segmented memory models

[Chuck Guzis]

On 4 Aug 2008 at 1:19, Roy J. Tellason wrote:

> I'm a little curious about all those restart instructions,  anyhow,  and why I 
> never saw any real use of them much.  I seem to recall one of them being used 
> for a debugger and that was about it.  Then there's the extra ones you get 
> with the 8085.  Anybody know of other uses for those?

There aren't any "extra" RST instructions for the 8085, just new 
vectors.  Recall that the 8085 has several external interrupt pins 
(IRQ 5.5, 6.5 and 7.5), which simply simulate calls to locations 
002C, 0034 and 003C respectively.

One early use for the RST instructions was before the day of the 8259 
PIC.  You could implement a rudimentary vectored interrupt scheme 
with a priority encoder and a buffer to gate a 1-byte RST instruction 
as part of the interrupt acknowledge protocol.  Note that the RST 
instruction takes the form 11nnn111, where nnn is the "number" of the 
RST.  A hangover from the 8008, whose RST was encoded 00nnn101.

Cheers,
Chuck