Re: DVG Multigame

> > select which bank of 8K the vector generator should use.
> >* Exclusive OR the PROM/VRAM output select (one half of L2) on the vector
> > generator, and have it drive !OE on the vector EPROM (27256).
> This seems like such a simple set of requirements, it seems a shame to
> go to all the trouble of making a PAL - a couple of 74-series TTLs should
> do just as well, I would think. An 'LS86 quad-XOR gate should handle the

Well, I'm a hardware newbie, so whaddya expect? ;-)

> OE* and inverted A15 problem, and an 'LS377 octal-D flip-flop to handle the
> latching. (Actually, any old D-type flip-flop package should work, since
> you don't seem to _need_ all eight bits, or any tristate outputs.)
>
> _Maybe_ a third quad-gate chip of some kind (NAND, NOR, etc.),
> depending on how a "write to the 8000-FFFFh region" is to be decoded.
> (Personally, if there's already some kind of Memory-Write-Enable signal
> available, I would just look for A15 to go high while MEMW* goes low, and
> have that condition generate a positive-going edge to the 'LS377 latch,
> thusly: <<warning - bad ASCII art ahead! :) >>

There isn't an A15/!MEMW qualifier unfortunately. A15 isn't used at all in
production.

> This, of course, assumes that both the CPU address and data buses
> become valid a sufficient amount of time before MEMW* goes low, so the '377
> latch can respond to the change in G* stage; if there's a timing issue
> there, it would be better to hold G* low and then NOR the MEMW* and
> CPU_A15* signals into the '377s CLK input.
> I know, it's not as sexy as a PAL, but I'm a great believer in keeping
> things simple and straightforward. :)

Oh, me too. I just don't know any better. ;-)

But how would we handle the 2 to 4 address decoder/enable on the vector
generator? Two bits going in to select which part of the 1K area the
vector generator owns and 4 not bits coming out. We need to do a !OE
on 3 of the 4... wait a sec... an inverter would work here.

Thanks for the * - Lemme go rethink this...

Asteroids is relocating just fine. Asteroids Deluxe, however, is an
entirely different story. It executes code out of the vector ROM! Bleugh!
That'll be tougher to move.

-->Neil

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Neil Bradley Seti@Home Hall of Shame on a 80386DX16, Intel 80387
Synthcom Systems, Inc. 32MB RAM, Win 95 Status: 48.425% complete
ICQ # 29402898 CPU Time: 1721 Hours 14 minutes 53.4 sec

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Received on Fri Mar 10 06:31:45 2000