- The Group Decode ROM: The 8086 processor’s first step of instruction decoding (righto.com)
A key component of any processor is instruction decoding: analyzing a numeric opcode and figuring out what actions need to be taken. The Intel 8086 processor (1978) has a complex instruction set, making instruction decoding a challenge. The first step in decoding an 8086 instruction is something called the Group Decode ROM, which categorizes instructions into about 35 types that control how the instruction is decoded and executed. For instance, the Group Decode ROM determines if an instruction is executed in hardware or in microcode. It also indicates how the instruction is structured: if the instruction has a bit specifying a byte or word operation, if the instruction has a byte that specifies the addressing mode, and so forth.