2.3.3. Calling subroutines

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 or register-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 MOV pc,lr instruction to return. By convention, registers r0 to r3 are used to pass parameters to subroutines, and to pass results back to the callers.

Note

Calls between separately assembled or compiled modules must comply with the restrictions and conventions defined by the procedure call standard. See the Using the Procedure Call Standard appendix in RealView Compilation Tools v2.2 Developer Guide for more information.

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.

Example 2.2. 

        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
        SWI     0x123456          ; ARM semihosting SWI

doadd   ADD     r0, r0, r1        ; Subroutine code
        BX      lr                ; Return from subroutine
        END                       ; Mark end of file
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