2 ms·
> The only advantage I see in x64 is that the complex instructions are a form of 'compression' for the micro-instructions. It's not a very good one. The x86_64
by duskwuff 6y ago
> The only advantage I see in x64 is that the complex instructions are a form of 'compression' for the micro-instructions.
It's not a very good one. The x86_64 ISA encoding evolved out of the original 8086 instruction encoding, and has inherited a lot of inefficiencies from there -- a lot of one-byte opcodes are wasted on rarely used bytewise instructions (like XLAT), privileged instructions (like the IN/OUT family), or on opcodes that are simply invalid in 64-bit mode.
Most instructions in real x86_64 code are two to four bytes long, not even counting immediate values. A focused attempt could probably do much better.