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AMD64 CPU registers

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The AMD64 CPU extends the Intel Pentium architecture by expanding most of the registers from 32 bits wide to 64 bits.

Intel X86-64 registers
63 ... 47 ... 31 ... 15 ... 07 ... 00 (bit position)
Main registers (8/16/32/64 bits)
    AX
  RAX   EAX AH AL   A register
    BX
  RBX   EBX BH BL   B register
    CX
  RCX   ECX CH CL   C register
    DX
  RDX   EDX DH DL   D register
Index registers (16/32/64 bits)
  RSI   ESI SI   Source Index
  RDI   EDI DI   Destination Index
  RBP   EBP BP   Base Pointer
  RSP   ESP SP   Stack Pointer
Instruction pointer (16/32/48 bits)
    RIP   EIP IP   Istruction Pointer


Another diagram illustrating the widened registers:

Intel 80386 registers
63 ... 31 ... 15 ... 07 ... 00 (bit position)
RAX A register (64 bits)
  EAX  (32 bits)
    AX  (16 bits)
    AH AL  (8 bits)