I love x86 complications (ARM is way more elegant in comparison), and I WILL be bothered to walk through mov %eax, (%r12), or 41 89 04 24 ---- 41 - being the amd64 prefix that 'unlocks' rax to being interpreted as r12 ---- 89 - still the same MOV instruction as the first one in mov %eax, (%rax) ---- 04 - The ModR/M byte that specifies the %eax part, and [--][--] as the 'Effective Address', which is another way of saying that we need a SIB byte because this (%r12) doesn't already have a simple encoding with the ModR/M byte alone (the same would be needed even if it was a simple (%esp)). ---- 24 - A 'Scaled Index' of none and as simias stated, RSP (but R12 in the context of the 41 prefix for this instruction). ---- All of this is easier to visualize when using the ModR/M and SIB tables in Volume II of the Intel manual. In my copy of the manual it's pretty early on in Chapter 2 (2.1.5 Addressing-Mode Encoding of ModR/M and SIB Bytes))