Four-Phase Systems AL1 Processor – 8-bits by Lee Boysel

A 8 bit alu/cpu before intel’s 8008. A nice 24 bit cpu for the 1970’s.

1 Like

The AL1 was an 8-bit processor with 8 registers (including the PC) and a full 8-bit ALU. Four-Phase used three of them in a system to form a 24-bit computer. Additional chips were also used, 3 x ROM chips, 3 x Random logic and associated clock generation for the 4-phase clock. The 4-Phase clocking is also notable, it allowed the use of minimum size transistors for most gates. This increases the chip density and performance greatly. The AL1 had over 1000 gates (~4000 transistors) with a die size of 130×120 mils (100mm2). This was essentially the complexity of Intels 8008 but with a die size of Intel’s 4-bit 4004. Furthermore, the speed was around 1 MHz, nearly 10-times anything else at the time, even on the same 10 micron process.

It would be interesting to know more about the instruction set. Also, how the 3 8-bit ALUs were integrated. – Was this somewhat like bit-slicing?

It’s looking to me like it’s an 8 bit slice, which makes it difficult to use a single one of them as a CPU in a useful way. But historically, it was important to try to do that!

Outside of a court case in 1990 where TI used it to try to invalidate an Intel patent, the AL1, and Four-Phase are relatively unknown in the IC realm

(Hmm, seems like the demo was successful, which went against TI in this case, so that quote needs a tweak.)

See this image from Ken Shirriff’s post (as always, well worth reading!):

The Four-Phase AL1 running as a single-chip processor in a patent litigation demo. From Boysel’s EECS presentation.


It was a bit slice (byte slice, in this case), but the term is normally used for a component which integrates a narrow (but complete) version of a computer’s datapath, with AMD’s 2901 being the best known.

The AL1 includes a program counter, which is why the demo with a single part being used as a microprocessor was possible. That is not how it was used in products that were actually sold, but it is something other “bit slices” couldn’t do.