ARM VERSION 1.2 - Manual (Page 50)

Writing ARM and Thumb Assembly Language
2-38 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
Converting to Thumb
There is no post-indexed addressing mode for Thumb LDR and STR instructions. Because
of this, you must use an ADD instruction to increment the address register after the LDR
and STR instructions. For example:
LDRB r2, [r1] ; load register 2
ADD r1, #1 ; increment the address in
; register 1.