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

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kenshirriff_at_oldbytes.space@momostr.pink

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nprofile1qqsd5q2y498lzv6s4q22yv6qz97v2sgfdp80vw0zm0g6zq86u2wczhqpz3mhxue69uhhyetvv9ujuerpd46hxtnfduqs6amnwvaz7tmwdaejumr0ds2lnuuk

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Last Notes

2026-09-12 16:30:34 UTC

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The 8087's microcode is too complicated and low-level to explain here. For details, see my latest article:
https://www.righto.com/2026/09/8087-microcode-reverse-engineering-fscale.html

2026-09-12 16:30:02 UTC

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I can now explain the microcode for an instruction called FSCALE that scales a number by a power of 2. It sounds simple, but the 8087 has an appalling number of special cases: positive and negative 0, two infinities, four rounding modes, three number sizes, six exceptions... so the code is complex.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/259/015/692/397/267/original/55969504f8dd2b1c.png

2026-09-12 16:28:40 UTC

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Zooming in on the microcode storage ROM, you can see the individual transistors, the pinkish blobs. The 8087 is very unusual, using four transistor sizes, giving it double density: 2 bits per transistor. The Opcode Collective examined all 13184 transistors, revealing the 0s and 1s of the microcode.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/259/010/801/958/715/original/834941bfb9a429eb.png

2026-09-12 16:27:01 UTC

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Under a microscope, you can see the circuitry of the 8087. The 8087 performs its calculations using low-level software called microcode, stored in the large ROM in the middle. Along with the "Opcode Collective", I'm trying to understand the algorithms in the microcode, from addition to arctangents.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/259/003/805/423/643/original/7a94ff23369f9bfa.jpg

2026-09-12 16:25:43 UTC

In 1980, Intel introduced the 8087 floating-point coprocessor chip. Plugging this chip into your IBM PC made math 100 times faster, good for CAD and spreadsheets. I opened up the chip to reverse-engineer exactly how it does complicated calculations. Let's take a look inside...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/258/998/658/375/022/original/b41ca3521f9fc1c7.png

2026-08-30 17:43:56 UTC

Before semiconductor RAM, most computers used magnetic core memory. This is the 128 KB core stack from the computer on Spacelab, the Space Shuttle's experimental laboratory. Let's take a closer look...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/185/696/405/906/640/original/b2ee8b4232a3449d.png

2026-07-31 16:40:36 UTC

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The IBM 604 calculator used about 1300 tubes and rented for $550 a month. It was a very popular system with over 5600 units produced. IBM advertised it as equivalent to 150 engineers.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/576/500/307/189/original/747a970b66c561f1.png

2026-07-31 16:39:34 UTC

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I've powered up the trigger module in this video. It is switched on or off by two inputs and remembers its state until switched again. I push two buttons to turn it on or off, and its state is displayed by two vintage neon bulbs. This may seem trivial, but the trigger (now called a flip-flop) is a key component of computers.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/571/863/271/726/original/07a1af6ed453ee13.mp4

2026-07-31 16:38:27 UTC

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The tube module uses a "dual triode" tube, two amplifiers in one glass tube. In the first photo, the two vertical dark rectangles are the two plates. The second photo shows the tube powered up, with the heaters glowing orange at the top and bottom.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/566/706/359/580/original/4ead1939bfa3c5c5.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/568/432/919/285/original/1d5074f0e5ce82bf.png

2026-07-31 16:37:19 UTC

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Before transistors, vacuum tubes were widely used as switches and amplifiers. A heater heats up the cathode, which emits electrons. They are attracted to the plate, causing current to flow. A negative voltage on the grid blocks the electrons, controlling the current flow.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/564/788/749/890/original/2d7b1955194a9a0b.png

2026-07-31 16:36:42 UTC

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This module was part of IBM's 604 Electronic Calculating Punch. It read 100 punch cards a minute, performed calculations that were specified by a wiring panel, and punched the results on cards. In the lower left, you can see rows of tube modules with their handles sticking out.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/562/323/770/681/original/a408f85557acabac.png

2026-07-31 16:36:03 UTC

This vacuum tube module was used in an IBM programmable calculator from 1948. This module, called a "trigger", stored one bit of information. Let's take a closer look and see the module operate.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/117/015/559/214/037/952/original/b0cfbaf5ef80dc65.png

