ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.8.4 Block copy with LDM and STM

Example2-11 is an ARM code routine that copies a set of words from a source location

to a destination by copying a single word at a time. It is supplied as

word.s

in the

examples\asm

subdirectory of the ADS. Refer to Code examples on page2-2 for

instructions on how to assemble, link, and execute the example.

Example2-11 Block copy

AREA Word, CODE, READONLY ; name this block of code

num EQU 20 ; set number of words to be copied

ENTRY ; mark the first instruction to call

start

LDR r0, =src ; r0 = pointer to source block

LDR r1, =dst ; r1 = pointer to destination block

MOV r2, #num ; r2 = number of words to copy

wordcopy LDR r3, [r0], #4 ; load a word from the source and

STR r3, [r1], #4 ; store it to the destination

SUBS r2, r2, #1 ; decrement the counter

BNE wordcopy ; ... copy more

stop MOV r0, #0x18 ; angel_SWIreason_ReportException

LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit

SWI 0x123456 ; ARM semihosting SWI

AREA BlockData, DATA, READWRITE

src DCD 1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4

dst DCD 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

END

This module can be made more efficient by using

LDM

and

STM

for as much of the copying

as possible. Eight is a sensible number of words to transfer at a time, given the number

of registers that the ARM has. The number of eight-word multiples in the block to be

copied can be found (if r2 = number of words to be copied) using:

MOVS r3, r2, LSR #3 ; number of eight word multiples

This value can be used to control the number of iterations through a loop that copies

eight words per iteration. When there are less than eight words left, the number of words

left can be found (assuming that r2 has not been corrupted) using:

ANDS r2, r2, #7

Example2-12 on page 2-45 lists the block copy module rewritten to use

LDM

and

STM

for

copying.