Virtual PDP-11?

[Ed Lee]

I am aware of the fact that one can overlays programs on the PDP-11.
By compiling hardware management codes in programs, Berkeley
PDP-11 unix ( not BTL ) may run programs larger than the logical
address space.  However, this is only half of the story.
For applications with large data memory space, the compiler
cannot predict memory accessing patterns before executions.
Dealing with virtual data memory, the CPU must be able to save
all registers and restart instructions ( or to back-track the
effects of an instruction , or to halt the CPU while bringing
in and out data memory blocks, or ... ).

My definitions ( along with many others, but not all ) of TRUE
virtual memory systems are:

1) To access logical memory transparently ( regardless of actual 
physical memory size, usually smaller ).
2) To improve performance with increasing memory blocks ( without
changing the codes or recompiling ).

	Ed Lee
	fortune!lee

PS: One can really appreciate TRUE virtual memory systems while 
designing programs for graphic and image processings.  I can remember
the fun of declaring 2 512x512x8 virtual frame buffers on the VAX and 
the pain of using 3 512x8 line buffers on the PDP-11.