2026-06-28 16:30:48 UTC

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Credits: IOP boards provided by Richard, 4 Pi board from @npub1xvg…62t2.
Photo of the IOP and CPU courtesy of RR Auction. IOP documentation from Mike Stewart. Exploded diagram from https://ibiblio.org/apollo/Shuttle/IBM-74-A31-016%20-%20Space%20Shuttle%20Advanced%20System,%204%20Pi%20-%20Prototype%20Input,%20Output%20Processor%20(IOP).pdf#page=145

2026-06-28 16:29:58 UTC

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Want more photos? Or more info, such as how the I/O processor implements 25 virtual processors with two instruction sets on top of a single physical processor? See my latest article: https://www.righto.com/2026/06/space-shuttle-io-processor-boards.html

2026-06-28 16:29:31 UTC

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This diagram shows how the circuit boards (called pages) were plugged into the I/O Processor's aluminum-alloy case. For storage, the system used magnetic core memory, larger pages at the back. This system didn't use a microprocessor; it was built from 11 logic pages crammed with simple chips.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/828/677/897/496/599/original/bdb367657aa33065.png

2026-06-28 16:29:01 UTC

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IBM had a series of aerospace computers called System/4 Pi. These computers all used standard-sized boards (top). Except that the I/O Processor's boards (bottom) were one inch wider. I guess they needed the extra space for more circuitry.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/828/673/973/376/821/original/985dec12c33bec0c.png

2026-06-28 16:27:42 UTC

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The second board is memory (PROM), holding microcode for the I/O Processor, which is a (very strange) computer, independent of the CPU. Each gold-lidded chip holds 2K bits in tiny metal fuses. The chip is programmed by blowing the fuse for each 1 bit, literally burning the PROM.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/828/669/255/124/008/original/f5b63626e97c53c9.png

2026-06-28 16:25:31 UTC

The Space Shuttle had five general-purpose computers that controlled, monitored, and navigated the Shuttle. Each computer consisted of two boxes: the CPU (right) and the I/O Processor (IOP, left). The IOP connected the computer to 24 high-speed networks. Let's look at two boards from the IOP... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/828/658/722/675/690/original/bdfb7e5580604b74.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/828/660/601/395/337/original/ad557b5e6d305fc9.png

2026-06-13 16:21:58 UTC

Intel introduced the 8087 floating-point chip in 1980 to make math faster. The chip does tangents, square roots, and so forth, but fundamentally everything is computed through additions and subtractions, performed by an adder at the heart of the chip. Let's look at the adder's circuitry... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/743/713/202/353/709/original/a352daf872448c4b.jpg

2026-06-07 19:02:50 UTC

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Yes, the thyratron is very similar to an SCR (also known as a thyristor: thyratron transistor).

2026-06-07 17:29:42 UTC

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For more on the IBM 604 and this thyratron tube module, see my latest article: https://www.righto.com/2026/06/ibm-604-thyraton-tube-module.html

2026-06-07 17:29:23 UTC

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Pluggable tube modules were an important innovation. They made it easy to repair tube-based systems, they packaged components densely, and they paved the way for IBM's tube computers in the 1950s. IBM featured the modules in this 1952 ad from Time magazine: "Fingers you can count on". 4/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/710/002/138/525/318/original/ac23113bcb5f6077.png

2026-06-07 17:28:12 UTC

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The tube is not exactly a vacuum tube since it has a bit of xenon gas inside. It is called a thyratron: when it turns on, the xenon ionizes, allowing a large current to flow through the tube. In the IBM 604, thyratron tubes provided high-current pulses to the electromagnets to punch holes in cards.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/709/995/768/003/934/original/c8d064af13fc1d4e.png

2026-06-07 17:26:34 UTC

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The tube module acts as a high-current switch: I hooked it up to control a light bulb. The first button turns the module on with a small signal. The second button cuts the power to turn the module off. 2/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/709/988/997/608/709/original/0e140e699149097a.mp4

2026-06-07 17:24:35 UTC

The IBM 604 Electronic Calculating Punch was introduced in 1948. Not quite a computer, it was a programmable calculator that became very popular due to its low price of $550/month, and over 5600 were produced. I have a vacuum tube module from this system, so I powered it up... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/709/983/493/765/921/original/e4c470945bdfb3ad.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/709/984/028/780/884/original/381d7e497e49bec4.png

2026-06-02 19:00:20 UTC

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I think the 8087 replaced empty registers with NaN for the FXCH exchange instruction for consistency. As you say, the empty value could have been swapped. But then the instruction is a special case compared to every other instruction that converted an empty value to NaN.

