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PostPosted: Thu Oct 13, 2011 9:30 pm 
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Just a crazy idea to build retro 8-bit computer based on Z80 and 2C0x PPU
Discuss :wink:


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PostPosted: Thu Oct 13, 2011 9:37 pm 
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That's easy. NES PPU already was connected to ZX Spectrum in 1990s indirectly (complete with CPU, so it was possible to load and run simple NES games from a floppy), and one guy also connected PPU to a Soviet 8080-based computer directly.


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PostPosted: Thu Oct 13, 2011 9:49 pm 
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Ewww....what a downgrade. :P


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PostPosted: Thu Oct 13, 2011 11:54 pm 
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A Z80 would be able to copy one byte from an increasing source to a fixed destination address every 22 cycles, vs every 8 cycles on the 6502. So it would need to be 3x as fast to be competitive.
Plus there's no sprite DMA, that's exclusive to the 2A03.

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PostPosted: Fri Oct 14, 2011 12:04 am 
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I/O mapped - OUTI or OTIR, 16t/byte

Memory-mapped - 18t/byte with dumb code:

Code:
ld a,(hl)
inc l
ld (de),a


or 12t/byte with no so dumb code:

Code:
pop bc
ld (hl),c
ld (hl),b


Z80 is usually clocked at 3.5 Mhz, 2A03 clock frequency is twice lower. So Z80 is either as fast as 2A03, or faster, with tricky code.


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PostPosted: Fri Oct 14, 2011 12:43 am 
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I guess that beats the LDI \ DEC DE thing I was thinking of.

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PostPosted: Fri Oct 14, 2011 1:12 am 
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Shiru wrote:
That's easy. NES PPU already was connected to ZX Spectrum in 1990s indirectly (complete with CPU, so it was possible to load and run simple NES games from a floppy), and one guy also connected PPU to a Soviet 8080-based computer directly.

:shock: do you have link to this info or any photos? I guess ZX Spectrum used as "soft cart" without any mappers

Z80 have /BUSAK signal and 3.5MHz should make it equal to 2A03, but depends on code 6502 core pretty fast


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PostPosted: Fri Oct 14, 2011 1:35 am 
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No photos, all the info is in Russian. Original article was published in 1995 in a local ZX Spectrum magazine. Here is Google translated article. The device wasn't popular, very few people actually made it. The author recently gave an audio interview, he said he abandoned the idea in late 1990s, but still gets emails with questions about the device.


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PostPosted: Fri Oct 14, 2011 2:30 am 
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Looks like a hybrid of two computers :? But i thought about using PPU only with Z80 as CPU and add some RAM and ROM. Just for fun. Challenge. Not after supporting any legacy, just print "Hello, world!" maybe something else :roll:

P.S. Z80 have DMA, ZX Spectrum - not :wink:


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PostPosted: Fri Oct 14, 2011 2:54 am 
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The second thing I've mentioned, a Soviet computer, had only NES PPU directly connected, without 2A03. It was rather simple device, I think it is not a big problem to connect the PPU to any 8-bit computer, or design a new one.

There is an addon device for ZX Spectrum with Z80 DMA on-board, by the way, and also a DIY sound card with 8237 DMA. These were developed about at the same time as the NES+ZX thing.


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PostPosted: Fri Oct 14, 2011 3:54 am 
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Is there any link to article about Soviet computer? Any software or at least sample code or so?


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PostPosted: Fri Oct 14, 2011 4:31 am 
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No, it was a hobbyist project of a guy who done it in late 1990s, and posted on a forum in 2000s, text description and some schematics. It was few years ago, I can't recall where it was.


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PostPosted: Fri Oct 14, 2011 6:59 am 
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Dwedit wrote:
A Z80 would be able to copy one byte from an increasing source to a fixed destination address every 22 cycles, vs every 8 cycles on the 6502.

That depends on what kind of divider is in front. A 2A03 takes 96 cycles because of that divide-by-12 in front.

Quote:
Plus there's no sprite DMA, that's exclusive to the 2A03.

Nor audio. But just as audio can be handled by a separate chip, so can hardware accelerated memcpy.

But if you want a Z80 and something very similar in capability to an NES PPU, there's always the SMS ;-)


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PostPosted: Fri Oct 14, 2011 7:41 am 
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I want computer with keyboard :oops:


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PostPosted: Fri Oct 21, 2011 5:28 am 
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Shiru wrote:
80sFREAK wrote:
Is there any link to article about Soviet computer? Any software or at least sample code or so?
No, it was a hobbyist project of a guy who done it in late 1990s, and posted on a forum in 2000s, text description and some schematics. It was few years ago, I can't recall where it was.

It was me, LOL.
tepples wrote:
Quote:
Plus there's no sprite DMA, that's exclusive to the 2A03.
Nor audio. But just as audio can be handled by a separate chip, so can hardware accelerated memcpy.
But if you want a Z80 and something very similar in capability to an NES PPU, there's always the SMS ;-)

Z80 have OUTR/INIR. ;)


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