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RAM : 736KB on IBM PC 5150

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    #16
    Originally posted by Trixter View Post
    Some DOS versions won't let this work even if it's valid. I did the same thing (added 64K to A000 in my 5160) but IBM PC DOS 2000 refuses to work (I can't remember if it hangs on the reboot, or if it simply doesn't use the extra).
    I wonder if this is related to an issue I've seen with the same DOS on the Tandy 1000's with DOS loaded high; if I run a piece of software that uses the Tandy graphics modes (which requires changing the top of the DOS memory space downwards at least 16k from the boot state) "something" gets clobbered with DOS'es memory management because running "mem /c" will no longer show the contents of UMBs, and commands like "loadhigh" no longer work. Drivers and whatnot loaded high previously continue to work, but I will say non-scientifically the computer gets a little "crashy". (I generally reboot after running Tandy graphics programs just to be safe.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

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      #17
      Originally posted by Eudimorphodon View Post
      ..if there's any software out there that actually hits that phantom page then you're asking for trouble having RAM from the expansion card sitting behind it.
      You would have to write twe different values - one to the main CGA window and a different one to the aliased window / back to back - then read from either. It's a very contrived use case. I doubt even a memory pattern tester would ever do that. Anything that would, would be a test program inspired form this thread. Even then, it's not a zero impedance short. It would have the resistance of the traces/connectors from one opposing driver (CGA card) to the other (memory chip) + the ESR of the transistors themselves.

      At any rate, it's moot. I disagree it would harm anything by doubly mapping RAM like this.

      Originally posted by Trixter View Post
      Some DOS versions won't let this work even if it's valid. I did the same thing (added 64K to A000 in my 5160) but IBM PC DOS 2000 refuses to work (I can't remember if it hangs on the reboot, or if it simply doesn't use the extra).
      Are you sure it's a DOS issue and not just a BIOS scan issue? I'm pretty sure most of not all DOS versions always trust the top RAM value in the BDA (via Int 12h) for the limit of memory they can use. Even if there is more RAM detected there, you'd want DOS not to use it if BIOS has it reserved for other purposes (like the MDA adapter!) For example, the PCjr BIOS limits it's scan and thus BDA reporting to 640K - even when a side-car adds more.
      Last edited by eeguru; July 1, 2020, 10:19 AM.
      "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

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        #18
        Originally posted by eeguru View Post
        Are you sure it's a DOS issue and not just a BIOS scan issue? I'm pretty sure most of not all DOS versions always trust the top RAM value in the BDA (via Int 12h) for the limit of memory they can use.
        It's not a BIOS scan issue because the BIOS stops at 640k on the system I'm testing. The use of the extra memory is changing the BDA value and soft-rebooting. IBM PC DOS 7 just hangs when I do that. I'll test other DOSes to see what happens.
        Offering a bounty for:
        - A working Sanyo MBC-775 or Logabax 1600
        - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

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          #19
          Originally posted by Eudimorphodon View Post
          So, yeah, if there's any software out there that actually hits that phantom page then you're asking for trouble having RAM from the expansion card sitting behind it.
          I know of at least one piece of software that does... 8088 MPH. It's quite useful if you're reprogramming the CRTC start address register - no matter which address corresponds to the top left, you have a contiguous range of addresses corresponding to the screen.

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            #20
            Originally posted by eeguru View Post
            At any rate, it's moot. I disagree it would harm anything by doubly mapping RAM like this.
            I'm sold that's unlikely to physically harm anything and I'll agree it's probably a "contrived" use case that someone would write to location X in the CGA space and then read the same cell from address X+16k (and care that it's different), but *technically* it is a compatibility-breaking condition, so it seems like from that standpoint it's pretty hard to argue that having overlapped RAM is in any way a "best practice".
            My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

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              #21
              I would assume that both chips will almost never switch at the exact same moment. So there is always some time where the drivers work against each other (slightly increasing current), even if they arrive at the same result shortly thereafter. Which for me is additional stress on the components, even if it doesn't harm in the short term.

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                #22
                Originally posted by Trixter View Post
                I'll test other DOSes to see what happens.
                704K.COM just tests to see if there is memory at A000; if there, it adjusts the BDA and then warm reboots. So, what happens on various DOSes? On my 5160 with 64K at A000, and an empty CONFIG.SYS file:

                MS-DOS 3.3: Successful, 651K free RAM after clean boot and running 704K.COM
                MS-DOS 6.22: Successful, 642K free RAM after clean boot and running 704K.COM
                DR-DOS 6.0: Produces erroneous error message "BOOT Diskette defective. Press F1 to retry or any to continue." after 704k's reboot. (F1 repeats message, other key goes to ROM BASIC)
                PC-DOS 7.0 Revision 1 (IBM PC DOS 2000): Locks up the system after 704k's reboot

                So I'm not sure what the unsuccessful DOS versions are doing, as 704k.com is extremely simple and doesn't do anything wacky. Maybe the typical warm boot process (set the BDA flag, jmp to fff0:0) doesn't work on all DOS versions, perhaps.

                Why is an additional 64K lower DOS RAM appealing to me personally? I like to code in the Turbo Pascal IDE, and it doesn't use UMBs, only EMS. While I have an EMS board installed, it doesn't free up all that much lower DOS RAM when using the TP IDE. An extra 64K would mean I could debug my larger projects on the 5160 itself.
                Offering a bounty for:
                - A working Sanyo MBC-775 or Logabax 1600
                - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

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                  #23
                  There are some non-PC DOS releases that went further, I have seen all the way up to 768KB conventional RAM for instance. However, most of those machines were phased out and replaced by PC-compatible machines somewhere around DOS version 3.

                  Of course those machines didn't neccesary have graphics memory in the A and B segments, or any standard BIOS. In some cases IO.SYS managed the memory-count on boot instead.

                  768KB could be possible for the PC as well, if someone made a BIOS-overlay to use, say for example, the naive mode of the PGC as display output.
                  Current systems owned by me:
                  Vintage:IBM PC/XT submodel 087 ( 1983 ), [Kon]tiki-100 rev. C (1983), Compaq Portable I ( 1984 ), IBM PC/XT submodel 078 ( 1985 ), IBM PC/XT286 ( ~1986 ), 3x Nintendo Entertainement Systems ( 1987 ).
                  Obsolete:Commodore A500 ( ~1990 ), IBM PS/2 model 70/386 type 8570-161 ( 1991 ), Atari Lynx II ( ~1992 ), Generic Intel 486SX PC ( ~1993 ), AT/T Globalyst Pentium w/FDIV bug MB ( 1994 ), Compaq 486DX4 laptop ( ~1995 ).

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