X86-64 Instruction Encoding: Difference between revisions

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| b0100||4 bits||Fixed bit pattern

| W||1 bit||When 1, a 64-bit operand size is used. Otherwise, when 0, the default operand size is used (which is 32-bit for most but not all instructions, see [[#Operand-size and address-size override prefix|this table]]).

| R||1 bit||This 1-bit value is an extension to the ''MODRM.reg'' field. See [[#Registers|Registers]].

| X||1 bit||This 1-bit value is an extension to the ''SIB.index'' field. See [[#64-bit addressing|64-bit addressing]].

| B||1 bit||This 1-bit value is an extension to the ''MODRM.rm'' field or the ''SIB.base'' field. See [[#64-bit addressing|64-bit addressing]].