virtual memory 4516

the earlier 3033 had a hack on this in the late 70s related to 24-bit addressing. instructions were 24-bit both real addressing and virtual address.

i've commented before that the 3033s were feeling some pressure from 4341 clusters and the 16mbyte real memory address .... i.e. you configure a six 4341 cluster, each 1mip (6mips aggregate), each 16mbyte real (96mbytes aggregaet), each having six i-o channels (36 i-o channels aggregate) for about the same cost as a 4.5mip, 16mbyte 3033.

the 370 PTE (page table entry) had two undefined bits ... the standard PTE was 12bit real 4k page numbers (i.e. 12bit*12bit ... gives 24bit, 16mbyte addressing). IDALS actually had field for doing i-o with real address up to 31bits. so 3033 came out with 32mbyte real support ... and used one of the unused PTE bits to allow defining 13bit (8192) 4k real pages. IDALs could move pages to-from disk ... above the 16mbyte line.

however, there was some number of things that required real addressing of contents of virtual pages ... and for this there was a game played with page table entries .... that had a virtual page point to a real page below the 16mbyte line and a virtual page above the 16mbyte line ... and would copy the contents of the virtual page (above the line) to the virtual page (below the line).

misc. past posts mentioning the 3033 hack for 32mbyte real storage: to Fix for small clusters addressing via paged memory? via paged memory? thereof dual-core design