ARM VERSION 1.2 manual Using two register-based structures

Writing ARM and Thumb Assembly Language

2.10.7Using two register-based structures

Sometimes you need to operate on two structures of the same type at the same time. For example, if you want the equivalent of the pseudo-code:

newloc.x = oldloc.x + (value in r0); newloc.y = oldloc.y + (value in r1); newloc.z = oldloc.z + (value in r2);

The base register needs to point alternately to the oldloc structure and to the newloc one. Repeatedly changing the base register would be inefficient. Instead, use a

non register-based map, and set up two pointers in two different registers as in Example 2-26.

Example 2-26

	MAP	0	; Non-register based relative map used twice, for
Pointx	FIELD	4	; old and new data at oldloc and newloc
Pointy	FIELD	4	; oldloc and newloc are labels for
Pointz	FIELD	4	; memory allocated in other sections
	; code
	ADR	r8,oldloc
	ADR	r9,newloc
	LDR	r3,[r8,Pointx]	; load from oldloc (r8)
	ADD	r3,r3,r0
	STR	r3,[r9,Pointx]	; store to newloc (r9)
	LDR	r3,[r8,Pointy]
	ADD	r3,r3,r1
	STR	r3,[r9,Pointy]
	LDR	r3,[r8,Pointz]
	ADD	r3,r3,r2
	STR	r3,[r9,Pointz]

ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-63