Talk:Diode matrix

PAL v PLA

I think that this page is confusing Programmable Array Logic with Programmable Logic Array FrankFlanagan (talk) 13:51, 30 April 2011 (UTC)[reply]

You may be right.

What can we do to make this less confused?

I see that Dieter Mueller hand-built a few boards out of discrete transistors that are more like a PLA than a PAL[1][2].

I only hesitate to tear out all mention of PAL in this article, and replace it with PLA, because I suspect that replacing "PAL chips" with "either PLA chips or PAL chips" would be more correct.

In particular, the "Programmable Array Logic" and "programmable logic array" articles imply that some minicomputers used one or more "PAL chips" in the CPU, while other minicomputers used one or more "PLA chips" in the CPU.

--DavidCary (talk) 17:13, 4 July 2011 (UTC)[reply]

The microprogrammed machines that I was involved in designing Mentec PDP-11#M11 and Mentec PDP-11#M1used SRAM for control store. I am aware of PLAs being used see for instance: http://www.computerhistory.org/collections/accession/102660026, Microcode and in part for the J11 http://simh.trailing-edge.com/semi/j11.html.
Equally I have used PAL's as part of CPU implementations, but for small pieces of fast combinatorial logic, registers and simple fast state machines.
My not being aware of PALs being used to implement microcode storage is more likely to be evidence of gaps in my knowledge than anything else, but as the architecture of PALs is not really that close to a diode matrix, I think that PAL could usefully be replaced with PLA. My 2 cents only. FrankFlanagan (talk) 21:24, 4 July 2011 (UTC)[reply]

Thank you for the reference links. I inserted all of them into the microcode article, where they seemed most appropriate.

After reading the book The Soul of a New Machine, and more recently the Programmable Array Logic article, both of which point out that the Eagle (Data General Eclipse MV/8000) used lots of one-time-programmable PAL chips, somehow I leaped to the incorrect conclusion that the Eagle stored all its microcode in one or more of those PAL chips. The microcode#Writable control stores article now specifically points out that the Eagle used SRAM chips, like the machines you were involved in designing.

I thought I knew of a microprocessor that had a section of its control unit arranged much like a PAL. But I recently learned it's not exactly a mask-programmed PAL or PLA or ROM, but yet another mask-programmed member of that family -- see talk:programmable logic device#another type of PLD for details.

Is there a specific reason that the Mentec designers and the Eagle designers chose to put the control store in SRAM rather than PAL or PLA? (What's the best place to document the advantages, disadvantages, and tradeoffs of the various ways to implement a control store? The control store article? The Microprocessor Design/Microcode Wikibook?)

Since I no longer know of any specific example of a CPU that uses a PAL as the control store, I've changed "PAL" to "PLA" in this article as you originally suggested.

I hope that, if the control store of some CPU was stored in PALs, someone will eventually edit these Wikipedia articles to fill in the gaps in our knowledge. --DavidCary (talk) 13:18, 10 July 2011 (UTC)[reply]