lee@fortune.UUCP (Ed Lee) (11/29/83)
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.