30 Dec
2009
30 Dec
'09
5:28 p.m.
On 29 Dec 2009 at 13:37, jim s wrote:
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.
The "if" and "where" of byte-vs-word operations are determined on the
186 by BHE/ and A0. Very straightforward. If they're both low, a
word is being requested. If either (but not both) is high, the upper
or lower byte of a word is being signalled (both high is a "reserved"
condition).
One of the attractions of the 186 over the 86 is the insanely simple
interfacing. No bus controller needed; easy to decipher signals--and
6 programmable memory chip selects and 4/5 peripheral chip selects.
My gripe was the 2-channel DMA controller. If one channel was used
for DRAM autoinitialize refresh, then that left you with only a
single DMA channel. In that respect, the NEC V50 was much superior.
--Chuck