phil@pepsi.amd.com (02/23/90)
Actually, this is general information on 16-bit devices on AT buses.
The AT bus allows either 8 or 16-bit devices to be used. In order to
tell the difference, the selected device must return a signal which
declares it capable of performing a 16-bit memory operation when
it sees its address on the bus. The problem is that there are two sets
of address lines, the old XT ones and the newer and faster AT ones. The
required timing of the 16-bit ack means you must use the new fast AT
address lines. But these lines only go down to A17, meaning a block
of 128Kbytes.
Suppose you have an 8-bit EMS board at D000:0-FFFF and a 16-bit VGA
BIOS at C000:0-7FFF. If your VGA board goes and asserts the 16-bit
ack for any address between C000:0-D000:FFFF, then an EMS access
will be trashed.
Does this kind of behavior correspond to any observed problems?
--
Phil Ngai, phil@amd.com {uunet,decwrl,ucbvax}!amdcad!phil
When guns are outlawed, only governments will have guns.