Why does operand 1 in a modr/m byte change depending on the decoding mode

Question

I'm trying to figure out why the instruction changes from add [eax], al to add [rax], al when changing the decoding mode from x86 to x64.

The instruction bytes are 00 00

I think it might be because its being used to specify a memory location but I couldn't find anything to prove it

Answer 1

This is because the default address size is 64 bits in 64-bit mode and 32 bits in 32-bit modes.

You can apply a 67 address-size override prefix to select an address size of 32 bits in 64-bit mods, however 64-bit address size is not available outside of 64-bit mode.