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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.


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

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

Add two arguments

        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         ; ARM semihosting (formerly SWI)
doadd   ADD     r0, r0, r1        ; Subroutine code
        BX      lr                ; Return from subroutine
        END                       ; Mark end of file
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