On Thu, 10 May 2018 09:57:22 +0200, Bernd Oppolzer <[email protected]> wrote:
>just for the record: > >to support 32 bit virtual addresses, it is not really needed >that the underlying hardware supports 32 bit real addresses. >It would be possible to support 32 bit virtual on a 31 bit real machine, >if the DAT tables accept 32 bit virtual addresses as arguments >and yield 31 bit real addresses. So your argument, that the address lines >on the real IBM hardware only supports 31 bits is a weak one. Thanks for adding that clarity. Yes, at any point in time IBM could have added a PSW.31 bit to their allegedly 31-bit hardware which would have seen the entire upper 12 bit bits of the address used as an index into the segment table, and voila, 32-bit virtual addressing. >That said, Paul's statements are anyway strange sometimes, because >he claims that his operation system (which he calles PDOS, IIRC) >will not make use of address translation ... PDOS/370 uses a S/370 DAT allowing for the use of 64 MiB of real memory for 16 MiB address spaces. PDOS/380 uses a split DAT (S/370 below 16 MiB and XA above 16 MiB). PDOS/390 uses a pure XA DAT. I intend to change PDOS/380 so that it no longer really uses DAT, but I have not yet done so. Future PDOS developments are totally irrelevant to the discussion on allowing 32-bit virtual memory for 32-bit programs (radical!) though. BFN. Paul. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