2026-05-30 17:48:19 UTC

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I did this work with the Opcode Collective, especially Smartest Blob and Gloriouscow.
The full ROM image: https://8087.martypc.net/#dzi=tiles%2Ftiles.dzi
A description of the 8087 microcode: https://github.com/a-mcego/granite/blob/main/tools/8087mc/bin/8087.md

2026-05-30 17:47:35 UTC

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We are reverse-engineering the meaning of the 8087's microcode. For more details, see my latest article:
https://www.righto.com/2026/05/microcode-inside-intel-8087-floating.html

2026-05-30 17:47:12 UTC

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A close-up of the microcode ROM shows the individual bits. Unlike most ROMs, the 8087 uses four transistor sizes, letting it store two bits per transistor. This complex, semi-analog approach was necessary to fit all the data into the chip; the 8087 pushed the limits of 1980 technology. 3/n
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/664/776/128/210/361/original/1e680c65c0f94805.png

2026-05-30 17:46:40 UTC

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A microscope reveals the complex circuitry of the 8087 chip. The microcode ROM in the center holds the micro-instructions. It executes these micro-instructions to add, subtract, compute square roots, arctangents, and so forth. 2/n
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/664/772/103/548/879/original/247ac681f90b6cb6.jpg

2026-05-30 17:44:23 UTC

In the 1980s, if you wanted your IBM PC to be faster for spreadsheets or CAD, you could buy the Intel 8087 floating-point chip. Internally, the chip runs special code called microcode. We opened the chip, extracted the microcode, and are figuring out how it works. 1/n
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/664/764/125/789/529/original/dbdbf19b55d610a4.png

2026-05-24 00:45:41 UTC

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Well, I'll use Armour Etch and Whink, which are like the consumer-friendly versions of HF. I've found that N-doping vs P-doping are visible without even trying, so maybe implants could also be visible. Of course, the implant concentration might be too low to see. But I wouldn't completely rule out reading the 286 ROM without giving it a try.

2026-05-24 00:23:38 UTC

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Has anyone tried etching the 286 to see if the implant ROM can be made visible without too much trouble? I could give it a try once I get the 8087 analysis finished :-)

2026-05-24 00:00:48 UTC

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Yes, the chips are TTL, specifically 5400-series. But that's hard to see because the part numbers are mostly JM38510 military numbers. And 7714 is a date code (1977 week 14), not a part number.

2026-05-23 23:54:03 UTC

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This wire looks like it has always been yellow.

2026-05-23 17:30:17 UTC

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Those documents should be very interesting. Do the documents describe programming the Spacelab computers in assembly, HAL/S, or something else?

2026-05-23 16:55:08 UTC

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Credits: thanks to Steve Jurvetson for providing the Spacelab computer. Thanks to Kyle Owen for the photo of the IBM AP-101SL computer.

2026-05-23 16:54:05 UTC

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For more on the Spacelab computer, see my latest article: https://www.righto.com/2026/05/reverse-engineering-spacelab-computer.html

2026-05-23 16:53:26 UTC

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Around 1991, the Spacelabe computers were upgraded, replacing the French Mitra 125 MS computers with more powerful IBM-made AP-101SL computers. The new computers still used simple ICs, but the "flat-pack" ICs were packed more densely. They also used semiconductor memory instead of magnetic core.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/624/925/956/285/669/original/6832fc08bb99f8a8.png

2026-05-23 16:52:04 UTC

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This circuit board had a few bugs, which were fixed on the back with yellow "bodge" wires.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/624/922/991/217/626/original/4c0fb14cccedea95.png

2026-05-23 16:51:10 UTC

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The boards need a lot of chips because a chip didn't do much back then. Even the ALU chips had just 170 transistors. Multiplexers (mux) select which inputs to add, registers hold temporary values, and logic gates (NAND, inverters) tie things together.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/624/919/229/764/269/original/92aeb7ca8945dd07.png

2026-05-23 16:50:20 UTC

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The Arithmetic/Logic Unit (ALU) is the heart of a computer, performing arithmetic. The ALU is a tiny part of a modern processor, but it occupied three circuit boards in the Spacelab computer. The larger chips are '181 ALU chips, each adding four bits; 8 chips let you add 32 bits.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/624/916/290/654/904/original/e8128ba960c8ccbc.png

