ISA vector generator card...

Hi all,

Wanted to bounce a couple ideas off you techy-types.

I have a decision to make for how to hook the ISA bus up to the IDMA
port on the DSP. I've decided to I/O map it, since that should be the
least-likely to hose any other PC functions. (I don't really want to
steal memory addresses to memory map it...)

Anyway, I can either hook the thing up as a 16-bit ISA card and directly
write to the IDMA port 16 bits at a time, or...

I can hook up an 8255 PIA to an 8-bit ISA port and use the 24 lines of
I/O on the 8255 to make a 16 bit bus + control lines and load the DSP
that way.

Direct to the ISA bus would be the fastest way to transfer data. With
an 8MHz bus clock, ISA can move about 3.2Mbytes/sec with I/O. That
means it would take about 9.7ms to load all 16Kwords of DSP RAM. Since
very few applications would need to re-load ALL of the vector RAM every
frame this is probably plenty fast. (Like I said, that's about 4000
vectors, and the only thing that I think comes close is the Death Star
explosion in Star Wars-- NeilB said that was about 3-4K vectors. ? Been
a while, I might be remembering that incorrectly.) The downside is that
it's a 16-bit ISA card (which isn't really all that bad).

Now if I just made an interface out of an 8255 PIA on an 8-bit bus, that
would really cut the throughput down-- maybe take ~70ms to load all of
DSP RAM. (That's like 14 frames per second, but only if you're
re-writing ALL the RAM. If a "real" game uses maybe 25% of that, that's
about 18ms to reload everything) The upside to the 8255 PIA approach is
that anything that can talk to the 8255 could be a "host" to the vector
generator. I'm not sure what this buys us though...

Any suggestions?

-Clay

Clayton N. Cowgill Engineering Manager
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Received on Tue Mar 24 13:08:09 1998