Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override

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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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