PC-card - VGA + MDA card

From: Ruud Baltissen (g.baltissen_at_hccnet.nl)
Date: 2001-05-12 23:00:45

Hallo allemaal,

Solved one problem, bumped into another one. When you insert a 16-bit 
card it tells the PC by negating MEMCS16. It does this by decoding 
A17..A23. This is a 128 KB range. So adressing the $Cxxxx range = 
CGA and MDA, this line is activated AFAIK. So this als must happen if a 
MDA card is inserted. Now see next table:

SBHE  A0  Transfer
  0       0     word
  0       1     byte D8..15
  1       0     byte D0..7
  1       1     not valid for 16 bit card

I could imagine that, when writing a byte to an odd address, you output it 
on both the lower and upper part of the databus, you could write data to 
the MDA card. But what about reading ???

FYI: SBHE is activated through a buffer by the processor. 

Any comment, even vague thoughts or ideas are welcome.


Groetjes, Ruud

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