Gregory's ServerThe fingernail-sized Pentium chip under a microscope. Some parts, like the cache and datapath were highly optimized by hand. Less critical areas, outlined in red, were designed by software using standard cells. The rows of cells are visible as gray columns. 2/21
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 Here is a closeup of seven standard-cell blocks in the Pentium. The gray blobs are regions of silicon, with shiny polysilicon wiring over top to make transistors. There are about 30 transistors here; unlike modern processors, you can still see the transistors in the Pentium. 4/21 Why the name "Pentium"? Intel sued AMD, claiming that the Am386 chip violated Intel's patent. AMD won when a judge decided that the number 386 couldn't be trademarked. Intel pivoted, abandoning the x86 numbering scheme for the trademarkable "Pentium" name. 5/21 The Pentium used four layers of metal on top of the silicon to connect everything together. In this closeup, you can see the metal layers, from M4 on top to M1 on the bottom, on top of the polysilicon and silicon. It's a 3-d structure, kind of a confusing jumble. 6/21 So what's inside a standard cell? The simplest circuit is an inverter, turning a 0 into a 1 and vice versa. It uses two transistors. A "0" in turns on the PMOS transistor at the top, making a high output. A 1 turns on the NMOS transistor at the bottom, giving a low output. 7/21 The standard cell for an inverter has these two transistors, shown on the right. The M1 metal layer (left) connects the transistors to create the inverter. The vias connect the metal layer to metal wires on top, which is how the cells are connected together. 8/21 NAND gates are very important in computer circuitry since you can build anything from NAND gates. Here's the standard cell for a NAND gate with four transistors (right) connected by metal wiring (left). The Pentium used CMOS logic, built from NMOS and PMOS transistors. 9/21 This photo shows three NAND gate standard cells, snapped together. Each cell is mostly identical, but the metal wiring is modified based on how the gate connects to the other metal layers. The thick metal bands at the top and bottom are the power and ground lines. 10/21 CMOS circuitry lets you create complicated gates, such as this five-input OR-NAND gate, with inputs A-E. Standard cell libraries usually include a wide variety of gates, not just common ones like AND and OR. 11/21 To store a bit of data in a circuit, the Pentium uses latches or flip-flops. Controlled by the clock, the latch remembers one value. (This is why your processor's clock speed is important.) The latch uses a multiplexer built from pass transistors, too hard to explain here. 12/21 The flip-flop is a very important circuit, a more controllable latch. Here's one type of standard-cell flip-flop in the Pentium. With standard cells, the designer doesn't need to worry about this complex wiring; the circuit is pre-designed in the library. 13/21 The software for standard cells is very complex. "Automated place and route" software first places the cells in rows to minimize the distances between circuits. Then the wiring is routed between the cells as densely as possible. Both are NP-complete, so heuristics are used. 14/21 The Pentium was a BiCMOS chip, using CMOS like current processors but also fast Bipolar transistors which made some signals 35% faster. BiCMOS doesn't help nowadays for digital circuits, but is commonly used in analog integrated circuits, especially high-speed ones. 15/21 This cell is a BiCMOS buffer, sending a signal at high speed to other parts of the chip. The large boxy transistor in the upper left is a bipolar NPN transistor. Its construction is very different from the long, thin CMOS transistors. Bipolar is fast, but power-hungry. 16/21 Here is a BiCMOS inverter, using two NPN transistors, visible at the top. 17/21 With its high performance, the Pentium processor had a large impact on the computer market. The Pentium even made it into pop culture; you can rock out to the song "It's all about the Pentiums". Intel started using standard cells with automated place and route software for the 386; designing the chip manually would have been too slow. One of the engineers working on this was Pat Gelsinger, who is now Intel's CEO. Maybe standard cells can make you a CEO too! 19/21 It's hard to explain circuits in detail on Twitter, so if you want more details on the standard cells in the Pentium, see my blog post: My earlier Twitter thread on standard cells in the 386: https://x.com/kenshirriff/status/1753840893281403373 21/21 @kenshirriff Thank you for posting on Mastodon; I will catch this on the blog. Twitter links are effectively useless unless you are a logged-in Twitter user (try it!) so consider preservation there-of nil  @kenshirriff By the time we dusted off the P54C to use as the heart of Larrabee/Knights/XeonPhi, the whole thing had been converted through about three different design languages. Also, Larrabee was one of the first/largest test projects for automated synthesis at Intel. Some of the SIMD stuff was still done by hand, but the P54C core was all automatic synthesis and doesn't look nearly as pretty. There were lots of teething problems! But it worked in the end. @TomF Interesting! Can you say more about the different design languages? Also, do you know why the chip was called the P54C? P5 makes sense for the Pentium but P54C seems random. @kenshirriff There's also P54CS and P54CQS, but I don't know what the letters mean, sorry - way before my time. The original P54C was done in Intel's in-house design language, which was then retired in the 90s I think. Fortunately the design had been ported to more modern design languages like VHDL or Verilog in the meantime, so then we took that version, ported it yet again to the absolutely newest language that Intel was using. @kenshirriff Interesting song. And funny how Bill Gates has a cup with "think similar" on his desk.  I was selling Intel product at the time. We were told the trademark determination happened during Intel's transition from x386 to x486(x86,x286,x386,x486*) so, AMD went 486 and Intel skipped 4 and went to 5, being a pentagon... hence... the trademarkable Pentium. @avoca @kenshirriff maybe I'm misunderstanding what you said but Intel definitely *did not* skip 486 models. There was a whole line of them that preceded the Pentium. I owned and/or used several of them. They "skipped" 586 as a numbering/naming scheme and went to Pentium, but it was after years of 486 being the top of the line chip. https://en.m.wikipedia.org/wiki/I486 Apologies if I'm simply misunderstanding you.  @kenshirriff I forget which magazine it was (perhaps the UK edition of Computer Shopper), but one columnist complained it sounded like a new fragrance by like Dior. I could see Johnny Depp doing a black and white advert for Pentium.  @kenshirriff I suspect that 30-odd years later the successor to that block based assembly has noticeable similarities to LLMs assembling components with a variety of designs that do the same thing electrically. |
Standard cells use a library of standardized blocks for gates and other circuits. Software builds your circuit by snapping the blocks into rows and laying out the wiring between the circuits. Quick and error-free, but not as dense and efficient as manual circuit design. 3/21