INSTRUCTION SET REFERENCE, A-L
3-6 Vol. 2A
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r/m16 — A word general-purpose register or memory operand used for instructions whose operand-size
attribute is 16 bits. The word general-purpose registers are: AX, CX, DX, BX, SP, BP, SI, DI. The contents of
memory are found at the address provided by the effective address computation. Word registers R8W - R15W
are available using REX.R in 64-bit mode.
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r/m32 — A doubleword general-purpose register or memory operand used for instructions whose operand-
size attribute is 32 bits. The doubleword general-purpose registers are: EAX, ECX, EDX, EBX, ESP, EBP, ESI,
EDI. The contents of memory are found at the address provided by the effective address computation.
Doubleword registers R8D - R15D are available when using REX.R in 64-bit mode.
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r/m64 — A quadword general-purpose register or memory operand used for instructions whose operand-size
attribute is 64 bits when using REX.W. Quadword general-purpose registers are: RAX, RBX, RCX, RDX, RDI,
RSI, RBP, RSP, R8–R15; these are available only in 64-bit mode. The contents of memory are found at the
address provided by the effective address computation.
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m — A 16-, 32- or 64-bit operand in memory.
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m8 — A byte operand in memory, usually expressed as a variable or array name, but pointed to by the
DS:(E)SI or ES:(E)DI registers. In 64-bit mode, it is pointed to by the RSI or RDI registers.
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m16 — A word operand in memory, usually expressed as a variable or array name, but pointed to by the
DS:(E)SI or ES:(E)DI registers. This nomenclature is used only with the string instructions.
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m32 — A doubleword operand in memory, usually expressed as a variable or array name, but pointed to by the
DS:(E)SI or ES:(E)DI registers. This nomenclature is used only with the string instructions.
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m64 — A memory quadword operand in memory.
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m128 — A memory double quadword operand in memory.
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m16:16, m16:32 & m16:64 — A memory operand containing a far pointer composed of two numbers. The
number to the left of the colon corresponds to the pointer's segment selector. The number to the right
corresponds to its offset.
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m16&32, m16&16, m32&32, m16&64 — A memory operand consisting of data item pairs whose sizes are
indicated on the left and the right side of the ampersand. All memory addressing modes are allowed. The
m16&16 and m32&32 operands are used by the BOUND instruction to provide an operand containing an upper
and lower bounds for array indices. The m16&32 operand is used by LIDT and LGDT to provide a word with
which to load the limit field, and a doubleword with which to load the base field of the corresponding GDTR and
IDTR registers. The m16&64 operand is used by LIDT and LGDT in 64-bit mode to provide a word with which to
load the limit field, and a quadword with which to load the base field of the corresponding GDTR and IDTR
registers.
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moffs8, moffs16, moffs32, moffs64 — A simple memory variable (memory offset) of type byte, word, or
doubleword used by some variants of the MOV instruction. The actual address is given by a simple offset
relative to the segment base. No ModR/M byte is used in the instruction. The number shown with moffs
indicates its size, which is determined by the address-size attribute of the instruction.
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Sreg — A segment register. The segment register bit assignments are ES = 0, CS = 1, SS = 2, DS = 3, FS = 4,
and GS = 5.
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m32fp, m64fp, m80fp — A single-precision, double-precision, and double extended-precision (respectively)
floating-point operand in memory. These symbols designate floating-point values that are used as operands for
x87 FPU floating-point instructions.
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m16int, m32int, m64int — A word, doubleword, and quadword integer (respectively) operand in memory.
These symbols designate integers that are used as operands for x87 FPU integer instructions.
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ST or ST(0) — The top element of the FPU register stack.
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ST(i) — The i
th
element from the top of the FPU register stack (i ← 0 through 7).
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mm — An MMX register. The 64-bit MMX registers are: MM0 through MM7.
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mm/m32 — The low order 32 bits of an MMX register or a 32-bit memory operand. The 64-bit MMX registers
are: MM0 through MM7. The contents of memory are found at the address provided by the effective address
computation.
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mm/m64 — An MMX register or a 64-bit memory operand. The 64-bit MMX registers are: MM0 through MM7.
The contents of memory are found at the address provided by the effective address computation.