ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

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. Refer to B

and BL on page4-58 for further information.

The

instruction:

•places the return address in the link register (lr)

•sets pc to the address of the subroutine.

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. Refer to the Using

the Procedure Call Standard in ADS Developer Guide for more information.

Example2-2 shows a subroutine that adds the values of its two parameters and returns

a result in r0. It is supplied as

subrout.s

in the

examples\asm

subdirectory of ADS. Refer

to Code examples on page2-2 for instructions on how to assemble, link, and execute the

example.

Example2-2

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

MOV pc, lr ; Return from subroutine

END ; Mark end of file