Re: Apple II GS Questions

[spec@vax2.concordia.ca (Mitchell Spector)]

In article Jeff Lemke <lemke@terra.cira.colostate.edu> writes...

>Brian Hammack wrote:
>> It compares to a //c.  The GS does 4096 colors, 11 voices, 8 megs RAM,
>> runs 16-bit forked files, slices, dices, julliens, and plain rocks.
>
>The IIgs may have a palatte of 4096 colors to choose from, but it cannot
>display more than 16 of those colors at a time at most, if I remember
>right,

    In pratical terms, without _any_ pixel placement restrictions
given, that would be that would be correct. You can use 16 colors
out of the 12-bit (4096 colors) palette at any given time.

    However that limit is 16 colors per SCANLINE, not per SCREEN.
Without taking up any extra CPU time, you can have 16 different
color palettes which each hold 16 different colors, which makes
it possible to display 256 colors on screen (16 * 16 = 256). This
is essentially like 16 independent pictures, pasted one below the
other in vertical strips. You do truely get 256 colors on screen
at once, but your extremely limited in where you can place all
those colors (I can tell you from experience, it is a huge pain
trying to paint graphics with 256 colors and only being able to
use 16 colors horizontally).

    If you really push the video hardware, you can use a different
palette on all 200 scanlines lines, making it possible in *theory*
to have 3200 distinct colors on screen (16 * 200 = 3200). Unlike
the above, this is very CPU intensive and virtually disables the
machine. Most of the time you end up with far less than 3200 colors
because palettes/colors get repeated on different lines (in the 
case of a GIF for example).

>and displays even fewer colors at higher resolutions.

    Yep, when talking about no pixel placement restrictions once
again, your limited to 4 colors in the 640x200 mode. It is possible
to have 16 dithered colors because of the way this mode functions,
but colors look sick and pale IMO, and the affects of dithering get
undone on modern displays (higher dot pitch).

    This again is a colors per scanline limitation, since you can
change palettes 16 times (64 pure colors) or 200 times (800 pure
colors). To date, I think I've seen a total of one program use
multiple palettes in the 640 mode. Most programs use that gross
16 color dithering mode.

>Not enough video RAM, I guess.

    The VGC doesn't have any dedicated video RAM. I think it uses
memory from main RAM (32K I believe). I'm not so sure that is the
reason more colors per scanline can't be used, it is most likely
a limitation of the VGC controller itself.

>That's a real graphic display crippler,

    Yep, the GS really should have been able to do 256 colors per
scanline from the start, I can't imagine it would have been too
difficult to have engineered that (or what it desparately lacked,
400 lines of resolution vertically, which was the real crippler).
If it supported 320x200x256 and 640x400x16 back in the late 80's,
more people would have taken it seriously. Of course the GS Finder
would have looked pixel for pixel like the Macintosh Finder, so
maybe that is why it the VGC never got updated.

>but it can be cured with a rather expensive Second Sight board.

    In theory, but that has yet to happen since so few pieces
of software have been written to support it. :)

>Also, it seems I heard from someone that there is some kind of a
>drawback or two to having more than 6 or 7 MB of RAM, but I don't
>remember what that is.

    You mean 4 or 5 MB of RAM. The IIgs will only DMA the first
4 megabytes in finds in the memory expansion socket, so that is
a problem if you have a board in your system that uses DMA. The
ROM 3 motherboard comes with 1 MB built-in, so you can have up
to 5 MB of DMA memory in that system.

    I personally don't see a need for more than a card 4 MB card 
in the IIgs, other than gee-whiz value  (there are a few rare 
exceptions to that,  but it should be stressed they are very, 
very few).

Mitchell Spector
spec@vax2.concordia.ca