Twenty years ago, Sun had special RAM in their Elite3D cards to do write-only alpha blending and z-buffering (saturated updates), instead of the much slower read-modify-write: one PCI write instead of one read + CPU computation + one write. That might have been UPA or even Sbus instead of PCI, but you get the idea. ATI and E&S later used it, too.

http://www.michaelfrankdeering.com/Projects/HardWare/p3DRAM/...

This looks like a variant of the Sun/Mitsubishi work, after a quick skim.