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TT98.004
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1991-08-26
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American Megatrends, Inc.
Tech Tip #98.004
01/20/91
Problem:
8 bit card with rom and a VGA card with rom at C000. The only way to
get the card to work was to put the VGA card in a 8 bit slot.
Solution:
IBM specifications, IBM specs for PC/AT, are as follows
1. Signal LA16 is not available on the I/O bus.
2. All I/O cards are supposed to use LA<17:23> address lines
to do local chip select, etc.
3. MCS16# signal is used to distinguish that the access is to
a 16 bit device.
Case 1:
- 16 bit VGA card, C000 area
- 8 bit controller, D000 area.
When you try to access the 8 bit controller, you will access
location in the D000 segment. Since the lack of LA16 prevents
distinguishing accesses between C000 and D000 areas, the VGA card
generates MCS16# signal, to indicate that the access is to a 16 bit
device. Now you are addressing the 8 bit device, but the CPU
assumes that the device accessed is a 16 bit. This causes all
sorts of problems.
Case 2:
- 8 bit VGA card, C000 area.
- 8 bit controller, D000 area.
In this case, as in the one above, whenever you access the 8 bit
controller at D000 area, the VGA card also gets selected because
of the lack of LA16, but since in this case the VGA card is in 8
bit mode therefore the situation described above does not happen.
The only solution that can be offered at this point is to configure
the 8 bit controller in the E000 area.