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 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.
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.
PDF version