Arm VERSION 1.2 Manual de usuario descargar pdf (Pagina 49)

Writing ARM and Thumb Assembly Language

An LDR Rd, =label example: string copying

Example 2-10 shows an ARM code routine that overwrites one string with another

string. It uses the

LDR

pseudo-instruction to load the addresses of the two strings from a

data section. The following are particularly significant:

DCB

The

DCB

directive defines one or more bytes of store. In addition to integer

values,

DCB

accepts quoted strings. Each character of the string is placed

in a consecutive byte. Refer to DCB on page 7-18 for more information.

LDR/STR

The

LDR

and

STR

instructions use post-indexed addressing to update their

address registers. For example, the instruction:

LDRB r2,[r1],#1

loads r2 with the contents of the address pointed to by r1 and then

increments r1 by 1.

Example 2-10 String copy

AREA StrCopy, CODE, READONLY

ENTRY ; Mark first instruction to execute

start LDR r1, =srcstr ; Pointer to first string

LDR r0, =dststr ; Pointer to second string

BL strcopy ; Call subroutine to do copy

stop MOV r0, #0x18 ; angel_SWIreason_ReportException

LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit

SWI 0x123456 ; ARM semihosting SWI

strcopy

LDRB r2, [r1],#1 ; Load byte and update address

STRB r2, [r0],#1 ; Store byte and update address

CMP r2, #0 ; Check for zero terminator

BNE strcopy ; Keep going if not

MOV pc,lr ; Return

AREA Strings, DATA, READWRITE

srcstr DCB "First string - source",0

dststr DCB "Second string - destination",0

END

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Arm VERSION 1.2 Manual de usuario Pagina 49

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