Subject: Re: [stella] Visual questions From: Glenn Saunders <cybpunks@xxxxxxxxxxxxx> Date: Tue, 25 Jul 2000 10:42:50 -0700 |
Well, I only use VDEL for the 6-digit score routine. It's net effect is a delay in scanline drawing, but I usually use separate scanline counters for each graphic's vertical positioning. Are you going to mention what "really" happens with VDEL? The fact that there are 2 more pre-loading graphics registers, GRP0A and GRP1A?
But, as far as VDEL's original intended use (it's net effect), it is dependent upon where we are drawing in the scanline.
Oh, what the heck, here's some rambling: If we turn on VDEL, we enable caching of GRP0 and GRP1 in their 'A' registers. For example, by doing a store to GRP0, we actually are storing our data into GRP0A (not displayed), and we inadvertantly move whatever was in GRP1A to GRP1.
GRP0 and GRP1 are what's actually displayed. Now, if we do this store in the scanline AFTER the P1 graphic, P1's graphics will change on the next scanline. If we do it before, P1's graphic will change on that very same scanline.
Whatever P0's graphics were should stay the same. VDEL only works if on every other scanline, i.e. the odd ones you do a STA(or STX, or STY) GRP0, and on the other scanlines, i.e. the even ones, you do a STA(or X orY) GRP1, not both on each scanline, otherwise the data from both will be changed because the cached copies will be updated into the regular graphics registers. Whew!
0 0 0 0 0 0 0 0 LINE PAIR 0 (contains write (all zeros) 0 0 0 0 0 0 0 0 (TIA holds data)
0 X 0 0 0 0 X 0 LINE PAIR 1 (contains write) 0 X 0 0 0 0 X 0 (TIA holds data)
0 0 X X X X 0 0 LINE PAIR 2 (contains write) 0 0 X X X X 0 0 (TIA holds data)
0 0 0 0 0 0 0 0 LINE PAIR 3 (contains write (all zeros) 0 0 0 0 0 0 0 0 (TIA holds data)
0 0 0 0 0 0 0 0 (TIA returns delayed data from last scanline) 0 X 0 0 0 0 X 0 LINE PAIR 1 (contains write)
0 X 0 0 0 0 X 0 (TIA returns delayed data from last scanline) 0 0 X X X X 0 0 LINE PAIR 2 (contains write)
0 0 X X X X 0 0 (TIA returns delayed data from last scanline) 0 0 0 0 0 0 0 0 LINE PAIR 3 (contains write (all zeros)
0 0 0 0 0 0 0 0 (TIA returns delayed data from last scanline) 0 0 0 0 0 0 0 0
Glenn Saunders - Producer - Cyberpunks Entertainment Personal homepage: http://www.geocities.com/Hollywood/1698 Cyberpunks Entertainment: http://cyberpunks.uni.cc
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