[stella] WIP: Salmon Run: Asymmetrical Playfield

[SeaGtGruff at aol.com]

In a message dated 3/14/2007 11:36:37 PM Eastern Daylight Time,  
thom.cherryhomes at gmail.com writes:

i'm  going to try the following approach:

use a set of pointers to read a  set of tables, that are 32x4 bytes
each, read those pointers on each pass  of the kernel, and change them
according to a random counter. since I am  using a reflected playfield,
I'll write them PF0 (which will always be F0),  PF1, PF2, PF2, PF1,
which according to my calculations, leaves enough to  stuff data for
PF0 and PF1 at least.

upon overscan, the scroll  counters will be updated as per usual, and
the kernel will loop  again.

-Thom



If PF0 is going to be #$F0 all the time, then once you set it to #$F0 just  
before the first line of the playfield (i.e., just after displaying the score), 
 you can just leave it alone until the bottom of the playfield (i.e., just 
before  displaying the "salmons remaining")-- there's no need to write to it at 
all  during the river kernel. In fact, if you don't mind using PF0 as a sort 
of "side  border," then you can just set it to #$F0 at the beginning of your 
program and  then leave it alone, letting it remain as is for the score, river, 
and lives  remaining.
 
With a reflected playfield, you don't need to write to the PF registers in  
order from left to right-- i.e., you can write to the PF registers "out of  
sequence" depending upon the earliest and latest times that you can  update them 
without having an undesired effect:
 
Left copy of PF1:
 
First pixel begins at screen position 16
= color clock 84 (i.e., 68 + 16)
= machine cycle 28 (i.e., 84 / 3)
Must be written to no later than machine cycle 27 and 1/3 (i.e., 28 -  2/3)
 
Left copy of PF1:
Last pixel begins at screen position 44
= color clock 112
= machine cycle 37 and 1/3
Must not be changed until after machine cycle 36 and 2/3
 
Left copy of PF2:
 
 
First pixel begins at screen position 48
= color clock 116
= machine cycle 38 and 2/3
Must be written to no later than machine cycle 38
 
Left copy of PF2:
Last pixel begins at screen position 76
= color clock 144
= machine cycle 48
Must not be changed until after machine cycle 47 and 1/3
 
Right copy of PF2:  
 
First pixel begins at screen position 80
= color clock 148
= machine cycle 49 and 1/3
Must be written to no later than machine cycle 48 and 2/3
 
Right copy of PF2:
Last pixel begins at screen position 108
= color clock 176
= machine cycle 58 and 2/3
Must not be changed until after machine cycle 58
 
Right copy of PF1:  
 
First pixel begins at screen position 112
= color clock 180
= machine cycle 60
Must be written to no later than machine cycle 59 and 1/3
 
Right copy of PF1:
Last pixel begins at screen position 140
= color clock 208
= machine cycle 69 and 1/3
Must not be changed until after machine cycle 68 and 2/3
 
Of course, the machine instructions take whole cycles, so you have to drop  
any fractional portions. Keeping in mind that you don't want to write to PF1 
(or  PF2) for its *right* copy until its *left* copy has finished being 
displayed,  and vice versa, this gives you the following windows for updating each PF  
register:
 
Write to PF1 for the *left* copy:
Anytime from cycle 69 through cycle 27
 
Write to PF2 for the *left* copy:
Anytime from cycle 59 through cycle 38
 
Write to PF2 for the *right* copy:
At cycle 48-- no ifs, ands, or buts
 
Write to PF1 for the *right* copy:
Anytime from cycle 37 through cycle 59
 
So you can start writing to the PF registers for a particular scan line  
*before* the previous scan line ends, and you can use the following order and  
times (or other order and times that adhere to the preceding rules):
 
Write to PF2 for the *left* copy-- on cycle 59 or later
Write to PF1 for the *left* copy-- on cycle 69 or later
Write to PF1 for the *right* copy-- on cycle 37 through 59
Write to PF2 for the *right* copy-- on cycle 48 exactly
 
This should give you plenty of time to load and store the PF registers, as  
well as the players.
 
Michael
 






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