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Thread: DOS mysteries

  1. #41
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    But such disks were not bootable, obviously.

    Back in the old 1.x and 2.0 days, we wrote our own IO.SYS, so the check probably wasn't there.
    Last edited by Chuck(G); January 22nd, 2020 at 03:23 PM.

  2. #42

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    Quote Originally Posted by Chuck(G) View Post
    But such disks were not bootable, obviously.

    Back in the old 1.x and 2.0 days, we wrote our own IO.SYS, so the check probably wasn't there.
    The checks were added in DOS 3.20, prior versions had no validity checks at all. I just checked DOS 3.1 and there is no such code in there. My guess is that they added the standard E9h/EBh validity checks and came across some odd data disks with the 69h and had to add the 69h check as well. Who knows why the curious comment about a "direct jump" was added.

  3. #43
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    That's true--there was no standard to DOS 1.x disks wrt boot sector data. Originally, you read the second sector and looked at the FAT ID byte and worked from there.

    But then, FAT ID F9h kind of messed things up. There's code in DOS to try to sort that out.

    I've still got the IOSKEL source to the system I worked with--DOS 2.0. Looks nothing at all like the PC.

  4. #44
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    Quote Originally Posted by Chuck(G) View Post
    For all I know, the next two bytes could be 62 6d...
    Hah, a rose by any other name, eh?

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