I've read that some fake sound chips from eBay actually used a gate array. Not original parts, not a 100% equivalent drop-in, but close enough to work & convince most buyers. Don't know if true but sounds plausible. And if so, a (relatively) simple cpu wouldn't be much of a stretch.
Ken's article doesn't mention it, but there's another reason to use a gate array: pre-fabrication, and through that, easier stock-keeping.
Say you have ~100 different IC's like the one described. One could then do all the manufacturing steps to produce eg. 100k of those gate arrays, except the last fabrication step. And possibly keep a large stock of those 'blank' gate arrays.
Then (when it's known which of those 100 IC's is needed), apply only the last production step to those 'blanks', and presto: selected IC ready - in large volume if needed. Or produce IC's where original part has become hard-to-find.
For a mil-spec part, that flexibility might be among reasons to go for a gate array.
These days, something like a fuse-based FPGA might be used instead?
The thing is gate arrays are sort of something between a full custom part and an FPGA, they cost less per chip than a gate array, but more per chip than full custom. On the other hand NRE (up front 1 time cost)) is a lot lower than for full custom (I've built both).
These parts are mil-spec which means that their volumes will be lower, it may have made sense to build a mil-spec pad ring then spin out a range of low volume mil-spec 7400 parts from it
I have heard of this structure also referred to as a PLA - programmable logic array, and I think a ULA is specifically for a PLA that is embedded in the empty space of a bigger design (so that in case something is broken, a debug is one mask, not a full respin).
I am not sure that Wikipedia is out-of-date enough to have the precise terminology, but I also may be wrong.
The terminology is a bit of a mess, but usually a PLA is highly structured with an AND plane and an OR plane, so it implements sum-of-products logic. A gate array is more general, with arbitrary connections.
I know it's not adding very much to the conversation but I just have to give kudos to the author of this article.
It's always nice to see what the magic smoke inside those chips look like before I let it out!
So great job I look forward to reading more of your articles!
I'm thinking of buying a bunch of 74xx ICs and buttons and led+resistors somewhere and let him mess with them. It's stone age digital tech according to current norms, but I'd rather see him do it with physical components than behind a screen.
I'm not sure what you're asking. IDT used the same gate array for numerous 7400-series products. As a result, simple chips would waste most of the die, while complex chips would use most of it. The tradeoff is that using a gate array saves design costs, although each chip is more expensive to manufacture due to the wasted silicon. Since IDT was selling into low volume, price-insensitive markets (military), the tradeoff was worthwhile.
I may be dating myself here but I seem to recall Intel offering a hobbled 486 processor in which the math processor was disconnected for no other reason than marketing. Regarding the Gate array Technology it seems we've come full circle with the new risk V processors being offered up at incredibly cheap prices.
Why use a $20 Arduino when you could have just gotten a $1 custom PCB, $0.50 microcontroller, and a handful of jellybean parts to do the same job? Because one costs less in Non-Recurring Engineering (NRE) than the other, you'd have to design the schematic and layout and then select parts that are affordable and available. Sometimes its just makes more sense to use a reusable component than doing something custom, especially when you don't plan to sell a lot of them. Someone needed a 1-to-4 decoder and for some reason, couldn't find one on the market that fit the application. The Gate Array already existed and was cheap to configure so that's what they went with. It's overkill but so is getting a brand new die made for such a throwaway part of the design.
That's [ULA], isn't it? This tech was also known as "Gate Array", before FPGA came along.
[ULA] https://en.wikipedia.org/wiki/Uncommitted_logic_array