This post delves into the intricate world of microcode and the microcode ROM in the original Pentium processor. Contrary to common belief, machine instructions are executed through simpler micro-instructions specified by microcode. The microcode ROM holds 4608 micro-instructions, denser than the 8086’s ROM. The Pentium’s ROM boasts unique layouts of transistors to store the micro-instructions efficiently. The article also touches upon the functionality of the Microcode Address Register, row-select drivers, and output circuitry. The processor’s BiCMOS architecture and testing mechanisms, including shift registers and XOR gates, illuminate the complex testing processes involved in ensuring the integrity of the ROM’s contents.
https://www.righto.com/2025/03/pentium-microcde-rom-circuitry.html