ARM VERSION 1.2 manual Example 2-8 Thumb code jump table

Writing ARM and Thumb Assembly Language

Converting to Thumb

Example 2-8 shows the implementation of the jump table converted to Thumb code.

Most of the Thumb version is the same as the ARM code. The differences are commented in the Thumb version.

In Thumb state, you cannot:

•increment the base register of LDR and STR instructions

•load a value into the pc using an LDR instruction

•do an inline shift of a value held in a register.

			Example 2-8 Thumb code jump table
	AREA	Jump, CODE, READONLY
	CODE16		; Following code is Thumb code
num	EQU	2
	ENTRY
start
	MOV	r0, #0
	MOV	r1, #3
	MOV	r2, #2
	BL	arithfunc
stop	MOV	r0, #0x18
	LDR	r1, =0x20026
	SWI	0xAB	; Thumb semihosting SWI
arithfunc
	CMP	r0, #num
	BHS	exit	; MOV pc, lr cannot be conditional
	ADR	r3, JumpTable
	LSL	r0, r0, #2	; 3 instructions needed to replace
	LDR	r0, [r3,r0]	; LDR pc, [r3,r0,LSL#2]
	MOV	pc, r0
	ALIGN		; Ensure that the table is aligned on a
			; 4-byte boundary
JumpTable
	DCD	DoAdd
	DCD	DoSub
DoAdd	ADD	r0, r1, r2
exit	MOV	pc, lr
DoSub	SUB	r0, r1, r2
	MOV	pc, lr
	END

2-34 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B