ARM VERSION 1.2 manual Calling subroutines

Writing ARM and Thumb Assembly Language

2.3.3Calling 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 page 4-58 for further information.

The BL 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.

Example 2-2 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 page 2-2 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
	MOV	pc, lr	; Return from subroutine
	END		; Mark end of file

ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-17