3 ms·

There’s also no R9H ... R15H. These “registers", if they existed, would be bits 8-15 of each of R8, R9, etc. Without talking to engineers at AMD, it’s probabl

There’s also no R9H ... R15H. These “registers", if they existed, would be bits 8-15 of each of R8, R9, etc. Without talking to engineers at AMD, it’s probably impossible to say precisely why they weren’t provided, but there is generally little/no reason to ever want to use the existing *H registers in 64-bit code, so it’s not surprising that they were omitted. Some combination of low/no demand and added encoding/decode/retirement complexity is an excellent reason to leave things out of an architecture.

pbsd 14y ago
There are also no RAH, RBH, .... You can only access AH by using the old 32-bit encoding. This happens because in 32-bit x86, you only have access to AL, AH, BL, BH, CL, CH, DL, DH (no lower bytes of EDI, ESP, etc). These are 8 different possibilities, that are encoded by ModR/M's 3 Reg bits. In amd64, they decided to regularize the instruction set: those 3 bits are now used to encode the lower byte of EAX--EDI (AL, CL, ..., DIL). To get R8L, ..., R15L you use the REX.R prefix, which acts as the 4th bit.