What is non-aligned access? (ARM/Keil)

Question

I\'m using Keil to write Assembly for ARM 7.

I have the following runtime error:

Non-aligned Access: ARM Instruction at 000000F8H, Memory Access at 7         


        

Answer 1

The issue is that the address you use for a 32-bit (4-byte) memory operation must be aligned to a 4-byte boundary. This means the address must be a multiple of 4, or if you prefer, the bottom two bits of the address must be zero.

In this case, the closest 4-byte aligned addresses would be 0x7F7F7F7C or 0x7F7F7F80.

Similarly, LDRH/STRH require 2-byte alignment, whereas LDRB/STRB can operate anywhere (1-byte alignment == unaligned).

In general the compiler/assembler takes care of making sure your variables are aligned correctly for the size they are - you should only run into this if you're generating addresses yourself (as per the question).