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:

  • AH and AL are the High and Low halves of the AX register.
  • BH and BL are the High and Low halves of the BX register.
  • CH and CL are the High and Low halves of the CX register.
  • DH and DL are the High and Low halves of the DX register.

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:

  • SIL for RSI
  • DIL for RDI
  • BPL for RBP
  • SPL for RSP

The same applies to registers R8 through R15: their respective lower byte parts are named R8BR15B.