2026-05-23 16:49:16 UTC

The Space Shuttle could hold a flying laboratory called Spacelab in its cargo bay. Three French-made computers ran Spacelab. I opened up a Spacelab computer and found that instead of a microprocessor, it is built from a multitude of simple chips. Let's take a closer look at the computer...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/624/912/700/890/522/original/214214663330bbfb.png

2026-04-19 04:43:56 UTC

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This vintage Navy film explains how differential gears work: https://www.youtube.com/watch?v=gwf5mAlI7Ug

2026-04-18 21:04:08 UTC

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The SR-71 used a different system, the NAS-14V2 astroinertial navigation system. Nortronics built the SR-71 system and Kollsman built the B-52 system. The B-52 system is said to have also been used in the Hound Dog cruise missile. A very detailed document on the SR-71 star tracker is here: https://audiopub.co.kr/wp-content/uploads/2021/10/NAS-14V2-ANS-System.pdf

2026-04-18 18:51:50 UTC

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They say that the accuracy was within 0.1 degree.

2026-04-18 18:50:17 UTC

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Although this unit looks like a gyroscope, it is not at all a gyroscope. There is nothing spinning.

2026-04-18 16:56:48 UTC

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For more on the electromechanical Angle Computer, see my article: https://www.righto.com/2026/04/B-52-star-tracker-angle-computer.html

2026-04-18 16:56:12 UTC

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This close-up of the Angle Computer shows the complexity of the gears inside. The thicker brass-colored cylinders are differential gears that add two signals using rotations.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/426/751/314/490/067/original/d77aab2736354f53.png

2026-04-18 16:53:41 UTC

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The back of the Angle Computer has devices called synchros that read out the star's position relative to the aircraft. At the bottom, motors drive the mechanism.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/426/742/261/129/496/original/2747151a1f50fc9f.png

2026-04-18 16:51:21 UTC

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Inside the Angle Computer, a star pointer corresponds to the position of the star on the celestial sphere. Gears move the pointer to match the star's coordinates (called the declination and Local Hour Angle).
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/426/726/425/451/167/original/4485ea1300549ada.png

2026-04-18 16:46:37 UTC

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The "Astro Tracker" was the heart of the celestial navigation system. The 4-inch clear bubble stuck out from the top of the aircraft to view the stars. Inside, a telescope and photomultiplier tube tracked a star, steered by motors and prisms.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/426/708/404/473/733/original/e91c4920c6f9ebea.png

2026-04-18 16:42:04 UTC

In the 1960s, the B-52 bomber could navigate by the stars. Celestial navigation requires spherical trigonometry, so an analog Angle Computer solved these equations electromechanically. Let's look inside...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/426/689/277/443/748/original/ba8e31cf1e24b027.png

2026-03-29 22:22:12 UTC

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Fair enough :-)

2026-03-29 17:30:27 UTC

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I forgot to explain the 4 Pi name. IBM's System/360 mainframes got their name because they supported a full 360º circle of applications, from business to scientific. The 4 Pi aerospace computers, however, were designed for the 3-D world, a "sphere" of applications. While a circle has 360º, a sphere has 4π steradians of solid angle.

2026-03-29 17:11:56 UTC

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There were many more models of System/4 Pi computers, including refrigerator-sized computers used on submarines; see my article for details: https://www.righto.com/2026/03/ibm-4-pi-computer-history.html

2026-03-29 17:10:28 UTC

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In 1991, the Space Shuttle upgraded its computer to the AP-101S, replacing two 60-pound boxes with one box. Since there were five computers on the Shuttle, this change freed up 300 pounds for additional cargo. The CPU was implemented in three boards full of chips, one below. These boards were much larger than the earlier 4 Pi boards.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/560/937/224/462/original/52c1c1399755da8c.png

2026-03-29 17:05:00 UTC

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The Space Shuttle used a more advanced System/4 Pi computer called the AP-101B. A separate computer, the I/O Processor, provided 24 data buses to all parts of the Shuttle. These boxes still used individual TTL chips and magnetic core memory.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/536/095/110/947/original/e1a3f1289b90810a.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/541/468/850/195/original/8e27dd8ace3af0aa.png

2026-03-29 16:59:44 UTC

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IBM's 4 Pi computers were built from simple TTL integrated circuits and used magnetic core memory for storage. "Flat-pack" integrated circuits on multi-layer circuit boards provided dense logic. The board below was round to fit inside a tactical missile, but most of the computers used standardized rectangular boards called "pages".
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/507/522/967/045/original/1e4a3ca7fe6f2bf6.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/520/789/902/233/original/78fc68c03f8df9f3.png

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...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/469/577/392/917/original/dc58a800e6094d8b.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/313/477/112/469/007/original/bd51682e80a94354.png

2026-02-14 22:09:42 UTC

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The 8087 doesn't help with 16-bit integer math.

