REVERSE STRING

                AREA program,CODE,READONLY
                ENTRY
main            push {r4-r7, lr}
                ldr r0, =string
                ldr r1, =string1
                strb r1, [r0]
                mov r1, #' '
                strb r1, [r0, #1]
                ldr r1, =string2
                strb r1, [r0, #2]
                mov r1, #0
                strb r1, [r0, #8]
                mov r0, r0
                bl reverse_string
                pop {r4-r7, pc}

reverse_string  push {r4-r7, lr}
                mov r4, r0
                mov r5, #0
find_length     ldrb r6, [r4, r5]
                cmp r6, #0
                beq done
                add r5, r5, #1
                b find_length
done            sub r5, r5, #1
                mov r6, #0
reverse_loop    cmp r6, r5
                bge done2
                ldrb r7, [r4, r6]
                ldrb r6, [r4, r5]
                strb r6, [r4, r6]
                strb r7, [r4, r5]
                add r6, r6, #1
                sub r5, r5, #1
                b reverse_loop
done2           pop {r4-r7, pc}

string DCB "hello world", 0
string1 DCB "hello", 0
string2 DCB "world", 0
                END