Ken Shirriff

Computer history. Reverse-engineering old chips. Restored Apollo Guidance Computer, Xerox Alto. Ex-Google, Sun, Msft. So-called boffin.

Blog: https://righto.com
Bluesky: https://bsky.app/profile/righto.com
Github: https://github.com/shirriff

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npub1mgq5f2207ye4p2q55ge5qytuc4qsj6zw7cu79k735yq04c5as9wqqysjel

NIP-05 Address

kenshirriff_at_oldbytes.space@momostr.pink

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nprofile1qqsd5q2y498lzv6s4q22yv6qz97v2sgfdp80vw0zm0g6zq86u2wczhqpz3mhxue69uhhyetvv9ujuerpd46hxtnfduqs6amnwvaz7tmwdaejumr0ds2lnuuk

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Last Notes

2026-03-29 16:53:11 UTC

In 1967, IBM introduced the System/4 Pi line of aerospace computers, packing mainframe performance into a compact box. 4 Pi computers powered everything from military aircraft to the Space Shuttle to sonar systems on submarines. Thread...
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2025-08-09 16:42:20 UTC

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The silicon die of the 386 processor is mounted in the middle of the ceramic package. Tiny bond wires connect the die to two tiers of contacts that surround it. Intricate but hidden wiring inside the ceramic connects these contacts to the 132 gold-plated pins. 2/N
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2025-07-11 00:55:22 UTC

The Globus is a navigational instrument that uses a rotating globe to show the position of the Soyuz spacecraft above the Earth. Inside the Globus, a complicated system of gears and motors positions the globe. Jon Bruner from @npub1jsp…ggr4 created a three-dimensional X-ray scan for us. 1/4
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/831/862/779/820/382/original/e45eb7c1d2002304.mp4

2025-06-20 02:57:39 UTC

Last week, the vintage IBM 1401 computer at the Computer History Museum started behaving strangely: it wouldn't halt. More specifically, if you had two HALT instructions in a row, it would halt for the first, but when you continued, it crashed mysteriously. Here's how we fixed it.... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/713/444/182/512/594/original/f544d480f5bac21d.png

2025-05-16 21:55:49 UTC

Tape drive repair at the Computer History Museum. I helped figure out why the tape drives wouldn't backspace. The problem turned out to be a bad germanium transistor on an inverter card. 1/5
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/519/735/109/459/784/original/a9d30dd925d284c4.mp4

2025-05-10 16:46:32 UTC

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The Kindle 11 uses an MT8113 processor. This is a 1 GHz dual-core Cortex A-7, which is 32-bit like the 386. The 386 ran at up to 40 MHz. That would put your Kindle at roughly 50 times as fast. (Apples and oranges blah blah blah.)

2025-05-10 16:28:35 UTC

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This close-up of the 386 processor's die shows the different parts of the prefetch unit. There's a lot going on just to hold 16 bytes of instructions.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/484/477/253/419/159/original/ea7fe8c5ea38a887.png

2025-05-10 16:24:11 UTC

Anyone use Intel's 386 processor from 1985, the first 32-bit x86? To improve performance, it preloaded instructions into a 16-byte prefetch queue (highlighted in red). Why does such a small queue take up so much of the chip? Let's look under the microscope at its complicated circuitry... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/484/459/842/162/685/original/1e8828f331603f65.png

2025-05-01 17:42:53 UTC

Intel's 386 processor (1985) moved the x86 architecture to 32 bits, but it needed to be backward compatible with earlier 16 and 8-bit processors. As a result, it needed complicated circuitry for its internal registers: six different circuits for 30 registers. Let's look at the silicon circuits. 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/433/808/533/161/186/original/96d10f995fda6bdc.png

2025-04-13 16:19:29 UTC

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Now the failures were very strange: the computer booted up and I could type in a simple "hello world" program. But when I ran it, the program printed random floating point numbers and then crashed. There was also a mysterious 2 by 2 checkerboard character near the middle of the screen.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/331/558/943/447/469/original/a352f0ce5992d915.png

2025-04-13 16:16:17 UTC

The Commodore PET, along with the Apple II and the Radio Shack TRS-80, formed the "1977 Trinity" that made home computers popular. The PET was very quirky, combining a tiny keyboard, a cassette drive for storage, and a monitor in a trapezoidal case. I got one, but it didn't work. Let's fix it... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/331/547/469/609/433/original/b049b0f426942a31.png

2025-03-31 17:42:53 UTC

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Zooming way in shows individual bits inside the ROM. I removed the metal layers so you can see the underlying silicon. Each bump is a transistor; the pattern of transistors stores the 0's and 1's in the ROM. 3/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/258/277/723/204/435/original/e1b60deef7602b59.png

