Subject: [stella] Games that do bad things to HMOVE... From: bwmott@xxxxxxxxxxxx Date: Thu, 23 Apr 1998 00:50:35 0400 (EDT) 
I've been working on improving the TIA emulation for the next release of Stella. One game that's given my TIA emulation trouble in the past was Heman's title screen. Usually the text that's displayed is distorted a lot. I started looking at a trace of the code that generates the title screen and this is what I found out: The upper part of the title screen (M Network Presents) is generated by P0 and P1 with NUSIZx set to two close and VDELPx turned on. P0 and P1 are positioned using the following code: PC=f622 IR=85 <$nn 2 STA> A= 7 X= 0 Y=ff PS=20 SP=fd Cyc=8284 0 PC=f624 IR=ea <implied 0 NOP> A= 7 X= 0 Y=ff PS=20 SP=fd Cyc=8286 2 PC=f625 IR=a8 <implied 0 TAY> A= 7 X= 0 Y= 7 PS=20 SP=fd Cyc=8288 4 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 6 PS=20 SP=fd Cyc=8290 6 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 6 PS=20 SP=fd Cyc=8293 9 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 5 PS=20 SP=fd Cyc=8295 11 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 5 PS=20 SP=fd Cyc=8298 14 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 4 PS=20 SP=fd Cyc=8300 16 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 4 PS=20 SP=fd Cyc=8303 19 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 3 PS=20 SP=fd Cyc=8305 21 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 3 PS=20 SP=fd Cyc=8308 24 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 2 PS=20 SP=fd Cyc=8310 26 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 2 PS=20 SP=fd Cyc=8313 29 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 1 PS=20 SP=fd Cyc=8315 31 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 1 PS=20 SP=fd Cyc=8318 34 PC=f626 IR=88 <implied 0 DEY> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8320 36 PC=f627 IR=d0 <$nn f628 BNE> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8322 38 PC=f629 IR=85 <$nn 10 STA> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8325 41 PC=f62b IR=85 <$nn 11 STA> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8328 44 STA RESP0 @ cycle 41 clock location 41 * 3 = 123 + 5 = 128 (60) STA RESP0 @ cycle 44 clock location 44 * 3 = 132 + 5 = 137 (69) Several lines of code down they finally do: STA HMOVE (HMP0 = $80 HMP1 = $90) Motion registers were set before position code above So after the HMOVE I guessed the positions to be: 128 + 8 = 136 137 + 7 = 144 and the final pixel locations to be: P0 = 136  68 = 68 P1 = 144  68 = 76 However, using these locations there doesn't seem to be anyway for the text to work out since during the drawing of the text P0 and P1 are being hit at the wrong cycle times (i.e. while they're being displayed). I thought about this and tried a few different approaches to solve the problem but nothing worked :( Then I realized that after the position code above they hit HMOVE in a very strange way. The code is: PC=f62d IR=85 <$nn 2 STA> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8360 PC=f62f IR=a2 <#$nn f630 LDX> A= 7 X= 9 Y= 0 PS=20 SP=fd Cyc=8362 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 9 Y= 0 PS=20 SP=fd Cyc=8364 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 8 Y= 0 PS=20 SP=fd Cyc=8366 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 8 Y= 0 PS=20 SP=fd Cyc=8369 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 8 Y= 0 PS=20 SP=fd Cyc=8371 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 7 Y= 0 PS=20 SP=fd Cyc=8373 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 7 Y= 0 PS=20 SP=fd Cyc=8376 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 7 Y= 0 PS=20 SP=fd Cyc=8378 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 6 Y= 0 PS=20 SP=fd Cyc=8380 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 6 Y= 0 PS=20 SP=fd Cyc=8383 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 6 Y= 0 PS=20 SP=fd Cyc=8385 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 5 Y= 0 PS=20 SP=fd Cyc=8387 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 5 Y= 0 PS=20 SP=fd Cyc=8390 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 5 Y= 0 PS=20 SP=fd Cyc=8392 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 4 Y= 0 PS=20 SP=fd Cyc=8394 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 4 Y= 0 PS=20 SP=fd Cyc=8397 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 4 Y= 0 PS=20 SP=fd Cyc=8399 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 3 Y= 0 PS=20 SP=fd Cyc=8401 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 3 Y= 0 PS=20 SP=fd Cyc=8404 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 3 Y= 0 PS=20 SP=fd Cyc=8406 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 2 Y= 0 PS=20 SP=fd Cyc=8408 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 2 Y= 0 PS=20 SP=fd Cyc=8411 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 2 Y= 0 PS=20 SP=fd Cyc=8413 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 1 Y= 0 PS=20 SP=fd Cyc=8415 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 1 Y= 0 PS=20 SP=fd Cyc=8418 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 1 Y= 0 PS=20 SP=fd Cyc=8420 PC=f632 IR=ca <implied 0 DEX> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8422 PC=f633 IR=10 <$nn f634 BPL> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8425 PC=f631 IR=ea <implied 0 NOP> A= 7 X= 0 Y= 0 PS=22 SP=fd Cyc=8427 PC=f632 IR=ca <implied 0 DEX> A= 7 X=ff Y= 0 PS=a0 SP=fd Cyc=8429 PC=f633 IR=10 <$nn f634 BPL> A= 7 X=ff Y= 0 PS=a0 SP=fd Cyc=8431 PC=f635 IR=85 <$nn 2a STA> A= 7 X=ff Y= 0 PS=a0 SP=fd Cyc=8434 74 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. 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? 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. Oh well, it's bed time :) Brad  Bradford W. Mott (bwmott@xxxxxxx) Computer Science Department http://www4.ncsu.edu/~bwmott/www North Carolina State University   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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