2026-02-14 22:08:57 UTC

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Early calculator chips are pretty weird. They operate in decimal and are serial, so they operate on one bit at a time with a one-bit ALU. So all the data is constantly in motion.

2026-02-14 17:45:31 UTC

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Instruction decoding in the 8087 has many more complications: hardcoded instructions handled by the "Bus Interface Unit", instructions that load a constant from the constant ROM, and handling the instruction queue. For more details, see my latest blog post:
https://www.righto.com/2026/02/8087-instruction-decoding.html

2026-02-14 17:43:59 UTC

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The microcode inside the 8087 chip uses conditional jumps for many purposes. Some conditions examine the instruction, so the microcode can act differently for different instructions. Here's the PLA that stores target addresses for microcode jumps. Target 0 is special: it does a three-way branch depending if the instruction is multiplication, division, or addition/subtraction.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/070/210/295/765/263/original/97320097bb05411c.png

2026-02-14 17:38:43 UTC

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Like many processors, the 8087 uses a "Programmable Logic Array" (PLA) to decode the bit patterns in instructions. A PLA consists of grids of transistors, with some of the transistors missing. The pattern of transistors controls the PLA's function. The grid is a dense and flexible way of implementing this logic.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/070/191/247/489/545/original/f545e53425b42f7a.png

2026-02-14 17:32:53 UTC

In 1980, Intel released the 8087 floating-point chip, making math much faster. I'm reverse-engineering this chip, 46 years later. Most of its instructions are implemented in microcode, but some are implemented in hardware. Let's look at the circuitry that decodes instructions and decides what to do...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/070/157/546/401/938/original/d0355c62ff44b38c.jpg

2026-02-05 05:10:34 UTC

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Thanks, I meant the 1402 card reader/punch. I've edited it now.

2026-02-04 22:29:15 UTC

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The card reader/punch (IBM 1403) can punch 250 cards per minute, so the heart cards zipped out at high speed. However, this machine can't print text on cards, so I needed to go back to the keypunch...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/014/711/450/155/721/original/d90d0f36f1425b31.jpg

2026-02-04 22:26:04 UTC

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To duplicate the heart cards, I toggled a short program into the 1401 computer to read a card and print out copies. The 1401 has a special feature called "column binary" that allows it to read and punch cards even if the hole pattern isn't valid.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/014/701/175/234/140/original/1a6d83ce385e38b6.jpg

2026-02-04 22:23:00 UTC

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To make the heart, I had to figure out the right sequence of characters to form the hole pattern. Inconveniently, some holes don't form valid characters, so I had to "multi-punch" in some columns. This worked for one card, but the keypunch can't duplicate multi-punched cards.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/014/674/913/974/809/original/8b3dcdb557214c3c.jpg

2026-02-04 22:16:41 UTC

The Computer History Museum has a presentation tonight on the history of computer dating: "Algorithms of Love". I made a special punch card for the event. It wasn't easy to make this card so I'll explain how I did it...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/116/014/664/909/132/010/original/2081b2230f6ba348.jpg

2026-01-23 17:51:13 UTC

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If you want more on the 8086 processor's ALU, see my latest blog post:
https://www.righto.com/2026/01/notes-on-intel-8086-processors.html

2026-01-23 17:49:35 UTC

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The control signals in the 8086 need to be amplified to drive the circuitry for 16 bits. The driver circuit is very unusual: it is a "bootstrap" circuit that uses stray capacitance to boost the output voltage higher than normal, similar to a charge pump. The transistor with +5 on the gate acts as a diode, preventing the extra voltage from escaping. On the die, you can see that the drive transistors are much larger than the regular transistors.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/661/335/648/174/original/86972c1629f867b7.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/661/833/356/559/original/f4ad96cd678a030a.png

