The revolutionary Intel 8008 microprocessor is 45 years old today (March 13, 2017), so I figured it’s time for a blog post on reverse-engineering its internal circuits. One of the interesting things about old computers is how they implemented things in unexpected ways, and the 8008 is no exception. Compared to modern architectures, one unusual feature of the 8008 is it had an on-chip stack for subroutine calls, rather than storing the stack in RAM. And instead of using normal binary counters for the stack, the 8008 saved a few gates by using shift-register counters that generated pseudo-random values. In this article, I reverse-engineer these circuits from die photos and explain how they work.
The image below shows the 8008’s tiny silicon die, highly magnified. Around the outside of the die, you can see the 18 wires connecting the die to the chip’s external pins. The 8008’s circuitry is built from about 3500 tiny transistors (yellow) connected by a metal wiring layer (white). This article will focus on the stack circuits on the right side of the chip and how they interact with the data bus (blue).
For the 8008 processor’s birthday, I’m using the date of its first public announcement, an article in Electronics on March 13, 1972 entitled “8-bit parallel processor offered on a single chip.” This article described the 8008 as a complete central processing unit for use in “intelligent terminals” and stated that chips were available at $200 each.1
You might think that an intelligent terminal is a curiously specific application for the 8008 processor. There’s an interesting story behind that, going back to the roots of the chip: the Data point 2200 “programmable terminal”, introduced in June 1970. The popular Data point 2200 was essentially a desktop minicomputer with its processor consisting of a board full of simple TTL chips. The photo below shows the CPU board from the Data point 2200. The chips are gates, flip flops, decoders, and so forth, combined to build a processor, since microprocessors didn’t exist at the time.
Processors typically use a stack to store addresses for subroutine calls, so they can “pop” the return address off the stack. This stack is usually stored in main memory. However, the Data point 2200 used slow shift-register memory2 instead of expensive RAM for its main storage, so implementing a stack in main memory would be slow and inconvenient. Instead, the Data point 2200’s stack was stored in four i3101 RAM chips, providing a small stack of 16 entries. 3 4 The i3101 was Intel’s very first product, and held just 64 bits. In the photo above, you can see the chips in their distinctive white packaging each with a large “I” for Intel. 5
To keep track of the top of the stack, the Data point 2200 used a 4-bit up/down counter chip to hold the stack pointer. The clever thing about this design is there’s no separate program counter (PC) and stack; the PC is simply the value at the top of the stack. You don’t need to explicitly push and pop the PC onto the stack; for a subroutine call you just update the counter and write the subroutine address to the stack.
The story of the 8008’s origin is that Data point went to Intel and asked if Intel could build a chip that combined the stack memory and the stack pointer onto a single chip. Intel said not only could they do that, they could put the whole processor board onto a single chip! This was the start of Intel’s 8008 project to duplicate the Data point 2200’s processor board onto a chip, keeping the Data point 2200 instruction set and architecture.6 After various delays, Intel completed the 8008 microprocessor, but Data point rejected it. Intel decided to sell the 8008 as a general-purpose processor chip, sparking the microprocessor revolution. Intel improved the 8008 with the 8080 and then the 16-bit 8086, leading to the x86 architecture that dominates desktop and server computers today.
Read more: Analyzing the vintage 8008 processor from die photos