Four-Phase AL1
If one person could be considered the father of MOS/LSI processors, it would be Lee Boysel. While working at Fairchild, he came up with the idea of a MOS-based computer and methodically designed and built the necessary cutting-edge chips (ROM in 1966, ALU in 1967, DRAM in 1968). Along the way he published several influential articles on MOS chips, as well as a 1967 "manifesto" explaining how a computer comparable to the IBM 360 could be built from MOS.
Four-Phase AL4 arithmetic-logic chip (variant of AL1)
Boysel left Fairchild and started Four-Phase Systems in October 1968 to build his MOS-based system. In 1970, he demoed the System/IV, a powerful 24-bit computer. The processor used 9 MOS chips: three 8-bit AL1 arithmetic / logic chips, three microcode ROMs, and three RL random logic chips. This computer sold very well and Four-Phase became a Fortune 1000 company before being acquired by Motorola in 1981.
Die photo of Four-Phase AL1 arithmetic-logic chip. Courtesy of Computer History Museum.
As described earlier, Boysel used an AL1 chip as a processor in a courtroom demonstration system in 1995 to show prior art against TI's patents. Given this demonstration, why don't I consider the AL1 to be the first microprocessor? It used an AL1 chip as the processor, along with ROM, RAM, and I/O and some address latches, so it seems like a single-chip CPU. But I've investigated this demonstration system closely, and while it was a brilliant hack, there's also some trickery. The ROM and its associated latch are actually set up as a microcode controller, providing 24 control lines to the rest of the system. The ROM controls memory read/write, selects an ALU operation, and provides the address of the next microcode instruction (there's no program counter). After close examination, it's clear that the AL1 chip is acting as an Arithmetic/Logic chip (thus the AL1 name), and not as a CPU.
There are a few other things that show the AL1 wasn't working as a single-chip computer. The die photo published as part of the trial has the components of the AL1 chip labeled, including "Instruction Register 23 bits". However, that label is entirely fictional—if you study the die photo closely, there's no instruction register or 23 bits there, just vias where the ground lines pass under the clock lines. I can only conclude that this label was intended to trick people at the trial. In addition, the AL1 block diagram used at the trial has a few subtle changes from the originally-published diagram, removing the program counter and adding various interconnections. I examined the code (microcode) used for the trial, and it consists of super-bizarre microcode instructions nothing like the AL1's original instruction set.
Detail of AL1 die photo showing fictional 'Instruction Register 23 bits' label.
While the demo was brilliant and wildly successful at derailing the Texas Instruments lawsuits, I don't see it as showing the AL1 was a single-chip microprocessor. It showed that combined with a microcode controller, the AL1 could be used as a barely-functioning processor. In addition, you could probably use a similar approach to build a processor out of an earlier ALU chip such as the 74181 or Fairchild 3800, and nobody is arguing that those are microprocessors.
Looking at the dates, it appears that Viatron (described below) shipped their MOS/LSI computer a bit before Four-Phase, so I can't call Four-Phase the first MOS/LSI computer. However, Four-Phase did produce the first computer with semiconductor memory (instead of magnetic core memory), and thus the first all-semiconductor computer.
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