Post

Ken Shirriff
@righto.com

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...

Ken Shirriff
@righto.com

Unlike most processors, the 8087 organizes its registers into a "stack", pushing numbers onto the top of the stack and popping them off. Here's a close-up of the eight registers, organized in a grid of cells. Each register holds an 80-bit number, so the registers are very tall.

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L Break Into Program
@breakintoprogram.co.uk

It sounds like you are describing Reverse Polish Notation there. I wonder if the engineers took inspiration from their HP scientific calculators? (I had one in college, given to me by a friend of the family. Fellow students would only borrow it once, more fool them).

Ken Shirriff
@righto.com

Each bit is stored in a "static RAM" cell, consisting of two inverters in a loop. This circuit has two stable states, holding a 0 or a 1. The implementation is complicated: silicon with polysilicon lines on top to make transistors. Horizontal metal wires connect everything.

Ken Shirriff
@righto.com

This diagram, based on the 8087 patent, shows the implementation of the stack. You saw the registers (yellow) earlier. This photo shows the three-bit circuitry that controls the stack (purple, green, and blue). The schematic shows one bit in detail.

Ken Shirriff
@righto.com

Intel was hesitant to produce the 8087 chip, considering it complex, risky, and with an unknown market. Intel's Israel site took on the project; the die is marked "i/IL". The chip was a highly profitable success. Now, almost all computers use floating-point systems based on the 8087.