MS-DOS 1.x

edited February 2021 in Product Comments

imageMS-DOS 1.x

Originally 86-DOS, written by Tim Paterson of Seattle Computer Products, DOS was a rough clone of CP/M for 8086 based hardware. Microsoft purchased it and licensed it to IBM for use with Microsoft's IBM PC language products. In 1982, Microsoft began licensing DOS to other OEMs that ported it to their custom x86 hardware and IBM PC clones.

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    In order to make 160kb and 320kb DOS disks readable in WINIMAGE,
    the following changes should be make to the floppy disk header.
    This is the first 30 bytes of the disk. You will need a hex editor for this.
    These modifications should work for most disks in WINIMAGE, and most disks
    should boot in DOS. There is one known exception, although it is readable
    in WINIMAGE, COMPAQ will not boot up if this change is made. Also this
    will not work for the infamous boot disks, since there are no files to see.
    As always NEVER attempt this on the original disks, make a copy.

    5¼" disk
    ADDRESS | BYTES | 160 | 320 |
    ------- | ----- | ----- | ----- |
    00-02 | 00-02 | DO NO CHANGE | jump code
    03-0A | 03-10 | OPTIONAL INFO | optional OEM name and version
    0B 0C | 11-12 | 00 02 | 00 02 | bytes per sector
    0D | 13 | 01 | 02 | sectors per allocation unit
    0E 0F | 14-15 | 01 00 | 01 00 | reserved sectors
    ------- | ----- | ----- | ----- |
    10 | 16 | 02 | 02 | number of FATs
    11 12 | 17-18 | 40 00 | 70 00 | maximum number of root directory entries
    13 14 | 19-20 | 40 01 | 80 02 | number of sectors in logical image
    15 | 21 | FE | FF | media descriptor byte
    16 17 | 22-23 | 01 00 | 01 00 | number of sectors occupied by a single FAT
    18 19 | 24-25 | 08 00 | 08 00 | sectors per track
    1A 1B | 26-27 | 01 00 | 02 00 | number of heads
    1C 1D | 28-29 | 00 00 | 00 00 | # of hidden sectors
  • @Mousy54 Thanks! I'll use it when I need it.
  • In other words, you just add an appropriate BIOS parameter block (BPB) to its standard location from DOS 2.0 and later.
  • AH, I don't know, could be.
    Although it is probable they are out there, I don't recall ever seeing a 160kb disk in DOS 2.x and above and as I remember all the 320kb disks I looked at cannot be naturally seen by WINIMAGE. I have also seen 180kb and 360kb that cannot be seen by WINIMAGE.
    What I did is read up about all the specs for all DOS disks from 160kb to 2.88mb.
    Once I understood how the disks where laid out, I created a small table listing all the parameters. After that I thought why not try it out on the invisible disks and it worked. Since I have not seen it anywhere on the internet, I thought I would share the information for others to use if they wish to.
  • The disks themselves aren't really "invisible", WinImage just relies on the BPB exclusively to figure out all the parameters. Without the BPB, you'd have to check the media descriptor byte in the FAT, image size, etc. to be sure what kind of disk it is, which WinImage doesn't bother doing.
  • I know that they are not invisible, just called them that, bad chose in words I guess. Yes, that is what I kind of figured out after all my reading.
  • edited March 2021
    BTW: MS-DOS 1.25 [Zenith Z-100 PC] (5.25-320kb) contains 360kb images, not 320-kb images.
  • Thanks for pointing that out. I've updated the download. It is actually supposed to be 320k, but that was a crufty copy. Oddly this adaptation of 1.25 actually supports 360k.

    This 1.25 floppy disk was actually bundled with Zenith's MS-DOS 2.11, I presume to increase software backwards compatibility for their machine. So it is also in the 2.11 download, but I've separated out a copy to make it easier to find.

    The file contents of the crufty copy were otherwise identical.

  • edited March 2021
    I thought it should be a 320k. I adjusted my copy to 320k before I commented and mostly it matches your new copy.
  • For MS-DOS 1.25 [Compaq OEM r1.10 Rev B] (5.25-320k):
    the MS-DOS 1.25 [Compaq OEM r1.10 Rev B].txt file states "This archive contains one 320k disk image in raw and ImageDisk disk format.
    The imageDisk is missing.
  • Thanks for pointing that out. I usually include an ImageDisk image with any disk that can not open in WinImage (which includes most DOS 1.x disks).

    I'll get that re-uploaded but in the mean time, you can convert a 320k IMG file to Imagedisk using the command:
    BIN2IMD DM=5 N=40 SS=512 SM=1-8 /2
  • Thanks
    Yes I noticed that almost all of them include an ImageDisk file.
    I'll give BIN2IMD a go.
  • "The command interpertor supported the following commands" needs to be "The command interpreter supported the following commands"
  • Ok, corrected. Thanks for pointing that out.
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