2026-01-23 17:44:15 UTC

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Each bit of the 8086's ALU is implemented with an unusual circuit. It uses two lookup tables (trapezoids), loaded from six control signals. This reprograms the ALU's logic to implement the desired operation. This is similar to how an FPGA uses lookup tables to implement different logic functions.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/643/865/364/083/original/bfe68d6627e856d7.png

2026-01-23 17:41:29 UTC

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What's the XI mux? To make the 8086's microcode more compact, it often doesn't specify the ALU operation, using a special code "XI" instead. The hardware looks at the machine instruction, determines if it is an ADD, SUB, XOR, or so forth, and substitutes the correct ALU operation. Thus, one microcode routine can implement 8 different machine instructions.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/631/671/040/629/original/fa7522f7b00f42db.png

2026-01-23 17:35:57 UTC

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The 8086's microcode specifies what ALU operation to perform along with which temporary register to use. The control circuitry holds this information in latches. A PLA (Programmable Logic Array) decodes the instruction into dozens of control signals. The Lookup Table (LUT) PLA generates six signals that configure the ALU to perform the desired operation.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/606/310/637/415/original/2c3ef21544a485a0.png

2026-01-23 17:31:57 UTC

The arithmetic/logic unit (ALU) in the Intel 8086 processor (1978) is more complicated than you might expect, performing 28 different operations from addition and logical AND to shifts and BCD adjustment. A special control circuit reconfigures the ALU for each operation. Let's look closer...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/945/586/323/699/700/original/102fa0e7b5e6779e.jpg

2025-12-30 19:51:11 UTC

If you're interested in obscure details of the microcode in the Intel 8087 floating-point chip, I have a new blog post...
https://www.righto.com/2025/12/8087-microcode-conditions.html

2025-12-09 18:33:38 UTC

In 1980, Intel announced the 8087 Math Coprocessor, a chip that made floating-point 100 times faster. I opened up the chip, took photos of the silicon structures, and analyzed its circuitry. It's a very complex chip for its time. Let's take a look inside...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/691/041/901/490/910/original/1cdcfcb63068720f.png

2025-09-15 17:22:01 UTC

I've been using HTML forever, but I just learned that <div /> isn't the same as <div></div>.
In the first case, the div isn't closed and everything that follows is inside it.

2025-09-10 16:28:19 UTC

Can anyone identify these mystery terminals? The first is from "Los placeres ocultos" (1977). The second is from "Policia" (1987). (Someone wants help identifying computer hardware in Spanish movies.)
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/180/922/622/920/348/original/3775c09f6a038a85.jpg
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/180/923/259/523/681/original/2499d8b2885e7420.jpg

2025-09-09 20:33:19 UTC

Can anyone recognize this front panel from a mainframe computer from a 1978 Spanish movie? The black color resembles IBM System/370 panels, but the knobs don't look right. Other information suggests maybe an IBM-compatible FACOM/Fujitsu computer.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/176/229/183/333/474/original/11621ba59a67038f.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/176/230/628/398/559/original/7f8cfa6e50c0e0d8.jpg

2025-09-06 15:30:13 UTC

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Marilou Schultz's rug is on display at SITE Santa Fe's current exhibition: "Once Within a Time"; let me know if you see it!
Photo credit: rug photo from First American Art Magazine.

2025-09-06 15:29:48 UTC

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To learn more about the 555 timer rug, see my latest post: https://www.righto.com/2025/09/marilou-schultz-navajo-555-weaving.html

2025-09-06 15:29:16 UTC

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Marilou Schultz first made a chip rug in 1994, when Intel commissioned a rug based on the Pentium as a gift to AISES (American Indian Science & Engineering Society). The Pentium weaving used natural dyes, while the 555 weaving uses aniline dyes and some metallic threads for more intense colors.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/158/057/875/855/920/original/e58557ce2c42d82a.png

2025-09-06 15:28:14 UTC

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Marilou Schultz based the rug on a photo by Antoine Bercovici (Siliconinsider). He used a special dark field microscope that produces a black background, highlighting the metal wiring on top of the silicon. The rug (left) mostly matches the photo (right), but there are some artistic changes.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/158/055/265/875/669/original/d17a9422b875d9bf.png

