X86-64 Instruction Encoding: Difference between revisions
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|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]]. |
|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]]. |
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Because the first four bits always equal 4, the existence of the REX prefix wipes out opcodes 0x40-0x4F, which were previously individual increment and decrement instructions for all eight registers. The Intel 64 and IA-32 Architectures |
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Software Developer’s Manual volume 2 states "The single-byte-opcode forms of the INC/DEC instructions are not available in 64-bit mode. INC/DEC functionality is still available using ModR/M forms of the same instructions (opcodes FF/0 and FF/1)." |
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==== Opcode ==== |
==== Opcode ==== |
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