Subject: Re: [stella] Games that do bad things to HMOVE... From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx> Date: Thu, 23 Apr 1998 09:36:15 -0400 (EDT) |
> PC=f622 IR=85 <$nn 2 STA> A= 7 X= 0 Y=ff PS=20 SP=fd Cyc=8284 0 > PC=f624 IR=ea <implied 0 NOP> A= 7 X= 0 Y=ff PS=20 SP=fd Cyc=8286 2 > PC=f625 IR=a8 <implied 0 TAY> A= 7 X= 0 Y= 7 PS=20 SP=fd Cyc=8288 4 Just out of curiosity, what debugger is generating that? (I really hope you aren't typing it... :) ) > So they waste 71 cycles and hit HMOVE ending on cycle 74. Thus they > are hitting HMOVE two cycles before HBLANK begins. > > Now, the best I can figure is that by hitting HMOVE at this time it > reduces the amount of motion applied to the objects by 6 clocks. > So that instead of moving P0 by 8 pixels and P1 by 7 pixels it > moves them by 2 and 1 pixels. > > So we have that because HMOVE occurs at cycle 74 we loose 6 color clocks > and the pixel locations are: > > 128 + 2 = 130 130 - 68 = 62 > 137 + 1 = 138 138 - 68 = 70 > > Using these pixel locations the drawing works out and the text is displayed > as it should be. Innnteresting. Do you always lose 6 pixels that way, in the same direction? That is, could you set HMP0 and HMP1 to move , say, seven pixels left each, and then do the early HMOVE, moving 13 pixels in a single HMOVE? > What do you all think? Is this what happens when you hit HMOVE near > the end of a scanline? > > Does this mean that if you hit HMOVE during let's say the middle of a > scanline it would have no effect? Or it might move the objects 30 or 40 pixels one direction. :) experiment :) > Finally, do you have any idea why they decided to do it this way? > I'm guessing that it somehow avoids the HMOVE BLANK lines and they > wanted this because they have a border around the title screen. Who says they knew what they were doing? It could've been just trial and error and it happened to work... I certainly did enough of that in the INV kernel :) If it does avoid the HMOVE lines, though, it might be worth keeping and looking into. (unfortunately, I can't think of a way to position a sprite variably and then do the HMOVE on cycle 74 that same scanline... anyone?) -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/stella.html
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