Some "inside" informations...
Subject: PM-transparency etc.
Szia Karesz,
Yesterday I've checked the PM-transparency problem; Actually I wasn't correct
when I wrote to you about this last year.
Unfortunately the thing looks not so good; only the four missiles can be
made transparent on a Gr.9 screen, and only if the GPRIOR register is set up
as follows:
- set GTIA mode to Gr.9 (bit 6=1, bit 7=0)
- set priority to 0 (bits 0-3 = 0)
- set 5th player mode for missiles (bit 4=1)
so GPRIOR has to be set to $50.
Then the color of the overlapping area will be:
color of missiles ($d019/$02c7) logical-ORed with
color of background ($d01a/$02c8).
I'm not 100% sure about the truth of the last sentence, but this was
my impression. I investigated these PM priority modes a couple of years ago,
but I forgot to write down anywhere - the same as for the HIP effect ;-)
Anyway, I have some older routines that I wrote in Atmas II, which make use
of this effect, so I'll check them out again, and make a small doc about it.
But you can also check it yourself.
BTW, I've also got Atari Intern, and it is one of my primary sources
about the Atari... It has the most comprehensive description of the Atari
hardware, which I have (I don't have the Atari Hardware Manual).
Of course (?), even Atari Intern does not describe things like this
priority-0/PM-transparency. (to say nothing of HIP, for example)
But doesn't matter, we have left with enough new things to explore. ;-)
BTW, we should collect all the undocumented features of Atari in a document,
(that is, make them *documented* ;-) )
and put it online on a web page to allow everybody to profit from them.
What's your opinion?
As for the illegal opcodes on your page, I also have a couple of other
docs describing these. There are some differences in the description of
some of the opcodes, especially for the more complicated ones - probably
because they can do really unpredictable things. (Known only for those,
who know the inner workings of the 6502.)
When I wrote MoniTOM, my monitor program back in 1992, I investigated most
of these instructions, and included them in both the assembler and
disassembler part of the program. (So you can use all the 256 instructions
with it!) Now, that I'm redesigning MoniTOM, have another chance to clear
this topic up - I plan to make redefinable even the mnemonic names of the
opcodes by a config program - this can be useful because the same opcode
is mentioned on 3-4 different names in different sources.
To tell the truth, I've used only a couple of these instructions so far.
Mostly in the boot blocks of Veronika and CE. But never in effects. I know,
Sanyi used the LAX ($xx),Y instruction in the landscape routine in Joyride.
(This is why it won't run on the 65816 turbo cards... ;-) )
Do you know where I can find this APE-debugger you wrote about?
Because I can't find anywhere.
I'd be really interested in it as you can bet... ;-)
Have you managed to depack the mails?
OK, that's all for now.
Udv, Tamas
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Tamas Bene ---
/__ __ __ _\
/ / /_|_ | |__\ Software, Hungary
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Last changes: 18 maj 1998
Feel free to contact me for any legal reason!
HeAvEn, Member of TaquarT