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Thread: DOS File Level access to Secondary (370H) FDC - Driver Nightmares

  1. #21
    Join Date
    Jan 2007
    Pacific Northwest, USA
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    Depends on the platform, but the most straighforward way, IMOHO, is to hook Interrupt 1ch This is a dummy routine called by the INT 8 servicer and normally contains only an IRET instruction.

    Why do this rather than 08h? Because there are some utilities and games that run the 8254 with a shorter interval to get better granularity. The usual way is to get the 8254 to interrupt, say, 4X more often, then program in a divide by 4 routine in the INT 8 servicer before passing the interrupt on to the regular time-of-day routines. 1Ch occurs down the chain, so no matter what you're running, the tick rate should be constant.

    Besides, this is why interrupt 1Ch is part of the BIOS.

  2. #22
    Join Date
    Feb 2017
    Guildford, U.K.


    Quote Originally Posted by pcdosretro View Post
    You would really need a device driver which accessed the secondary floppy controller (370h-377h) for DOS to see the drives since PC BIOSes only access the primary controller for INT 13h access which is what the DOS builtin disk driver uses. Even then DOS may not recognize the disks since they aren't in a standard floppy format (160KB, 180KB, 320KB, 360KB, 720KB, 1.2MB, 1.44MB, 2.88MB). I don't recall a specific media descriptor byte code for 8" floppy drives though there were two old unused codes (FAh and FBh).

    Would it be possible to use a null modem serial cable to transfer over whatever data you have from whatever computer used the 8" floppy drives to a PC ? Seems like that would be easier than trying to get an anachronistic device from the 70s to work with a PC from the 80s.
    I remembered my first "Personal Computer", the S-100 bus Morrow Designs Decision I, had a floppy controller that supported 4 diskette drives and provided both a 34-pin 5.25" FD header and, IIRC a 50-pin 8" FD header. I never thought about the differences at the time, but all 3 diskette drives (I had by that time installed the 5.25" FH 5MB HD in place of one 5.25" FD) worked in perfect co-operation. The 5.25" diskettes were 10spt, hard-sectored, but that's a minor issue in retrospect. I remembered the 8" (Tandon TM800?) drives were capable of 800KB double-sided , maximum.

    Media Descriptors:

    Last edited by WBST; January 22nd, 2020 at 01:07 AM.

  3. #23


    I hope to have the motor off timer implemented this weekend. The roadmap after that is:
    1) correct crashing in the error handler
    2) get formatting working
    3) make build with DSDD 8" parameters
    4) abstract driver to allow choosing between drive types with command line switch at loading

    Got a set of 10 NIB DSDD 8" disks for testing on ebay for $15 shipped, so that's nice


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