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 program-relative expression. See B, BL, BX, BLX, and BXJ for more information.

The BL instruction:

After the subroutine code is executed you can use a BX lr instruction to return. By convention, registers r0 to r3 are used to pass parameters to subroutines, and r0 is used to pass a result back to the callers.

Example 2.2 shows a subroutine that adds the values of two parameters and returns a result in r0. It is supplied as subrout.s in the main examples directory install_directory\RVDS\Examples. See Code examples for instructions on how to assemble, link, and execute the example.

        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