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Arm Compiler armasm User Guide : Register usage in subroutine calls

Register usage in subroutine calls

You use branch instructions to call and return from subroutines. The Procedure Call Standard for the Arm® Architecture defines how to use registers in subroutine calls.

A subroutine is a block of code that performs a task based on some arguments and optionally returns a result. By convention, you use registers R0 to R3 to pass arguments to subroutines, and R0 to pass a result back to the callers. A subroutine that requires more than four inputs uses the stack for the additional inputs.

To call subroutines, use a branch and link instruction. The syntax is: 

    BL  destination

where destination is usually the label on the first instruction of the subroutine.

destination can also be a PC-relative expression.

The BL instruction:

  • Places the return address in the link register.
  • Sets the PC to the address of the subroutine.

After the subroutine code has executed you can use a BX LR instruction to return.

Note

Calls between separately assembled or compiled modules must comply with the restrictions and conventions defined by the Procedure Call Standard for the Arm® Architecture.

Example

The following example shows a subroutine, doadd, that adds the values of two arguments and returns a result in R0:

        AREA    subrout, CODE, READONLY     ; Name this block of code
        ENTRY                     ; Mark first instruction to execute
start   MOV     r0, #10           ; Set up parameters
        MOV     r1, #3
        BL      doadd             ; Call subroutine
stop    MOV     r0, #0x18         ; angel_SWIreason_ReportException
        LDR     r1, =0x20026      ; ADP_Stopped_ApplicationExit
        SVC     #0x123456         ; AArch32 semihosting (formerly SWI)
doadd   ADD     r0, r0, r1        ; Subroutine code
        BX      lr                ; Return from subroutine
        END                       ; Mark end of file
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