The x86 encoding is fairly simple as there are only four kind of opcodes: * one byte with implicit operands, for example LOOP or RET or ADD AX,imm * one byte

The x86 encoding is fairly simple as there are only four kind of opcodes: * one byte with implicit operands, for example LOOP or RET or ADD AX,imm * one byte shortcut encoding with registers in bits 0-2 of the only opcode byte, these are 16-bit instruction only: INC/DEC, PUSH/POP, XCHG AX,rr (these instructions are also available as 2-byte encodings) * two byte with one register and one register/memory operand, both encoded in the second byte (bits 3-5 and 0-2/6-7, with bit 0 of the first byte determining the size and bit 1 of the first byte determining if the source is the register or the register/memory operand) * two byte with a single register/memory operand, in which case bits 3-5 of the second byte are part of the opcode Prefixes and immediate operands complicate things a bit but that's it. It's refreshingly simple compared to the 386 and later. An interesting tidbit is that registers are ordered AX/CX/DX/BX in the instruction set encoding because their function roughly matches AF/BC/DE/HL in the 8080 (BX comes last because like HL it can be used to address memory, even though the encoding of memory operands is completely different on the 8086)