Writing ARM and Thumb Assembly Language
2-18 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
2.3.4 An example Thumb assembly language module
Example 2-3 illustrates some of the core constituents of a Thumb assembly language
module. It is based on
subrout.s
. It is supplied as
thumbsub.s
in the
examples\asm
subdirectory of the ADS. Refer to Code examples on page 2-2 for instructions on how
to assemble, link, and execute the example.
Example 2-3
AREA ThumbSub, CODE, READONLY ; Name this block of code
ENTRY ; Mark first instruction to execute
CODE32 ; Subsequent instructions are ARM
header ADR r0, start + 1 ; Processor starts in ARM state,
BX r0 ; so small ARM code header used
; to call Thumb main program
CODE16 ; Subsequent instructions are Thumb
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 0xAB ; Thumb semihosting SWI
doadd
ADD r0, r0, r1 ; Subroutine code
MOV pc, lr ; Return from subroutine
END ; Mark end of file
CODE32 and CODE16 directives
These directives instruct the assembler to assemble subsequent instructions as ARM
(
CODE32
) or Thumb (
CODE16
) instructions. They do not assemble to an instruction to
change the processor state at runtime. They only change the assembler state.
The ARM assembler,
armasm
, starts in ARM mode by default. You can use the
-16
option
in the command line if you want it to start in Thumb mode.
BX instruction
This instruction is a branch that can change processor state at runtime. The least
significant bit of the target address specifies whether it is an ARM instruction (clear) or
a Thumb instruction (set). In this example, this bit is set in the
ADR
pseudo-instruction.
