3 Aug
2008
3 Aug
'08
11:47 p.m.
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