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Thread: EGA video corruption?

  1. #121

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    Hi,

    I looked at unpacked keen4 and code looks similar to keen dreams. But I totally cannot understand what it's trying to do pre-crtc start. I've attached a dosbox tracelog of the area.

    Could someone explain it to me better? Thanks
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  2. #122
    Join Date
    Aug 2009
    Location
    Bavaria / Germany
    Posts
    120

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    Ok, they are using a three-way timing sensitive routine to determine the vertical blanking start. Using multiple I/O reads to determine if the display still generates visible scanlines.
    a) depending a bit on CPU speed
    b) depending on bus speed -> PCI card
    c) depending on the horizontal frequency of the display (VGA and EGA differ by factor 2 here)

    So it's not really related to differences in the register latching in the EGA/VGA.

    There would be a lot of room to use an own routine. My suggestion, if Keen doesn't reprogram it, to use the system timer 0 to wait and check for a defined timespan. EGA scanline is 64s, VGA 31s. I guess we should watch for like 20s...
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