Weird :S If anyone wants too point out any flaws with my source :S
Code:
ldr r0, =0x121dcbf0
ldrb r0,[r0]
sub r0,#0x1
strb r0,[r0]
bx  lr

ldr r0, =0x121dcbf0
ldrb r0,[r0]
add r0,#0x1
strb r0,[r0]
bx  lr
(Thats EU)

I think maybe

Code:
ldr r0, =0x121dcbf0
ldrb r0,r1
sub r0,#0x1
strb r0,r1
bx  lr

ldr r0, =0x121dcbf0
ldrb r0,r1
add r0,#0x1
strb r0,r1
bx  lr
That might work better

This is a little of topic I think I'll start a new topic for this project.