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Thread: Seeking AT&T 6300 IRQ, DMA and BIOS Information

  1. Default Seeking AT&T 6300 IRQ, DMA and BIOS Information

    1. Is there a DOS program that can read the IRQ table and the DMA listing to show interrupts and addresses are assigned to an AT&T 6300 XT type computer system using DOS 6.22? I am looking for board conflicts.

    2. Is there anyway convert BIOS EPROM code back to assembly or basic or whatever was used to create it. I have a problem with parallel printing on an AT&T 6300 computer. I never had this issue before I upgraded my BIOS from version 1.21 to 1.43. I believe that there is some sort of error in the BIOS code keep in the BUSY line high or 1 even with no cable or loopback attached. I can see the BUSY line being high by using the Microsoft Diagnostics version 3 tool. I downloaded the 1.43 BIOS upgrade from the AT&T 6300 Shrine website. I am at a loss to determine why the parallel port is not functioning.

  2. #2
    Join Date
    Aug 2006
    Chicagoland, Illinois, USA
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    INFOPLUS (freeware) or CheckIt (there are old DOS versions floating around) should be able to do this for you in a basic sense, but it's better to inspect the boards you have and understand their dip switches/jumpers/etc. Is there something specific you're attempting that isn't working? We have some AT&T 6300 experts on the forum. Keep in mind that the AT&T 6300/Olivetti M24/Xerox 6060 have some hardware quirks, although keeping BUSY high is not one I'm familiar with.

    While there are ways to disassemble the BIOS, you're in luck, as both the original BIOS and the 1.43 BIOS sources are available:

    (moderators: move this thread to "PCs and Clones" if possible.)
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  3. #3
    Join Date
    May 2011
    Outer Mongolia


    I know for a fact that Check-It's ability to suss out if an interrupt is being used and correctly attributing it is pretty much non-existent. For instance, on my Tandy 1000 which has several oddities like COM2 using IRQ2 instead of IRQ3 to free up the latter for a network card, the best its IRQ display can do is it's smart enough to figure out that COM2 *isn't* on IRQ3. (Which it labels as "free" even if I have the network card driver loaded.) It does say that a "Mouse" is using IRQ2 if I have the driver loaded, which is something, I guess.

    (It also claims that my "hard disk" is using IRQ5 even though I'm using an XT-CF card that doesn't use an interrupt; presumably it's doing so because that's standard for an XT. It also lists the standard DMA assignments for memory refresh, floppy, and fixed disk even though my machine *doesn't even have a DMA controller*.)
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  4. #4
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    To throw another wrench into the picture, it's possible for some devices to share interrupts with others--much depends on the device and the driver.

  5. Default

    Thank you for recommending the Checkit and InfoPlus programs they were helpful in determining the parallel port problem as being a cable-related problem. I had an older Centronics cable that allowed the testing of the printer to proceed. Checkit kept illustrating an error with my Commerical loopback connector for parallel. I replied no to the loopback test and everything passed. The printer test was successful. I'll have some questions regarding the VGA card that I have been writing about in another post. Thanks to all that responded to the post.

  6. #6


    Check-It! 3.0 came with both parallel and serial loopback plugs in the box and is likely expecting those in its testing. I'd like to hope they are all wired in a standard fashion, but you never know.


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