[stella] cosmic ark stars missile 1

[Eckhard Stolberg]

The Cosmic Ark starfield effect should work for all moveable objects
alike. Also in your example code you should be able to replace the
#$60 with all other values exept #$70 of course. The effect isn't that
difficult to understand, if you know how the horizontal motion works
in the TIA.

The TIA is generating 228 clock pulses in each scanline (160 during
the visible part of the screen and 68 during the horizontal blank).
At every fourth of these TIA pulses a motion pulse is generated too,
which gets fed into the horizontal position counters of the moveable
objects under certain circumstances. This way the horizontal position
counter gets more clock pulses than the normal 160 it gets during the
visible part of the screen. This will result in a left shift. The
extended horizontal blank will prevent the normal TIA pulses to go
through to the position counters. Therefore the counter will get
fewer clock pulses than normal, which will result in an eight pixel
shift to the right.

When you trigger an HMOVE command, the latches that will allow the
motion pulses to go through to the position counters will be opened
for all moveable objects. Also a counter from 0 to 16 will be started
which gets advanced on each motion pulse. This counter continuously
gets compared to the values in the HMxx registers (with the highest
bit inverted). If there is a match, the latch for that object will be
closed and no more motion pulses can get through. So a HMxx value of
$80 means no motion pulses will get though, because the value matches
the initial state of the counter, and a value of $70 means all 15 motion
pulses will get through to the position counter of that object.

The tricky part is that the HMOVE counter will count from 0 to 15 only
once. And a latch will only close on a perfect match between the counter
and the HMxx value. So if you set the HMxx register to a high value
like $70 initially and then wait until the HMOVE counter has progressed
a bit but not reached $f0 ($70 with the top bit inverted) yet, you can
write a value that is smaller than the current state of the HMOVE counter
into a HMxx register and the compare will never match. Therefore this
object will keep getting motion pulses fed into it's position counter
until a new HMOVE command can close the latch.

During the visible part of the screen the motion pulses will overlap
perfectly with the normal pixel clock pulses and therefore have no effect.
Therefore the object will move 17 pixels to left in each scanline, which
is the number of motion clocks that fit into the horizontal blank. On
the TIA89 (the one that has problems with Kool Aide Man) however the
motion pulses will overlap with the normal pixel pulses in such a way
that the pulse will be cancelled out. Therefore the position counter will
get fewer pulses which will result in extra shifts to the right. As a
result the starfield effect will have a different pattern on these VCSs.

The pixel clock pulses are also used to advance the signals in the object
output circuit. Here (and that is true for both types of TIAs) the motion
pulses also overlap in such a way that the signals get delayed. That is
why you get some lines with no 'stars' and some lines with double wide
'stars' in the Cosmic Arc starfield.


Ciao, Eckhard Stolberg

----- Original Message ----- 
From: "Andrew Davie" <andrew at taswegian.com>
To: "'Atari 2600 programming list'" <stella at atari2600.org>
Sent: Tuesday, January 15, 2008 2:17 PM
Subject: [stella] cosmic ark stars missile 1


>I don't understand the magic behind cosmic ark stars, but I am using it
> successfully for Boulder Dash to give the extra-life shimmer.  However, 
> the
> code I use is using missile 0 and I really want to use missile 1.  But 
> when
> I change things from missile 0 to missile 1, I no longer get 'stars'; 
> just
> 'stripes'.  In other words the effect doesn't work.  I'm pretty sure it's 
> to
> do with the magic numbers in the following code.  I just don't know what
> magic numbers to use for M1 (or for ball).  Can anyone help?
>
>                jsr rndm
>                and #7
>                clc
>                adc #4
>
>                TAY
>         ;       STA     WSYNC
> .r22             DEY
>                BPL     .r22
>                STA     RESM0
>                LDA     #$70            ;this value is important for the
> effect
>                STA     HMM0
>
>                STA     WSYNC
>                STA     HMOVE
>                JSR     Trick2           ;waste 18 cycles and load move
> value
>                STA     HMM0            ;this is the tricky part
>
>
>                LDA     #$02            ;turn missile0 on
>                sta WSYNC                       ; 3
>                STA     ENAM0
>
>                rts
>
> Trick2:          NOP                     ;the tricky subroutine
>                NOP
>                LDA     #$60
>                RTS
>
>
> The above gives a reasonable random effect for me.  If I change the lda 
> #$60
> then it stops working.  If I change to missile 1, it stops working.
>
> Could someone please analyse this and/or suggest the correct values?
>
> Thanks
> A
>
>
>
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