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IRQ queery

Discussion in 'A+' started by robbo1962, Apr 8, 2006.

  1. robbo1962

    robbo1962 Byte Poster

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    Hi all, i am studying the IRQ section and have come up against a problem , as you know each peripheral has a IRQ number to communicate with the CPU, my problem is this, in the days when computers sometimes had 2 floppy disk drives how would you transfer data from one to the other given that there was only 1 IRQ assigned to the floppy drive (IRQ 6) as they both use the same interface, as far as i know there is no master/slave settings for floppys.Thanks in advance robbo1962
     
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  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Actualy - there is - sort of. Floppies have a setting to drive 0 or drive 1. The twist that was in the old cables was there to 'swap' the drive numbers. This was a trick started by IBM - they made all drives drive 1 and used the twist so that the drive number as it appeared to the controller depended on where on the cable the drive was plugged.

    Harry.
     
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  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Note that the Meyers book has a photo of the twisted cable, but doesn't really fully explain what is happening!

    Harry.
     
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  4. robbo1962

    robbo1962 Byte Poster

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    ok thanks for taking the time to answer, a bit clearer than i could find in the book. still not totally convinced that you dont need two seperate IRQ addresses,can't understand why they still produce 2 interfaces for the floppy drive anyway
     
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  5. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    There is only *one* controller for *both* floppy drives. It is the controller that needs the IRQ, hence only one. This mirrors the situation with IRQ14 which handles the first IDE controller. That controller also handles two drives.

    I'm not sure what you mean by "still produce 2 interfaces for the floppy drive", but the main reason with supporting this stuff is legacy. It is still possible to make a PC with 2 floppies.

    Harry.
     
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  6. Modey

    Modey Terabyte Poster

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    Well a few years ago, well ok maybe a bit longer than that, it was fairly common for a pc to have 2 floppy drives. You could even have a 3.5" drive and a 5.25" drive on the same cable (they used to come with 4 connectors on them, but still only 2 drives max could be connected.).

    It was easier to copy disks if you had two drives the same, no swapping source & target multiple times (you had to do this in DOS to copy a disk on one drive).

    I agree that these days two floppys would be unlikely and probably uncessary on a PC, but they still support the feature. Most motherboards now give you a floppy cable with a single connector on it, just for one drive, but if you put an old style cable on it, you could have yourself a coupple of 360k 5.25" drives if you were really lucky!
     
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  7. r.h.lee

    r.h.lee Gigabyte Poster

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    modey,

    You mention "If I were lucky." I had a bit of a practical "extreme A+ final exam" on floppies, cd-rom drives, and hard drives.

    So my goal was to install "Windows NT Server 4.0 evaluation" on my 486DX4/100 computer. At the time, I was studying towards my MCSE. Well, the evaluation required at least Windows 98 SE. So I had Windows 98 SE on CD. It was also the "Upgrade" version. So I say, great, so I need a full install OS before that. Well, I had some 3.5" floppies for MS DOS 6.22. Guess what? They were for MS DOS 6.22 Upgrade. So guess what? I had MS DOS 5.0 on 5.25" floppies. Fortunately, it was the actual "full install" version.

    So here's what I had to do.
    1. Plug in my 5.25" floppy drive to my computer using the twisted floppy cable.
    2. Boot my computer from the MS DOS 5.0 floppies disk.
    3. Format C:
    4. cd c:\
    5. md dos
    6. copy a:\*.* c:\dos\*.*
    7. Turn off computer.
    8. Remove the 5.25" floppy drive, then install the 3.5" floppy drive.
    9. Boot computer back up.
    10. Boot from the 3.5" floppy drive and upgrade MS DOS 5.0 to MS DOS 6.22 usng 3.5" floppy drsks.
    11. Use Soundblaster's Sound Card driver on 3.5" floppy disk to install drivers for the Soundblaster audio card which controlled the CD-ROM drive.
    12. Booted up MS DOS 6.22, then changed directories to the CD-ROM drive.
    13. Ran the install program on Win 98 SE CD.
    14. After installing Windows 98 SE, removed the Windows 98 SE CD then inserted the Windows NT Server 4.0 evaluation CD.
    15. Ran the installation process for Windows NT Server 4.0.

    There's a little bit of good news and bad news. The good news was, Windows NT Server 4.0 was properly installed onto the HD. The bad news was, the sound card, and the associated CD-ROM drive, are not part of the Windows NT 4.0 Hardware Compatibility List. That means, I can't use a CD to install anything to this computer.

    To this day, the server is still chugging along. :)

    So in summary, that "little twist" may seem like a trivial matter on the A+ test, but by understanding it much better, that knowledge may come in handy one day, like it did me. :)
     
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  8. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Wow! What a procedure! I used to have something like that if I needed to restore Windows as I had a long chain of 'upgrade only' versions.

    Eventualy I got fed-up, and bought a full version! :biggrin

    Harry.
     
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  9. r.h.lee

    r.h.lee Gigabyte Poster

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    Harry,

    That's the EXACT lesson that I learned too. :)
     
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  10. Modey

    Modey Terabyte Poster

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    Heh nice work around Lee. :)
     
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  11. robbo1962

    robbo1962 Byte Poster

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    thanks for all who replied,especially hbroomhall.. the penny has finally dropped and i can now see why only one IRQ is required for the floppy drives.
     
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