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Old Yesterday, 00:39   #41
Bruce Abbott
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Originally Posted by pandy71 View Post
Once again - CGA run asynchronously from main CPU - it use MB XTAL but beside this there no synchronization between CPU and 6845 - you can query Status register at $3DA and check status bits but not much above this - there is no HSync, VSync information so CPU is not aware which line, which part of line is active - CPU may only check status bit
Ah, so that's why CGA can't do raster bars...

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i can imagine ROM was limited size, time was also tight,
Yes, and they had set themselves a tough task - to provide a fully featured graphics API that didn't compromise on speed for the sake of hardware independence. Ultimately that turned out to be short-sighted, but who knew how far the Amiga would go? It lasted a decade before becoming a problem.

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as Amiga was closed architecture till A2000
Not true. Electrically the A1000 was just as expandable. It was just awkward having expansion boxes sticking out the side.

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then no need of external graphic too. Probably it was then sane approach to solve most of the problems in such way.
I agree. For the market they were targeting it was fine.

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This is why i think keeping legacy separated is probably best solution - new application should sue newgraphic.library where legacy application can use old graphic.library and perhaps do happy bit banging as glue logic will combine legacy layer with new one.
That's what we effectively have now with RTG libraries, though being better integrated into the OS would be nice...

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New CPU ISA seem to be perfect sharp transition point- sadly Commodore died and as such EOT
Sad, but for the best IMO. Custom chips had no future in the market that was developing, and Motorola killed 68k so there would be nothing left of the Amiga we knew and loved except for the OS. Then Microsoft launched Window 95 and even that was taken from us. The best thing for the Amiga to do at that time was to retire and go 'retro' while it still had an identity.
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Old Yesterday, 01:38   #42
pandy71
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Originally Posted by Bruce Abbott View Post
Ah, so that's why CGA can't do raster bars...
That's why you need to waste many CPU cycles to do crude "raster bar" and still you don't know where during line your beam is.
To do something useful with this people using timer (8254/8253) but this is workaround.

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Not true. Electrically the A1000 was just as expandable. It was just awkward having expansion boxes sticking out the side.
A500 too but this doesn't count as in overall it is workaround (fragile connection).
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Old Yesterday, 10:44   #43
Thomas Richter
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Ah, so that's why CGA can't do raster bars...
CGA (and by that VGA) includes a register that indicates whether the electron beam is in the active display area, and whether it is in the vertical sync or not. That is as far as it gets, there is no indication of the raster scan position anywhere in the CGA register map. Thus, what the "raster bars" do is that they busy-wait for the vertical sync to be indicated, and then busy-wait through cycle counting until the right spot on the display is reached, then change the color through the attribute controller.



CGA and VGA are quite limited.
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