Nothing in particular - VRAM was the core of all the cards thise days, the serial port shift register (inside the VRAM) fed muxes that in turn fed luts and out to the display.
The VRAM array was 2 sets of 96 bits wide - the accelerator was in part a 1:4 24:96 mux that drove the VRAM array - that's how the dumb frame buffer worked.
I'd spent a year before all this writing code to grab stats figure out where the mac graphics code spent it's time, initially we'd expected to optimise drawing vectors (ie lines - autocad was driving the parallel PC graphics market at the time) - my big discovery was that for the sorts of workloads that our customers were using 99% consisted of just 4 operations:
- solid fills (>90%)
- patterned fills
- copying within the FB (ie scrolling)
- writing text (quite low by important for the DTP market)
I wrote the spec for the first 2 cards (to do the first 3 operations) and the went off to design the 3rd chip (modelled in C, turned into an inch thick spec describing the state machines and data paths and then passed to someone else to make gates)
I added a hack on top of the copy primitive to accelerate text hiliting.
So that accelerator chip also contained a state machine did 4 simple things as fast as it possibly could (solid fills used magic modes in the VRAMS, it could fill at 1.5Gbytes/sec - faster than the high end 3D hardware at the time)
In the end the text fill was useful, but not that much faster (it turned 1-bit font images into 24-bit pixels on the fly), all that cool hardware was pissed away by the way the underlying apps used the font rendering subsystem (page maker invalidated the font cache every line, quark did an N-squared thing, etc)
my big discovery was that for the sorts of workloads that our customers were using 99% consisted of just 4 operations
Those are the same operations that most if not all of even the very cheapest "2D-only" integrated graphics are capable of, starting from the early 90s up to today.
well this was 1989 ..... as I said prior to this state of the are was fast vectors for CAD, and before any of that we didn't have many per-pixel framebuffers (memory was so expensive so you got stuff like CGA/etc or plain ttys)
I love Retrocomputing and it sounds like an awesome thing to look at.