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Notes on the Intel 8086 processor's arithmetic-logic unit
In 1978, Intel introduced the 8086 processor, a revolutionary chip that led to the modern x86 architecture. Unlike modern 64-bit processors, however, the 8086 is a 16-bit chip. Its arithmetic/logic unit (ALU) operates on 16-bit values, performing arithmetic operations such as addition and subtraction, as well as logic operations including bitwise AND, OR, and XOR. The 8086's ALU is a complicated part of the chip, performing 28 operations in total. 1 In this post, I discuss the circuitry that con...
Conditions in the Intel 8087 floating-point chip's microcode
In the 1980s, if you wanted your computer to do floating-point calculations faster, you could buy the Intel 8087 floating-point coprocessor chip. Plugging it into your IBM PC would make operations up to 100 times faster, a big boost for spreadsheets and other number-crunching applications. The 8087 uses complicated algorithms to compute trigonometric, logarithmic, and exponential functions. These algorithms are implemented inside the chip in microcode. I'm part of a group that is reverse-enginee...
The stack circuitry of the Intel 8087 floating point chip, reverse-engineered
Early microprocessors were very slow when operating with floating-point numbers. But in 1980, Intel introduced the 8087 floating-point coprocessor, performing floating-point operations up to 100 times faster. This was a huge benefit for IBM PC applications such as AutoCAD, spreadsheets, and flight simulators. The 8087 was so effective that today's computers still use a floating-point system based on the 8087. 1 The 8087 was an extremely complex chip for its time, containing somewhere between 40,...
Unusual circuits in the Intel 386's standard cell logic
I've been studying the standard cell circuitry in the Intel 386 processor recently. The 386, introduced in 1985, was Intel's most complex processor at the time, containing 285,000 transistors. Intel's existing design techniques couldn't handle this complexity and the chip began to fall behind schedule. To meet the schedule, the 386 team started using a technique called standard cell logic. Instead of laying out each transistor manually, the layout process was performed by a computer. The idea be...
Solving the NYTimes Pips puzzle with a constraint solver
pre { background: #f4f4f4; border: 1px solid #ddd; border-left: 3px solid #a03; border-radius: 2px; color: #666; display: block; font-family: monospace; line-height: 1.3; margin-bottom: 1.6em; max-width: 60em; overflow: auto; padding: 1em 1.5em; page-break-inside: avoid; white-space: pre-wrap; word-wrap: break-word; } The New York Times recently introduced a new daily puzzle called Pips . You place a set of dominoes on a grid, satisfying various conditions. For instance, in the puzzle below, the...
A Navajo weaving of an integrated circuit: the 555 timer
The noted Diné (Navajo) weaver Marilou Schultz recently completed an intricate weaving composed of thick white lines on a black background, punctuated with reddish-orange diamonds. Although this striking rug may appear abstract, it shows the internal circuitry of a tiny silicon chip known as the 555 timer. This chip has hundreds of applications in everything from a sound generator to a windshield wiper controller. At one point, the 555 was the world's best-selling integrated circuit with billion...
Why do people keep writing about the imaginary compound Cr2Gr2Te6?
I was reading the latest issue of the journal Science , and a paper mentioned the compound Cr 2 Gr 2 Te 6 . For a moment, I thought my knowledge of the periodic table was slipping, since I couldn't remember the element Gr. It turns out that Gr was supposed to be Ge , germanium, but that raises two issues. First, shouldn't the peer reviewers and proofreaders at a top journal catch this error? But more curiously, it appears that this formula is a mistake that has been copied around several times. ...
Here be dragons: Preventing static damage, latchup, and metastability in the 386
I've been reverse-engineering the Intel 386 processor (from 1985), and I've come across some interesting circuits for the chip's input/output (I/O) pins. Since these pins communicate with the outside world, they face special dangers: static electricity and latchup can destroy the chip, while metastability can cause serious malfunctions. These I/O circuits are completely different from the logic circuits in the 386, and I've come across a previously-undescribed flip-flop circuit, so I'm venturing...
A CT scanner reveals surprises inside the 386 processor's ceramic package
Intel released the 386 processor in 1985, the first 32-bit chip in the x86 line. This chip was packaged in a ceramic square with 132 gold-plated pins protruding from the underside, fitting into a socket on the motherboard. While this package may seem boring, a lot more is going on inside it than you might expect. Lumafield performed a 3-D CT scan of the chip for me, revealing six layers of complex wiring hidden inside the ceramic package. Moreover, the chip has nearly invisible metal wires conne...