2025-09-06 15:27:04 UTC

Celebrated Navajo (Diné) artist Marilou Schultz recently completed a striking weaving. Although this rug may appear abstract, it is a representation of the wiring inside an integrated circuit. It shows the 555 timer, said at one point to be the world's most popular IC. Let's take a closer look...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/158/050/948/973/697/original/1c67c7967bb775ca.png

2025-08-19 14:08:50 UTC

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That QR code is the coolest thing. Nice how the black parts show the underlying structure, including the alternating squares of the timing pattern, while the colored parts have the data

2025-08-18 18:49:18 UTC

The latest issue of the journal Science mentions a magnetic compound Cr2Gr2Te6. The element Gr confused me, but it turned out to be a typo for Ge, germanium. Strangely, I found multiple papers with the same typo in the same context, so I wrote a short blog post about it.
https://www.righto.com/2025/08/Cr2Ge2Te6-not-Cr2Gr2Te6.html
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/051/260/012/006/032/original/b4d763331f5a9876.png

2025-08-17 14:46:00 UTC

How did Intel's 386 processor connect to the outside world? Tiny wires connect to the bond pads around the edges. Special input/output circuits—visible in zoomed-in blocks—send and receive signals while protecting the chip from dangers that could destroy it. Let's take a look... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/044/643/703/148/847/original/ade5aadb4db85882.png

2025-08-09 21:29:33 UTC

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Here's one more scan of the 386 for your viewing enjoyment...
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/115/000/927/586/381/110/original/1a3b0b8050871852.png

2025-08-09 16:49:07 UTC

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For more photos and information on the 386 package, see my latest blog post. Thanks to Jon Bruner and Lumafield for the CT scans.
https://www.righto.com/2025/08/intel-386-package-ct-scan.html

2025-08-09 16:48:24 UTC

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Here are the remaining four layers inside the package: one signal layer and three power layers. There are four power layers altogether so the chip's I/O circuitry and logic circuitry can be powered separately. That way, current surges from I/O don't affect the CPU.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/814/940/400/506/original/e87e706cd64b30c0.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/815/489/913/022/original/3a0535796477af65.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/815/802/643/766/original/4f93d34930af14b8.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/816/463/499/658/original/dec0e7a371ccf122.png

2025-08-09 16:45:03 UTC

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One of the six power layers inside the Intel 386 processor's packaging. It's almost a solid conductive plane; the grid of holes joins the ceramic above and below. Most of the pins are connected to lower layers, so only a few pins (red dots) reach this layer.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/810/561/736/923/original/def2ccedc0cd3ceb.png

2025-08-09 16:43:59 UTC

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The CT scan of a signal layer inside the 386's package. The wide abstract-looking regions connect the pins (red dots) to the chip in the middle. Unexpectedly, I spotted thin wires running to the edges of the chip. These are used during manufacturing to electroplate the pins with gold.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/804/296/926/732/original/b2bbce69021da18e.png

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
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/800/784/471/090/original/c93a49f63aaa3e6e.png
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/801/823/991/849/original/44334b84e6e6e32c.png

2025-08-09 16:41:24 UTC

Intel's 386 processor was popular in late 1980s microcomputers. From the outside, the 386 chip is a boring ceramic square. But I obtained a 3-dimensional CT X-ray scan from Lumafield, revealing six layers of complex wiring hidden inside the ceramic package. Let's take a closer look... 1/N
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/999/798/287/025/630/original/b6b7263a4a45c5a4.png

2025-08-02 15:48:17 UTC

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The chip uses these components to turn an FM radio signal into audio by combining an oscillator, mixers, filters, amplifiers, and other circuits. If you want to know more about this FM radio chip, I go into great detail in my blog post: https://www.righto.com/2025/08/reverse-engineering-analog-TDA7000.html

2025-08-02 15:48:10 UTC

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The most unusual components in the chip are the "varicap diodes", which help adjust the radio's frequency. These special diodes act as capacitors with a value that can be controlled by changing the voltage.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/959/952/713/675/600/original/7618026ed7e72556.png

2025-08-02 15:47:30 UTC

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Another important part of the chip is the resistors, which impede the flow of electricity. They are typically long and zig-zagging. The ends of the resistors are connected to the metal wiring. The arrows indicate the holes in the insulating oxide layer where these contacts take place.
https://assets.oldbytes.space/assets.oldbytes.space/media_attachments/files/114/959/950/899/190/720/original/87eafe22cbeca0a6.png