Say someone
were to home-build a CPU from scratch using only individual
components, no ICs only modern descrete(?) components. How big would the
CPU be? For comparison lets say it would be an 8080 clone. Any guesses?
One of the vendors of bit-slice components (AMD 2900 or clone, I think)
offered a board that emulated an 8080. In addition to being faster than
an 8080, you could of course modify the microcode to add or change
instructions. IIRC, it looked like the board had about fifty chips.
If you didn't use a bit-slice, you'd have to use separate ALU, memory,
and shifter chips, and you'd wind up with even more.
The early microprocessor architectures were designed based on minimal
transistor count for a single-chip implementation. This does not result
in minimal chip count if you implement the equivalent in 7400-series
chips. It is quite possible to design useful processors with a lot fewer
TTL chips.
Er, Eric? He said, no ICs only modern discrete components...
As Eric says, an 8080 clone or similar would not be good to do with TTL chips or
something, but it might work well with discretes.
How big? Depends on the routing technology:
A few trannies to a PCB, standard modules where possible, do anything
complicated on the backplane => as big as a PDB8 (which someone mentioned) - say
a 2 foot cube.
Something more modern, say a multilayer pcb covered with densely packed surface
mount transistors => you might get it onto a board the size of an AT
motherboard, or possibly two such boards (allow for plugs on one to go straight
into sockets on the other, mount the boards side by side so you can place
connectors in arbitrary locations over the board)
Something in between is probably more practical - still pin through hole, single
or possibly double sided, but with modern, denser PCB (or pen-wired) layouts,
and not worrying about using standard modules (unlike DEC, you're not
mass-producing) => it would probably be a rack full of Eurocards. (20 * 5 * 10
inches, roughly). Hmm, perhaps a double rack. 20 * 10 * 10.
I haven't looked at transistor counts or packing densities so these are only
guesses.
My opinion (FWIW) is that while this might be a fun project, doing something
with a machine that was originally designed this way (PDP8 again) might be more
educational.
Philip.
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