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by PaulHoule
474 days ago
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My understanding is that the VAX from Digital was the mother of all "32-bit" architectures to replace the dead end PDP-11 (had a 64kbyte user space so wasn't really that much better than an Apple ][) and PDP-10/20 (36-bit words were awkward after the 8-bit byte took over the industry) The 68k and 386 protected mode were imitations of the VAX. Digital struggled with the microprocessor transition because they didn't want to kill their cash cow minicomputers with microcomputer-based replacements. They went with the 64-bit Alpha because they wanted to rule the high end in the CMOS age. And they did, for a little while. But the mass market caught up. |
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VMS is the only OS (that I know of) that uses all 4 processor privilege modes.
Side note: The 21064 has such bizarre IPR mappings, the read values have lots of bits scrambled around compared to their write counterparts. This is likely a hardware design decision affecting the programmer's model, if I had to guess.