(nasm.info)Section B.2.1


B.2.1. Register Values
----------------------

   Where an instruction requires a register value, it is already
implicit in the encoding of the rest of the instruction what type of
register is intended: an 8-bit general-purpose register, a segment
register, a debug register, an MMX register, or whatever. Therefore
there is no problem with registers of different types sharing an
encoding value.

   The encodings for the various classes of register are:

   * 8-bit general registers: `AL' is 0, `CL' is 1, `DL' is 2, `BL' is
     3, `AH' is 4, `CH' is 5, `DH' is 6, and `BH' is 7.

   * 16-bit general registers: `AX' is 0, `CX' is 1, `DX' is 2, `BX' is
     3, `SP' is 4, `BP' is 5, `SI' is 6, and `DI' is 7.

   * 32-bit general registers: `EAX' is 0, `ECX' is 1, `EDX' is 2,
     `EBX' is 3, `ESP' is 4, `EBP' is 5, `ESI' is 6, and `EDI' is 7.

   * Segment registers: `ES' is 0, `CS' is 1, `SS' is 2, `DS' is 3,
     `FS' is 4, and `GS' is 5.

   * Floating-point registers: `ST0' is 0, `ST1' is 1, `ST2' is 2,
     `ST3' is 3, `ST4' is 4, `ST5' is 5, `ST6' is 6, and `ST7' is 7.

   * 64-bit MMX registers: `MM0' is 0, `MM1' is 1, `MM2' is 2, `MM3' is
     3, `MM4' is 4, `MM5' is 5, `MM6' is 6, and `MM7' is 7.

   * Control registers: `CR0' is 0, `CR2' is 2, `CR3' is 3, and `CR4'
     is 4.

   * Debug registers: `DR0' is 0, `DR1' is 1, `DR2' is 2, `DR3' is 3,
     `DR6' is 6, and `DR7' is 7.

   * Test registers: `TR3' is 3, `TR4' is 4, `TR5' is 5, `TR6' is 6,
     and `TR7' is 7.

   (Note that wherever a register name contains a number, that number
is also the register value for that register.)


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