[jan-jaap]

I think in this case PMAP is a little different. This is halfway the graphics pipeline, virtual memory isn't used here

It's not too complicated to recognize the blocks in the Graphics Architecture diagram in the DE3 PCB:

The XBAR (crossbar) are probably the fat interconnect dots between the image planes and the MGP (Multimode Graphics Processor)
The system does not have enough VRAM to have a 1280x1024 RGB frame buffer. Your application doesn't need to run in RGB mode either. The image planes can contain indexed color data. If it's RGB itgoes straight to the DACs, if it isn't it must pass through a color map lookup table.

IMHO, the PMAP probably integrates the MGP and the color map. It might be sort for MGP-colorMAP

NB: the Window Planes should have been inside the green area. The DE3 has space for 20 VRAM SIMMs, you can see 15 are filled in this one.
5 SIMMs make up an 8-plane increment.
You always need 4 planes for window ID and 4 planes overlays/underlays. These make up 1 bank of 5 SIMMs.
This system has two more banks filled, so it has 16 usable bitplanes. If you scroll back this thread you can see this confirmed in the terminal window with 'hinv' and 'gfxinfo'.
If this system had the last bank filled as well, it would have 24 (usable) bitplanes. You could use this for an RGB visual. But not too fast: if you want double buffering you'll have to split your 24 planes into two 12-bit indexed visuals.

Similarly, with 16 bitplanes I could run 8 bit, dual buffered.