alt Hacker News> Unlike other partial register writes, when writing to an e register like eax, the architecture zeros the top 32 bits for free.
I’m familiar with 32-bit x86 assembly from writing it 10-20 years ago. So I was aware of the benefit of xor in general, but the above quote was new to me.
I don’t have any experience with 64-bit assembly - is there a guide anywhere that teaches 64-bit specifics like the above? Something like “x64 for those who know x86”?
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Chapter 3 of volume 1, ctrl+f for "64-bit mode", has a lot of the essentials including e.g. the stuff about zeroing out the top half of the register.
https://www.intel.com/content/www/us/en/developer/articles/t...
See https://github.com/MattPD/cpplinks/blob/master/assembly.x86.... - mostly focused on x86-64 (and some of the talks/tutorials offer pretty good overview)
It's not only xor that does this, but most 32-bit operations zero-extend the result of the 64-bit register. AMD did this for backward compatibility. so existing programs would mostly continue working, unlike Intel's earlier attempt at 64-bits which was an entirely new design.
The reason `xor eax,eax` is preferred to `xor rax,rax` is due to how the instructions are encoded - it saves one byte which in turn reduces instruction cache usage.
When using 64-bit operations, a REX prefix is required on the instruction (byte 0x40..0x4F), which serves two purposes - the MSB of the low nybble (W) being set (ie, REX prefixes 0x48..0x4f) indicates a 64-bit operation, and the low 3 bits of low nybble allow using registers r8-r15 by providing an extra bit for the ModRM register field and the base and index fields in the SIB byte, as only 3-bits (8-registers) are provided by x86.
A recent addition, APX, adds an additional 16 registers (r16-r31), which need 2 additional bits. There's a REX2 prefix for this (0xD5 ...), which is a two byte prefix to the instruction. REX2 replaces the REX prefix when accessing r16-r31, still contains the W bit, but it also includes an `M0` bit, which says which of the two main opcode maps to use, which replaces the 0x0F prefix, so it has no additional cost over the REX prefix when accessing the second opcode map.