[djmips]

It could be added and it was a feature on the Gamecube CPU for example - you were able to lock the cache and then, IIRC, half the cache was used as normal and half was at your disposal for very fast access to look up tables or data you were using over and over again. Some code to deform 3D meshes when animating them using a skeleton used the locked cache to speed up that operation greatly by having the bone matrices at hand.

A problem I imagine on modern processors is the cache might be shared amongst several cores and interrupts etc.

Back in the nineties, 3DFX made a speed of light rendering demo for the Voodoo1 where they took over the entire machine and through intimate knowledge of cache behaviour effectively set aside an area in the L1 cache that would contain some data they could stream in and speed up their triangle setup.