what was the last central processing unit made commercially available that was not on one chip?
The 8-bit discrete logic processor kit!
It is commercially available now (you can buy it from the website above).
It is multi-chip since it's made of 74LS chip.
And... it is definitely retro.
I believe the correct answer is the IBM Q System One.
It's a quantum computer which is currently offered for commercial use. Although I've never seen a specification of exactly what electronics it uses, I'm reasonably certain it uses more than one chip (I'm reasonably certain nobody has a clue of how to build such a CPU on a single chip at the present time).
The world of large computers is amazing. MCM systems of monstrous in the eyes of a PC user parameters were widely popular right up to the cloud revolution, and even now, taking into account legal restrictions, they are actively used in banks.
Just remember IBM POWER2+ in a six-chip configuration for one core (not counting L2 cache) (video) - it was launched ...
Honeywell had a CPU that was 36-bit from 1970-1989 (which one might say was ahead of its time as IBM said that on their 32-bit machines years ago they needed one more bit because they only used 31-bits and used the high order bit to determine the end of a list of addresses as passed as parms)
The CPU operated on 36-bit words, and addresses were 18 bits. ...
The VAX 9000, first ship 1990, must be a contender here.
Each CPU was implemented with 13 Multi-Chip Units (MCUs), with each
MCU containing several emitter-coupled logic (ECL) macrocell arrays
which contained the CPU logic.
By the time it was released, the 9000's price/performance had been eclipsed by systems based on NVAX, a single-chip ...
The last commercial non-integrated main processor introduction that I can recall was the one in Tandem NonStop Cyclone systems, introduced circa 1989. The CPU seems to have been 3 large printed circuit boards full of ECL gate arrays.
It depends. Some CPUs needed specific support chips, e.g. Intel 8080 (1974) needed (cited from Wikipedia) "i8224 clock generator/driver and the i8228 bus controller". Indeed, one of the advantages of Z80 was no need for these additional chips.
You are getting everything wrong.
The 286 has integrated segmentation unit and protected modes to allow multitasking OSes and more memory (up to 16MB). It was actually used for that purpose in early versions of OS/2 and Windows.
The definition of "workstation" is arbitrary. 640kB DOS was becoming cramped, and the 286 allowed to use more memory.
Having multiple graphics or sound chips in a system has exactly the effects you would expect. It would allow the composite system to perform multiple processing tasks at once, with the results of each contributing processor being combined into the final output (or spread across multiple outputs), producing more sophisticated visuals or sounds.
Double-SID mod for Commodore 64 is quite popular. Simultaneous work with CGA and MDA, albeit on different monitors, was also quite popular. Sharp X68000 deserves special attention, taking into account its popular hardware extensions. A classic example of doubling VDP is PC Engine SuperGrafx.
Considering the principles of operation of the graphic subsystems ...
CompuPro did this in the olden days, with 8-bit and 16-bit CPU slave cards with a 16-bit or 32-bit main CPU. But there were a few differences from what you are suggesting:
Each CPU had its own RAM. That was an absolute must. If two CPUs have to share RAM (except to a limited degree - e.g., DMA for disk access), they will slow each other down. RAM is already ...
how I can add additional slave processors (and possibly RAM) to this system through the mainboard's PCI card slots
While DMA works through PCI, this is not practical to sustain to have additional CPUs on the card access main memory.
So basically you'd need a complete second PC with RAM and one or more CPUs on the card. The best you can hope for is to use ...