Messages in this thread |  | | | Date | Sun, 30 Dec 2007 17:40:18 -0800 | | From | "H. Peter Anvin" <> | | Subject | Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override | |
David P. Reed wrote:
>
>
> H. Peter Anvin wrote:
>> Now, I think there is a specific reason to believe that EGA/VGA (but
>> perhaps not CGA/MDA) didn't need these kinds of hacks: the video cards
>> of the day was touched, directly, by an interminable number of DOS
>> applications. CGA/MDA generally *were not*, due to the unsynchronized
>> memory of the original versions (writing could cause snow), so most
>> applications tended to fall back to using the BIOS access methods for
>> CGA and MDA.
>>
> A little history... not that it really matters, but some might be
> interested in a 55-year-old hacker's sentimental recollections...As
> someone who actually wrote drivers for CGA and MDA on the original IBM
> PC, I can tell you that back to back I/O *port* writes and reads were
> perfectly fine. The "snow" problem had nothing to do with I/O ports.
> It had to do with the memory on the CGA adapter card not being dual
> ported, and in high-res (80x25) character mode (only!) a CPU read or
> write access caused a read of the adapter memory by the
> character-generator to fail, causing one character-position of the
> current scanline being output to get all random bits, which was then put
> through the character-generator and generated whatever the character
> generator did with 8 random bits of character or attributes as an index
> into the character generator's font table.
>
[Additional history snipped]
This is all true of course (and a useful history lesson to those not
familiar with it) but what I wrote above is still true: due to the lack
of synchronized memory (it doesn't have to be dual-ported, just
synchronized, if it has enough bandwidth), most DOS applications *in the
i386+ timeframe* just invoked the BIOS rather than dealing with the
synchronization needs themselves (anything compiled with a Borland
compiler using their conio library, for example.)
Hence the variety of software that poked directly at CGA/MDA as opposed
to EGA/VGA was smaller, but I never claimed it was uncommon.
-hpa
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