• Traditional uses are conventions (except RSP, keep that as the stack pointer)
• "AH" means high byte (bits 8-15) of AX, "AL" is low byte (bits 0-7)
• In 64-bit mode, operations on 32-bit registers zero the upper 32 bits

General Purpose

The Original 8 Registers (from 8086/80386 era)

• RAX - Accumulator Register
  • 64-bit: RAX
  • 32-bit: EAX (lower 32 bits)
  • 16-bit: AX (lower 16 bits)
  • 8-bit low: AL (bits 0-7)
  • 8-bit high: AH (bits 8-15)
  • Traditional use: Arithmetic operations, function return values
• RBX - Base Register
  • 64-bit: RBX
  • 32-bit: EBX
  • 16-bit: BX
  • 8-bit low: BL
  • 8-bit high: BH
  • Traditional use: Base pointer for memory access
• RCX - Counter Register
  • 64-bit: RCX
  • 32-bit: ECX
  • 16-bit: CX
  • 8-bit low: CL
  • 8-bit high: CH
  • Traditional use: Loop counters, shift/rotate counts
• RDX - Data Register
  • 64-bit: RDX
  • 32-bit: EDX
  • 16-bit: DX
  • 8-bit low: DL
  • 8-bit high: DH
  • Traditional use: I/O operations, extended arithmetic (with RAX for multiplication/division)
• RSI - Source Index
  • 64-bit: RSI
  • 32-bit: ESI
  • 16-bit: SI
  • 8-bit low: SIL (no high byte variant)
  • Traditional use: Source pointer for string/memory operations
• RDI - Destination Index
  • 64-bit: RDI
  • 32-bit: EDI
  • 16-bit: DI
  • 8-bit low: DIL (no high byte variant)
  • Traditional use: Destination pointer for string/memory operations
• RBP - Base Pointer
  • 64-bit: RBP
  • 32-bit: EBP
  • 16-bit: BP
  • 8-bit low: BPL (no high byte variant)
  • Traditional use: Base of the current stack frame
• RSP - Stack Pointer
  • 64-bit: RSP
  • 32-bit: ESP
  • 16-bit: SP
  • 8-bit low: SPL (no high byte variant)
  • Traditional use: Points to the top of the stack (DON'T misuse this one!)

The 8 New Registers (added in x86-64)

• R8 through R15
• These follow a simpler naming pattern:
• R8:
  • 64-bit: R8
  • 32-bit: R8D
  • 16-bit: R8W
  • 8-bit: R8B
• R9:
  • 64-bit: R9
  • 32-bit: R9D
  • 16-bit: R9W
  • 8-bit: R9B
• R10, R11, R12, R13, R14, R15: Same pattern - just replace the number
  • 64-bit: R10, R11, R12, R13, R14, R15
  • 32-bit: R10D, R11D, R12D, R13D, R14D, R15D
  • 16-bit: R10W, R11W, R12W, R13W, R14W, R15W
  • 8-bit: R10B, R11B, R12B, R13B, R14B, R15B
    Note: R8-R15 don't have the "high byte" variants (no R8H, R9H, etc.)

Special Purpose

• RIP:
  • Instruction Pointer
  • Has address of next instruction to execute
• RFLAGS
  • Flags register
  • Status flags (zero, carry, overflow, etc.)
  • FLAGS:
    • ZF
      • Zero Flag
      • Result is a zero
    • CF
      • Carry Flag
      • Unsigned overflow/borrow
    • SF
      • Sign Flag
      • Result is negative
    • OF
      • Overflow Flag
      • Signed overflow
    • PF
      • Parity Flag
      • Even number of 1 bits in result