I've come to understand that many CPUs were not single-chip, well into the integrated circuit era. Today we might assume that a microprocessor will come in a single package, but that wasn't a reasonable assumption in other decades, where the computer designer might even be creating the CPU as well, and not have access to a semiconductor fab.

Are you trying to make me feel old, simply because I once had the 74Fxxx TTL catalog nearly memorized? Or that I can do an instruction decoder and conflict interlock unit in 100K ECL?

Now get off my lawn or I shall be forced to shake my cane at you quite vigorously.

Honestly, those were fun, fun days.

It's surprising to me how in the 1970s, if someone needed a processor for something (even a video game or CNC controller), it was normal for them to invent a new instruction set and put together a processor from TTL chips. The barrier to entry was very low.

>...put together a processor from TTL chips. The barrier to entry was very low.

It was mostly because of the clock speeds, wasn't it? I seem to remember once hearing that below 20 Mhz you could play "TTL Lego" but above that you ran into problems with things like propagation delay.

The stated propagation delay for TTL chips was usually about 10 or 20 nanoseconds (sometimes larger for more complex logical operations, and frequently smaller in subsequent logic generations, like 5 nanoseconds or so). I just checked this in a late 1970s databook.

So, 20 MHz seems pretty credible for a region where you would see problems due to propagation delay—maybe even somewhere below that with the original 7400 series.

That's also a reason that I hadn't thought about so much why putting the whole CPU on a single die would look more and more practical over time: propagation delay in wires!

> The barrier to entry was very low.

If you had the money. Those chips weren't cheap back then (even the 74xx branch), and you still needed quite a few to do anything useful when it came to building a CPU.

Well, there's always the 74181, a 4 bit ALU[0] as used in the Alto[1] among others like the NOVA and VAX 11/780.

0. https://mil.ufl.edu/4712/docs/74LS181.pdf

1. https://en.wikipedia.org/wiki/74181#Computers