Conclusion
I took detailed die photos of the 8008 that reveal the circuitry it used.
While the 8008 wasn’t the first microprocessor or even the first 8-bit microprocessor, it was truly revolutionary, triggering the microprocessor revolution and leading to the x86 architecture that dominates personal computers today.
Notes and references
- According to the oral history of the 8008,
photos of the 8008 were obtained in October / November 1971 (page 6).
Chip designer Federico Faggin mentions that toward the end of 1971, “everything was working except for a few errors.” Faggin then debugged a problem with the dynamic memory losing data, making it ready for production (page 9). ↩ - Using the carry look ahead circuit avoids the delay from a standard ripple-carry adder, where the carries propagate through the sum. ↩
- The 8008’s ALU supports eight operations: add, subtract, add with carry, subtract with carry,
AND, OR, XOR, and compare.
It also implements left and right shift and rotate operations. The 8008 also has increment and decrement instructions, extending the Datapoint 2200’s instruction set. ↩ - Because silicon has higher resistance than polysilicon, most chips use the polysilicon and metal layers for wiring, not the silicon layer.
The 4004 and 8008 chips are unusual in that they prefer to use the silicon layer for wiring rather than polysilicon.
I expect this was due to the recent introduction of polysilicon: before polysilicon, routing needed to be done in the silicon layer and perhaps the chip designers were sticking with the older layout techniques. ↩ - The 8008 required 20 support chips according to chip architect Federico Faggin.
In contrast, the 4004 and earlier MOS computers such as the Four Phase and CADC were designed with a small number of MOS chips that worked together without extra “glue chips”.
In this sense, the 8008 was a step backwards architecturally, saying “here’s the CPU, you figure out how to make a computer out of it.” ↩ - For details on Intel’s insistence on 16 pins, see Oral History of Federico Faggin, page 55-56.
It was only when the 1103 memory chip required 18 pins that Intel reluctantly moved beyond 16 pins. And that was treated by Intel like “the sky had dropped from heaven,” resulting in “so many long faces”. ↩ - If two metal lines need to cross, one of them can be routed under the other by using the polysilicon layer. To be low resistance, this cross-under must be relatively wide, so cross-unders are avoided if possible. ↩
- The 8008 registers use the “3T1C” cell: three transistors and one capacitor (details).
The circuit doesn’t physically contain a separate capacitor, but uses the gate capacitance of the transistor.
One unusual feature of the 8008 cell is it uses one wire for both reading and writing the bit, while the typical 3T cell has separate wires for reading and writing.
The 4004 had separate wires, but the design changed slightly in the 8008. ↩ - Pull-up resistors in later chips such as the 6502 were implemented using depletion-mode NMOS transistors.
These yielded more faster, more efficient logic.
They were also wired differently, with the gate connected to the output rather than the power rail. ↩ - The 8008 architecture and the evolution of Intel’s microprocessors are discussed in detail in Intel Microprocessors: 8008 to 8086. ↩
- The second version of the Datapoint 2200 had a totally new implementation of the processor, still built from TTL chips. While the first version had a serial ALU (processing one bit at a time), the second version operated in parallel using 74181 ALU chips. As a result, the second version was much faster. ↩
- The extensive 4004 Anniversary Project has reverse-engineered the 4004 processor. The 4004 schematic is here. ↩
- The Motorola 6800 microprocessor originally used enhancement-mode transistors. To operate off a single +5V supply, it had a voltage-doubler circuit on the chip. ↩
- Interestingly, in 2007 Intel started using metal gates again in order to scale transistors further (details). In a way, semiconductor technology has gone full circle, back to metal gates, although now unusual metals such as hafnium are used. ↩
Original article by Ken Shirriff at his blog.
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