History of the 8008
The 8008’s complicated story starts with the Datapoint 2200, a popular computer introduced in 1970 as a programmable terminal.
(Some people consider the Datapoint 2200 to be the first personal computer.)
Rather than using a microprocessor, the Datapoint 2200 contained a board-sized CPU build from individual TTL chips.
(This was the standard way to build a CPU in the minicomputer era.)
Datapoint and Intel decided that it would be possible to replace this board with a single MOS chip, and Intel started the 8008 project to build this chip.
A bit later, Texas Instruments also agreed to build a single-chip processor for Datapoint.
Both chips were designed to be compatible with the Datapoint 2200’s 8-bit instruction set and architecture.
The 8008 processor was first described publicly in “Electronic Design”, Oct 25, 1970. Although Intel claimed the chip would be delivered in January 1971, actual delivery was more than a year later in April, 1972.
Around March 1971, Texas Instruments completed their processor chip, calling it the TMC 1795.
After delaying the project, Intel finished the 8008 chip later, around the end of 1971.
For a variety of reasons, Datapoint rejected both microprocessors and built a faster CPU based on newer TTL chips including the 74181 ALU chip.
TI tried unsuccessfully to market the TMC 1795 processor to companies such as Ford, but ended up abandoning the processor, focusing on highly-profitable calculator chips instead.
Intel, on the other hand, marketed the 8008 as a general-purpose microprocessor, which eventually led to the x86 architecture you’re probably using right now.
Although TI was first with the 8-bit processor, it was Intel who made their chip a success, creating the microprocessor industry.
A family tree of the 8008 and some related processors. Black arrows indicate backwards compatibility. Light arrows indicate significant architecture changes.
The diagram above summarizes the “family tree” of the 8008 and some related processors.10
The Datapoint 2200’s architecture was used in the TMC 1795, the Intel 8008, and the next version Datapoint 2200.11. Thus, four entirely different processors were built using the Datapoint 2200’s instruction set and architecture.
The Intel 8080 processor was a much-improved version of the 8008. It significantly extended the 8008’s instruction set and reordered the machine code instructions for efficiency. The 8008 was used in groundbreaking early microcomputers such as the Altair and the Imsai.
After working on the 4004 and 8080, designers Federico Faggin and Masatoshi Shima left Intel to build the Zilog Z-80 microprocessor, which improved on the 8080 and became very popular.
The jump to the 16-bit 8086 processor was much less evolutionary. Most 8080 assembly code could be converted to run on the 8086, but not trivially, as the instruction set and architecture were radically changed.
Nonetheless, some characteristics of the Datapoint 2200 still exist in today’s x86 processors.
For instance, the Datapoint 2200 had a serial processor, processing bytes one bit at a time.
Since the lowest bit needs to be processed first, the Datapoint 2200 was little-endian.
For compatibility, the 8008 was little-endian, and this is still the case in Intel’s processors.
Another feature of the Datapoint 2200 was the parity flag, since parity calculation was important for a terminal’s communication.
The parity flag has continued to the x86 architecture.
The 8008 is architecturally unrelated to Intel’s 4-bit 4004 processor.12. The 8008 is not an 8-bit version of the 4-bit 4004 in any way. The similar names are purely a marketing invention; during its design phase the 8008 had the unexciting name “1201”.
If you want more early microprocessor history, I wrote a detailed article for the IEEE Spectrum.
I also wrote a post about TI’s TMC 1795.
