Intel x86 Assembly Language & Microarchitecture Register Fundamentals 8-bit Registers
Example
The first four 16-bit registers could have their upper- and lower-half bytes accessed directly as their own registers:
AHandALare the High and Low halves of theAXregister.BHandBLare the High and Low halves of theBXregister.CHandCLare the High and Low halves of theCXregister.DHandDLare the High and Low halves of theDXregister.
Note that this means that altering AH or AL will immediately alter AX as well! Also note that any operation on an 8-bit register couldn't affect its "partner" - incrementing AL such that it overflowed from 0xFF to 0x00 wouldn't alter AH.
64-bit registers also have 8-bit versions representing their lower bytes:
SILforRSIDILforRDIBPLforRBPSPLforRSP
The same applies to registers R8 through R15: their respective lower byte parts are named R8B – R15B.