2025-03-31 17:42:20 UTC

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This closeup of the Pentium's microcode ROM shows that it consists of two rectangular banks. Each bank outputs 45 signals forming the 90-bit micro-instruction. These 90 bits control the rest of the CPU. The detail shows six of the bit signals exiting the banks. 2/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/258/274/690/941/830/original/0c40b8ddac8f6d1a.png

2025-03-31 17:41:27 UTC

The Pentium processor, like many others, implements its instructions in microcode. Each step of an instruction is described by a micro-instruction, stored in the chip in the microcode ROM.
This die photo shows the parts of the Pentium. Let's take a quick look at the microcode ROM...1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/258/272/215/801/588/original/f4ab1b4d01f20944.png

2025-03-23 16:23:38 UTC

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Here's the vintage keyset with my USB interface. I used an Arduino-like microcontroller (the Teensy) to make the keyset act like a standard USB keyboard. To get upper case and special characters, you click mouse buttons at the same time. Too much coordination for me!
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2025-03-02 18:29:59 UTC

Intel's Pentium processor (1993). Looking inside the chip , I found a large, complicated circuit just to multiply by 3 (lower right). Why? The Pentium uses a fast technique to multiply 64-bit numbers and it turns out that 3 is a special case. Let's take a closer look at multiplication... 1/N
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2025-02-01 18:56:41 UTC

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IBM's construction of computers from frames was a major innovation: the frames fit through doors and were transportable. Earlier computers were often constructed in place and difficult to move. Photo shows an IBM 701 at GE with frames for power and drum storage, and main frame in the center.
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2025-02-01 18:56:05 UTC

What is the origin of the word "mainframe"? Digging through archives, I traced it back to 1953. The IBM 701 computer was built from "frames": power frames, a storage frame, a drum frame, and the main frame. This 1953 drawing from the Installation Manual shows the dimensions of the "main frame". 1/n
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2024-12-28 19:01:57 UTC

In 1994, a math professor discovered that Intel's Pentium chip sometimes gave the wrong answer when dividing. Fixing this "FDIV" bug cost Intel $475 million. I analyzed the Pentium chip and found the bug. 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/731/993/398/850/101/original/0412813fdab454f4.png

2024-12-06 16:52:25 UTC

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The Pentium uses a division algorithm called SRT. It generates two bits at a time, making division twice as fast. SRT's secret is quotient digits can be negative: -2, -1, 0, 1, 2. A 2048-entry table gives the digit for a particular divisor and remainder. Unfortunately, 5 entries (red) were wrong. 2/9
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2024-12-06 16:48:26 UTC

Intel launched the Pentium processor in 1993. Unfortunately, dividing sometimes gave a slightly wrong answer, the famous FDIV bug. Replacing the faulty chips cost Intel $475 million. I reverse-engineered the circuitry and can explain the bug. 1/9
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/606/886/094/437/860/original/b48256b20c6778ef.jpeg

2024-11-23 20:53:37 UTC

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For more on antenna diodes in the Pentium, see my latest blog post:
https://www.righto.com/2024/11/antenna-diodes-in-pentium-processor.html

2024-11-23 20:53:18 UTC

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Here's a photo of the Pentium die with main functional blocks labeled. In this photo, the three layers of metal wiring almost completely hide the underlying silicon. 5/6
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/534/242/395/948/379/original/cca8c22a0a824929.jpeg

2024-11-23 20:44:34 UTC

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This closeup photo shows the three layers of wiring on the Pentium and the transistors underneath. The L-shaped hook towards the lower left is a connection to an antenna diode. 4/6
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/534/204/297/945/529/original/b710c3765f2a2768.jpeg

2024-11-23 20:40:35 UTC

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The solution is to add extra diodes to the circuitry, special devices that will drain away the dangerous charge.
These diodes are relatively rare, only needed when long wires on the chip are connected to a transistor gate on one end and the other end isn't connected yet. The long wire acts (metaphorically) as an antenna, so the protective diode is called an antenna diode. 3/6
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/534/185/150/732/411/original/fdec8a5f087a0bb6.jpg

2024-11-23 20:34:28 UTC

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The problem is that the chip manufacturing process uses plasma to etch away unwanted metal to produce wiring. However, long wires on the chip can pick up electrical charge from the plasma. This charge can damage the chip's transistors. In particular, each transistor has a thin layer of "gate oxide", just a few hundred atoms thick, that can easily be damaged. 2/6
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/534/165/523/356/108/original/e5e2049096b5e6b4.jpg

2024-11-23 20:30:28 UTC

I was studying the Pentium processor's die and I noticed strange connections that I didn't understand. These turned out to be "antenna diodes", special diodes that protect the chip from damage during manufacturing. Let's take a look... 1/6
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/113/534/096/290/461/228/original/1be8a8414350491e.jpeg