler instructions used to implement subrouting (method) calling
  • The branch and link (bl instruction in the ARM processor is used to invoke (= call) a method.

  • Syntax of the branch and link instruction:

    bl label Effect: (1) Save the Program Counter (PC) in the Link Register (LR) (2) Branch to memory location marked by the label "label"

    Explanation:

    Recall that the Program Counter (PC) contains the address of the next instruction that the CPU will fetch (and execute) after it processed the current instruction in the Instruction Register (IR) In other words: PC contains the location of the instruction following the bl instruction: Assembler program in memory: .... .... bl method <--- CPU currently executing the bl instruction next instr <--- The PC contains the address of this instruction ! .... By saving the value in the PC (in the LR register), we have saved the return address that the invoked method must use to return back to its caller !!!

  • The retrurn address saved in the LR register is a "bread crump" - using the analogy of Hansel and Gretel....

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• Example of the bl instruction
  • Consider the following ARM assembler program:

    main: mov r0, #1111 mov r1, #1111 bl myMethod mov r2, #2222 mov r3, #2222 myMethod: mov r8, #4444 mov r9, #4444

    I will use EGTAPI in class to show you the executing.

    I have taken some snapshot of the execution for my notes.

    The content of the registers at the moment the ARM processor executes the bl instruction is as follows:

    Notice that PC = 33280 which is the location of the "bl myMethod" instruction

    The "bl myMethod" instruction will call (= run, jump) the myMethod function which is located at address 33292 as given by EGTAPI:

    When we click STEP and execute the "bl myMethod" in EGTAPI, we will see these values in the registers:

    Notice that:

    PC = 33292 - the program made a jump to the myMethod function (So the instructions in the myMethod function will now be executed) Notice that the LR register contains: 33284 !!! This is the address of the instruction that follows the "bl myMethod" instruction !!! So the bl (branch and link) instruct has saved the return address in the lr register

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/bl.s

    How to run the program:

    To compile:   as255 bl          To run: use EGTAPI

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  OK, now that we left the return address in the LR register; let's see how we can use it to return to its caller.