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Writing ARM and Thumb Assembly Language
ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-17
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 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