At 11:47 27-10-99 +0200, Shevek wrote:
>On Tue, 26 Oct 1999, Marat Fayzullin wrote:
>
>> Before you start using GB CPU similarities to Z80 as an argument, I have
>> to warn you that GB CPU is has slightly *different* set of opcodes than
>> Z80, and Z80 has no ability to interrupt on certain opcodes chosen by the
>> programmer. So, you are back to square one, emulating GB CPU on MSX CPU.
>
>I would not know if it is possible, but the similarity does for sure make
>it easier to emulate it. Of course you need to check all opcodes (IN's and
>OUT's will be very different), but it is very fast if you can say: with
>all these opcodes we can just execute the command directly. As I said
>before, I don't know if it is possible to emulate it at reasonable speed
>(It seems that it isn't), but it is a very good argument the cpu's are
>alike.
Yeah. For instance, you can keep all GB CPU registers (including flags) in
the alternate Z80 registerbanks permanently. The GB CPU PC and SP can be
kept in Z80's IX and IY.
Also Gameboy uses 16kB ROM switching, easily converted to MSX memorymapper
switching.
Also, other computers normally don't have patternmodes, MSX does. For V9990
it would be easy to store all patterns, this means hardware tile-caching!
Even with SCREEN 4 it would be possible if you reduce colors per tile from
4 to 2.
As a result of this patternmode, MSX doesn't have to build the entire
screen every frame. And certainly not pixel-by-pixel, like PC's do!
MSX also has the advantage of hardware scrolling and VDP commands.
I'll immediately admit it wouldn't be possible to emulate the GB (let alone
GB Color) perfectly on current MSX systems, but an 'approximation' is
definately within the possibilities I believe.
Even if it could run only 1 game at 10% speed, it would be impressive, right?!
Greetz,
Patriek
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