segmented memory models

[Chuck Guzis]

On 3 Aug 2008 at 0:11, Roy J. Tellason wrote:

> I seem to recall something about the use of "reserved" interrupts or similar 
> that was the reason behind that -- apparently somebody (m$?) used 'em anyhow,  
> making the '186 not an option.

Right from the beginning, Intel reserved interrupt types 0-31 
(decimal), with 0-4 having defined functions on the 8086/88.  The 186 
used 5 for a bounds exception (BOUND instruction), 6 for unused 
opcode exception (useful!), and 7 for ESC opcodes if enabled.  The 
DMA controllers used 10 and 11 for signalling TC; and the timers used 
8, 18, and 19.  External interrupts  (INT0-INT3 pins) were mapped to 
12-15.

Of course, neither the DMA controllers nor the timers were anything 
like the 8237 or 8254 from a programming standpoint.

It seems to me that someone used the 80186 in 8086 compatibility mode 
with the traditional 82xx support devices to maintain compatibility 
(Tandy 2000?).  The 80186 was quite a bit faster than the 8086 for 
the same clock speed.

It was the 80C188 that seemed to find all sorts of applications in 
the embedded world.  It was available in several variations, 
including with an on-chip UART.

Cheers,
Chuck