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Thread: 8259A PICs in PC, XT and AT

  1. #11
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    Well, so far I have tested various 8259As, but no real Intels yet. Only NECs or integrated solutions (Faraday FE2010, Headland HT18/C, and some ALi 486 chipset).
    So far only the Headland-variation had slightly different behaviour. The NECs should have been pre-1985, but don't behave like the Headland does.
    Which makes me wonder what real Intels from pre-1985 DO behave like.
    A simple test-program with Turbo C++ source can be found here, if anyone is interested: https://www.dropbox.com/s/0h6p3fpmiy...8259A.zip?dl=0

  2. #12
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    I've put up my blog here for now: https://scalibq.wordpress.com/2015/1...-all-relative/
    I may add to it as testing reveals more quirks of the 8259A.

  3. #13
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    Did you try 8259 in popular Chips and Technologies 82C206 and clones ?


    P.S. There was also a bug in the AMD 9517 DMA controller used in early BM PC.
    http://www.vintage-computer.com/vcfo...T-s-and-clones

  4. #14
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    Quote Originally Posted by alecv View Post
    Did you try 8259 in popular Chips and Technologies 82C206 and clones ?
    No, I don't have any such machines myself. But if anyone cares to try, I have linked my test-tool with source and binaries above. I should add it to the blog as well, actually.
    It would be nice to have a good overview of what kind of chips you may encounter and what kind of quirks you may have. So far the Headland chipset was the only quirky one I found.

    Quote Originally Posted by alecv View Post
    P.S. There was also a bug in the AMD 9517 DMA controller used in early BM PC.
    http://www.vintage-computer.com/vcfo...T-s-and-clones

    Ah, interesting! PCs are really a mess, I'm surprised they work at all

  5. #15

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    I found an article in an old computer-magazin that mentioned "programming the hercules gfx".
    The article is in german, but the listing is perhaps usefull.

    here it is: http://forum.classic-computing.de/in...4166#post84166
    Last edited by dr.zeissler; December 16th, 2015 at 12:14 AM.

  6. #16
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    Quote Originally Posted by Scali View Post
    Well, so far I have tested various 8259As, but no real Intels yet. ...
    Which makes me wonder what real Intels from pre-1985 DO behave like.
    A simple test-program with Turbo C++ source can be found here, if anyone is interested: https://www.dropbox.com/s/0h6p3fpmiy...8259A.zip?dl=0
    I found that one of my IBM AT motherboards has two Intel branded 8259A's (P8259A with "I" symbol and "INTEL", copyright '85).
    Not "pre-1985", but because of your "no real Intels yet", I ran your test program on the motherboard.
    It outputted:

    AT-compatible machine
    New 8259A?
    Yes AEOI!
    New 8259A2?
    Yes AEOI2!

  7. #17
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    Quote Originally Posted by modem7 View Post
    I found that one of my IBM AT motherboards has two Intel branded 8259A's (P8259A with "I" symbol and "INTEL", copyright '85).
    Not "pre-1985", but because of your "no real Intels yet", I ran your test program on the motherboard.
    It outputted:

    AT-compatible machine
    New 8259A?
    Yes AEOI!
    New 8259A2?
    Yes AEOI2!
    Ah, thanks. At least my code is now officially verified on a real IBM AT, not just clones
    I have opened up some 8088 machines, and found that my 5160 and Philips P3105 both use a real Intel post-1985 8259A. They were detected as new 8259A, and AEOI was enabled properly.
    The biggest question I have left is: what happens when you have an AT with an old 8259A as slave?

    Edit: as an aside: I noticed that my 286 masked out the RTC interrupt by default. So I didn't just have to reprogram the RTC to generate interrupts for each tick, but I also had to change the interrupt mask to get it to work. Any idea if the real AT does the same thing? (I suppose the answer is hidden somewhere in the BIOS listing as well, but probably easier to just boot the system, open debug and do an in al, 0A1h to see what value it has).
    Last edited by Scali; December 16th, 2015 at 05:49 AM.

  8. #18

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    Tried on my IBMs: 5160, 5162, 5170 and PS/1 2011. All report New 8259A.

  9. #19
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    Quote Originally Posted by Scali View Post
    Edit: as an aside: I noticed that my 286 masked out the RTC interrupt by default. So I didn't just have to reprogram the RTC to generate interrupts for each tick, but I also had to change the interrupt mask to get it to work. Any idea if the real AT does the same thing? (I suppose the answer is hidden somewhere in the BIOS listing as well, but probably easier to just boot the system, open debug and do an in al, 0A1h to see what value it has).
    On one of my IBM ATs, I booted to BASIC then executed "print inp(&ha1)". 253 was the result.

  10. #20
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    Quote Originally Posted by modem7 View Post
    On one of my IBM ATs, I booted to BASIC then executed "print inp(&ha1)". 253 was the result.
    Ah right, thanks.
    So that means it indeed masks the RTC, and everything else, except for irq 9. I don't think anything is connected to that on a standard AT? But irq 2 is rerouted to irq 9, so perhaps they figured it shouldn't be used, and therefore shouldn't be masked?
    It is somewhat interesting... because this means that by default an AT won't generate any irq's above 7. So it pretty much acts like a standard PC/XT, unless you deliberately enable the extra stuff. That may have been a conscious decision by IBM.

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