29 Dec
2009
29 Dec
'09
1:37 p.m.
If I recall, the rule is that the S lines or equivalent will tell you
what to take on the output. I don't know if it is a violation to drive
the other 8 bits or not, but your hardware should only be told that
there is an 8 bit transaction going on.
Jim
Phill Harvey-Smith wrote:
While we're on the subject of 80186s I discovered
some odd (well
undocumented) behavior whilst experementing on my RM Nimbus (as I'm
writing a MESS driver).
If I load a value into AX, and then do an out instruction with AL, the
whole of AX seems to still appear on the data bus, something like:
MOV AX,055AAh
OUT 022,AL
It looks like to me that the whole of AX is actually getting put out,
and that the /BHE ine is set to signify only the bottom 8 bits are
valid. The Nimbus video controller seems to ignore /BHE and treat the
data bus as always 16 bits wide.
Is this known undocumented behavior ? I seem to remember that the Z80
has a similar undocumented behavior with the addressing of it's in and
out instructions also.
Cheers.
Phill.