the venerable 80186 (and 80188)

On 30 Dec 2009 at 9:49, Fred Cisin wrote: Somewhat different BIU architecture on the 80188. BHE/-S7 is always high on the 188, and data's never multiplexed onto A15-A8. The execution timing's the same on both chips, according to the datasheet. So if anything is being done with AH, you don't have any way of seeing it. What *is* documented is doing word I/O on the 188--it's converted by the BIU to two 8-bit transfers from port and port+1. The behavior that the OP noted with the 186 seems to be documented in graphic form in the BIU section of the 186 Microprocessor User's Manual at: http://www.intel.com/design/intarch/manuals/27083003.pdf Not only does BHE/ signal which half of the 16-bit bus to use during outpuf, but on input, the interface circuitry is allowe to drive all 16 data bus bits, even if BHE/ and A0 signal a byte transfer. So the behavior can be reciprocal. --Chuck