-----Original Message-----
From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx>
To: stella@xxxxxxxxxxx <stella@xxxxxxxxxxx>
Date: Friday, January 16, 1998 6:14 PM
Subject: Re: [stella] Screen shots
>>>Perhaps we wouldn't need any more than 320x200 anyway. Instead of
doubling
>>>the pixels to create 160x200, we just switch from venetian blinds to
>>>vertical blinds and put the even-numbered scanlines on even-numbered
>>>columns. Piece of cake.
>
>Er, _possibly_. Definitely easy to code. Problem is that the gaps in the
>blinds would be almost twice as large (we have to fit fewer pixels in a
>larger space horizontally than vertically.) Dunno how good it would
>look...
>
>>Too cool !! But if you're running in a tweaked mode, the difference
between
>>the even pixels and the odd pixels is merely a setting in a bank-switch
>>register. It just doesn't get much easier than that. May even run faster
>>than mode 13.
>
>No, it's still quite complicated, and definitely not faster than mode 13h.
>In mode 13h, it's three assembler instructions per pixel - load into
>register, store to memory, increment memory counter.
Actually, with z26, when it's doing background or playfield, and that's what
it's doing most of the time, it's 3/8 of an instruction per pixel, two
32-bit stores in a row, then we increment the pointer by 8. At first
glance, I don't see a way to do a left-pixel right-pixel kind of thing
without slowing it down by a fair amount. Of course, that doesn't mean a
way doesn't exist.
>With mode-X, each
>pair of pixels would be: load into register, store to video memory, load
>next pixel into register, store to system memory, increment counter, then
>at the end of the scanline, it's another load/store/inc counter cycle for
>each pair - total, eight instructions for two pixels plus about another ten
>per scanline to bankswitch. Not much slower, but tricky to code (but I'm
>up for a challenge - someone let me try to hack their emulator to do it :)
I'm not an expert in these tweaked modes, that's why I'm so excited that
someone else might like to take a try at it. But given that I'm not an
expert, I still suspect that a tweaked mode would be faster than mode 13.
You can play games with the registers to split even from odd. But beyond
that, I have other reasons for wanting tweaked modes, even if they're
slower.
If you start out doing 320 by 200 tweaked, once the addressing and rendering
issues are resolved, it's a small step to 320 by 240 or 360 by 240. I want
one or the other of these modes because then you can display a tall game.
Although most games fit in 200 scan lines, many of them don't. Even combat
doesn't really fit. Also, modes that are 240 scan lines tall, refresh at 60
Hz. Then instead of fighting a 70Hz monitor trying to get it to play a 60
Hz game, you can synchronize the game with the monitor AND get correct play
speed. Many visual artifacts disappear when the game is synchronized with
the monitor.
So if vertical blinds is the initial motivator to get us tweaked modes,
great, but the payoff will show up in other ways as well.
I tried to email the source for version 0.89 to the group, but I guess that
message got eaten. Source code for version 0.88 can be found on the home
page:
http://www.whimsey.com/z26.html
I'll be posting version 0.89 shortly, but there are no differences in the
graphics so you could still start getting an idea about how things work
anyway. The rendering engine is found in a file called tia.asm. I would be
very happy for you take a shot at it!
John
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