The 8086's microcode ROM holds 512 micro-instructions, each 21 bits wide. The microcode engine is assisted by two smaller ROMs: the "Group Decode ROM" to categorize machine instructions, and the "Translation ROM" to branch to microcode subroutines for address calculation and other roles.
See: [https://www.righto.com/2022/11/how-8086-processors-microcode-engine.html How the 8086 processor's microcode engine works] [https://righto.com/8086/groupRom.html Group Decode ROM viewer] [https://www.righto.com/2023/03/8086-register-codes.html Reverse-engineering the register codes for the 8086 processor's microcode] [https://www.righto.com/2023/07/undocumented-8086-instructions.html Undocumented 8086 instructions, explained by the microcode] [https://www.reenigne.org/blog/8086-microcode-disassembled/ 8086 microcode disassembled]
== Pipeline Design ==