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The x86 CPU supports two basic opcode sizes:
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standard one-byte opcode
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two-byte opcode consisting of a 0Fh opcode expansion prefix byte.
The second byte then specifies the actual instruction.
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x86 instruction format:
| <<< x86 Instruction Format Reference | Index | x86 ADD Instruction Opcode >>> |
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The x86 opcode bytes are 8-bit equivalents of iii field that we discussed in simplified encoding.
This provides for up to 512 different instruction classes, although the x86 does not yet use them all.
| <<< x86 Instruction Format Reference | Index | x86 ADD Instruction Opcode >>> |