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AMD64 CPU registers

Main article: X86-64 § Architectural features

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)