The stack circuitry of the Intel 8087 floating point chip, reverse-engineered

Summary

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 40,000 and 75,000 transistors, depending on the source.2 To explore how the 8087 works, I opened up a chip and took numerous photos of the silicon die with a microscope. Around the edges of the die, you can see the hair-thin bond wires that connect the chip to its 40 external pins. The complex patterns on the die are formed by its metal wiring, as well as the polysilicon and silicon underneath. The bottom half of the chip is the "datapath", the circuitry that performs calculations on 80-bit floating point values. At the left of the datapath, a constant ROM holds important constants such as π. At the right are the eight registers that form the stack, along with the stack control circuitry. Die of the Intel 8087 floating point unit chip, with main functional blocks labeled. The die is 5mm×6mm. Click for a larger image. The chip's instructions are defined by the large microcode ROM in the middle. This ROM is very unusual; it is semi-analog, storing two bits per transistor by using four transistor sizes. To execute a floating-point instruction, the 8087 decodes the instruction and the microcode engine starts executing the appropriate micro-instructions from the microcode ROM. The decode circuitry to the right of the ROM generates the appropriate control signals from each micro-instruction. The bus registers and control circuitry handle interactions with the main 8086 processor and the rest of the system. Finally, the bias generator uses a charge pump to create a negative voltage to b...