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9.16.4 i386-Mnemonics Instruction Naming

Instruction mnemonics are suffixed with one character modifiers which specify the size of operands. The letters b, w, l and q specify byte, word, long and quadruple word operands. If no suffix is specified by an instruction then as tries to fill in the missing suffix based on the destination register operand (the last one by convention). Thus, mov %ax, %bx is equivalent to movw %ax, %bx; also, mov $1, %bx is equivalent to movw $1, bx. Note that this is incompatible with the AT&T Unix assembler which assumes that a missing mnemonic suffix implies long operand size. (This incompatibility does not affect compiler output since compilers always explicitly specify the mnemonic suffix.)

When there is no sizing suffix and no (suitable) register operands to deduce the size of memory operands, with a few exceptions and where long operand size is possible in the first place, operand size will default to long in 32- and 64-bit modes. Similarly it will default to short in 16-bit mode. Noteworthy exceptions are

Different encoding options can be specified via pseudo prefixes:

Mnemonics of Intel VNNI/IFMA instructions are encoded with the EVEX prefix by default. The pseudo {vex} prefix can be used to encode mnemonics of Intel VNNI/IFMA instructions with the VEX prefix.

The Intel-syntax conversion instructions

are called cbtw, cwtl, cwtd, cltd, cltq, and cqto in AT&T naming. as accepts either naming for these instructions.

The Intel-syntax extension instructions

are called movsbw/movsxb/movsx, movsbl/movsxb/movsx, movsbq/movsxb/movsx, movswl/movsxw, movswq/movsxw, movslq/movsxl, movzbw/movzxb/movzx, movzbl/movzxb/movzx, movzbq/movzxb/movzx, movzwl/movzxw and movzwq/movzxw in AT&T syntax.

Far call/jump instructions are lcall and ljmp in AT&T syntax, but are call far and jump far in Intel convention. AT&T Mnemonic versus Intel Mnemonic

as supports assembly using Intel mnemonic. .intel_mnemonic selects Intel mnemonic with Intel syntax, and .att_mnemonic switches back to the usual AT&T mnemonic with AT&T syntax for compatibility with the output of gcc. Several x87 instructions, fadd, fdiv, fdivp, fdivr, fdivrp, fmul, fsub, fsubp, fsubr and fsubrp, are implemented in AT&T System V/386 assembler with different mnemonics from those in Intel IA32 specification. gcc generates those instructions with AT&T mnemonic.

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