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(nasm.info)Section B.1

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B.1. Key to Operand Specifications
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   The instruction descriptions in this appendix specify their operands
using the following notation:

   * Registers: `reg8' denotes an 8-bit general purpose register,
     `reg16' denotes a 16-bit general purpose register, and `reg32' a
     32-bit one. `fpureg' denotes one of the eight FPU stack registers,
     `mmxreg' denotes one of the eight 64-bit MMX registers, and
     `segreg' denotes a segment register. In addition, some registers
     (such as `AL', `DX' or `ECX') may be specified explicitly.

   * Immediate operands: `imm' denotes a generic immediate operand.
     `imm8', `imm16' and `imm32' are used when the operand is intended
     to be a specific size. For some of these instructions, NASM needs
     an explicit specifier: for example, `ADD ESP,16' could be
     interpreted as either `ADD r/m32,imm32' or `ADD r/m32,imm8'. NASM
     chooses the former by default, and so you must specify `ADD
     ESP,BYTE 16' for the latter.

   * Memory references: `mem' denotes a generic memory reference;
     `mem8', `mem16', `mem32', `mem64' and `mem80' are used when the
     operand needs to be a specific size. Again, a specifier is needed
     in some cases: `DEC [address]' is ambiguous and will be rejected
     by NASM. You must specify `DEC BYTE [address]', `DEC WORD
     [address]' or `DEC DWORD [address]' instead.

   * Restricted memory references: one form of the `MOV' instruction
     allows a memory address to be specified _without_ allowing the
     normal range of register combinations and effective address
     processing. This is denoted by `memoffs8', `memoffs16' and
     `memoffs32'.

   * Register or memory choices: many instructions can accept either a
     register _or_ a memory reference as an operand. `r/m8' is a
     shorthand for `reg8/mem8'; similarly `r/m16' and `r/m32'. `r/m64'
     is MMX-related, and is a shorthand for `mmxreg/mem64'.