Write a program to copy text from the location pointed at by r0 to the location pointed at by r1. The copied version must be in reverse order. Assume the string you are copying is non?null (it contains at least one character) and that it is terminated.

What will be an ideal response?

A simple way is to find the end of the string and then move a character from the end of the source string to the beginning of the destination and then continue character?by?character working in opposite directions. Note that this code uses some features not available with the Keil system.

; Point to source

; Point to destination

; REPEAT

; Read char from source

; UNTIL all last char found

; REPEAT

; Copy source character to destination

; Decrement source pointer, increment destination pointer

; UNTIL all done

AREA ReverseString, CODE, READWRITE

ADR r0,Source ;r0 points to source

MOV r3,r0 ;keep a copy of the start of the source

FindEnd LDRB r2,[r0],#1 ;read a byte and update pointer

CMP r2,#0x0D ;test for end

BNE FindEnd ;continue until terminator found

;pointer to source is one past the end of the string

ADR r1,Dest ;r1 points to destination

CopyBak LDRB r2,[r0,#-1]! ;read a char using predecrementing from source

STRB r2,[r1],#1 ;store at destination with post incrementing

CMP r0,r3 ;are we back at the start?

BNE CopyBak ;if not keep going

MOV r0, #0x18 ;angel_SWIreason_ReportException

LDR r1, =0x20026 ;ADP_Stopped_ApplicationExit

SVC #0x123456 ;ARM semihosting (formerly SWI)

AREA ReverseString, DATA, READWRITE

DCD 0xFFFFFFFF ;this is just a marker to help me read the code

Source DCB "Test",0x0D

Dest DCD 1,1,1,1,1 ;fill the target with 1s to help testing

END