1990s SGI

Yes, the O2 had a unified memory model.  2D, 3D, the video decompressor, and video inputs all
wrote to pbuffers, pbuffers were used as texture inputs, and one pbuffer was fed to each video
output DAC.  It allowed things to be plumbed any which way.

SGI's higher end graphics systems at the time, Impact and Reality Engine 2, were optimized to
render directly to the screen, but could also render to pbuffers.  They also didn't have the
video inputs by default.

Yes, I worked on O2, though not on the graphics subsystems.