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Thread: RAM : 736KB on IBM PC 5150

  1. #21

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    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.

  2. #22
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    Quote 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.
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  3. #23

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    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.

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