• 1993: Intel released the powerful Pentium processor, establishing a long-running brand.
• ROM in Pentium's FPU: Earlier, the author wrote about the ROM in the Pentium's floating point unit that holds constants like π.
• Microscopic Details: A photo shows the Pentium's silicon die under a microscope with the floating point unit and constant ROM labeled. The die has multiple layers including transistors fabricated on the silicon die, polysilicon wiring on top, and three layers of metal wiring (M1, M2, M3).
• ROM's Output Circuits: The circuitry for the ROM's output is a 86-times-repeated circuit. It includes an 8-to-1 multiplexer to select one bit out of eight words from the ROM, a latch to remember the read value, and a BiCMOS driver for long-distance signal transmission. The ROM is a grid of transistors representing data, with a precharge transistor to pull lines high before reading.
• Transistors and Inverters: CMOS circuitry uses PMOS and NMOS transistors. The detailed schematic shows various components like the ROM grid, multiplexer, keeper (implemented with an inverter and a PMOS transistor), latch (implemented with a multiplexer and inverters), and the BiCMOS driver (with an NPN transistor and three inverters).
• Conclusions: The functional blocks in the circuit are arranged with a constant width for each bit. The circuit to select a word from the ROM is about as complex as the entire MOS 6502 processor, illustrating Moore's law. The author plans to write more about the Pentium and readers can follow for updates.
• Notes: The 8-to-1 multiplexer and the latch's multiplexer use different switch implementations based on the characteristics of NMOS and PMOS transistors. Early NMOS processors used only NMOS for multiplexers.