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Preface Introduction Writing ARM Assembly Language
Introduction Overview of the ARM architecture Architecture versions ARM, Thumb, Thumb‑2, and Thumb‑2EE instruction ARM, Thumb, and ThumbEE state Processor mode Registers Instruction set overview Instruction capabilities Structure of assembly language modules Layout of assembly language source files An example ARM assembly language module Calling subroutines Conditional execution The ALU status flags Conditional execution Using conditional execution Example of the use of conditional execution The Q flag Loading constants into registers Direct loading with MOV and MVN Loading with MOV32 Loading with LDR Rd, =const Loading addresses into registers Direct loading with ADR and ADRL Loading addresses with LDR Rd, =label Load and store multiple register instructions Load and store multiple instructions available in Implementing stacks with LDM and STM Block copy with LDM and STM Using macros Test‑and‑branch macro example Unsigned integer division macro example Using frame directives Assembly language changes Assembler Reference ARM and Thumb Instructions Directives ReferenceCalling subroutines
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| Home > Writing ARM Assembly Language > Structure of assembly language modules > Calling subroutines | |||
To call subroutines, use a branch and link instruction. The syntax is:
BL destination
where destination
destination
The BL instruction:
places the return address in the link register
sets the PC to the address of the subroutine.
After the subroutine code is executed you can use a BX lr instruction to return. By convention, registers r0 to r3 are used to pass parameters to subroutines, and r0 is used to pass a result 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 Procedure Call Standard for the ARM Architecture specification, aapcs.pdf, on the ARM website for more information.
Example 2.2 shows a subroutine that adds the values of two parameters and returns a result in r0.
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
SVC #0x123456 ; ARM semihosting (formerly SWI)
doadd ADD r0, r0, r1 ; Subroutine code
BX lr ; Return from subroutine
END ; Mark end of file
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