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. Refer to B and BL 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.

Example 2.2 shows a subroutine that adds the values of its two parameters and returns a result in r0. Refer to 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             	; Next instruction to execute after return
		;...
doadd   ADD     r0, r0, r1        ; Subroutine code
        MOV     pc, lr            ; Return from subroutine
        END                       ; Mark end of file