How do I show sprites in the border on C64?
I\'ve seen cool C64 demos showing sprites in the border area of the screen. It shouldn\'t be possible; I think they managed to fool the graphics chip somehow. How exactly di
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It all relied on timing. The C64 had a method to query the exact vertical location of the electron beam while it was drawing the screen. When a new line started, you had to wait a few cycles (you could time this using the NOP instruction) and then you had to set a hardware register of the videochip which was responsible for setting the screenmode (and the border width). By timing it exactly right, and doing it every scanline again, the whole sideborder disappeared.
The bottom border went away with a similar trick. On the exact scanline where the vertical border started you too had to set the videomode which disabled the bottom border for that frame.
Indeed this whole thing had to be done in assembly. Otherwise you could never get the timing exactly right.
As a side note, I think the sideborder trick was credited to the 1001 Crew (a Dutch group). I'm not sure who pulled off the first bottom border trick.
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Timing was the key. The image was created in the border by changing the overscan (border) colour as the CRT's beam moved from left to right. There are two timing signals required to produce an image - vertical refresh and horizontal refresh. By detecting when the horizontal and vertical refresh occurs you can start a sequence of assembler instructions to change the border colour to produce an image. You need to work out the number of CPU clock ticks per border pixel and use that to create code that changes the border colour at the right point.
It doesn't work very well when it comes to writing games as the CPU overhead is too great for there to be any time lift to process user input and game state.
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That is a long time ago.
I know there was a solution that relied on the frequency of the monitor.
With a CRT, the current pixel is known even if it was outside of the normal screen. So you could manipulate the ray.
Somewhere in my junkpile there must be some C64 books.
Offtopic, but graphics with the VIC20 (the predecessor of the C64) was fun. There was no way to manipulate each pixel, but you could change the existing characters. So you filled the screen with all characters from 0 to ... and changed the characters to set pixels to the screen. ;-).
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For a good tutorial on the topic of opening the borders on the C64, check out Pasi Ojala's excellent article in C=Hacking Issue 6.
Without getting too technical, the trick uses a feature of the VIC chip to let you switch between 25/24 rows and 40/38 columns of text/graphics, and involves making this switch at the exact right moment to fool the VIC into thinking it has already switched the borders on when in fact it hasn't. Check out the above article for a more thorough explanation with code examples.
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As it was already said, you have to fool the VIC to think that the border already started, but the reason why I write this is because the top answer is a little bit inprecise: I was totally unable to find a register to make the border smaller, so this is the way you do it (at least for top and bottom): You wait until the VIC reached the 25th character row and then you enable 24 rows ($D011, bit 3). You can do the same thing for the left and the right border, just with 38 cols ($D016, bit 3), but to do that you need very precise timing and you also need to eliminate the bad lines by setting the vertical scroll register, so the scanline mod 8 is never equal to the scroll value. Of course, you can't use the normal display anymore because the bad lines actually aren't just bad, they are used to load character data I think, stuff that repeats for every 8th line in the non-border area. I personally was a little bit confused when I read the top answer, I hope that can help. (Also, the top answer has a mistake: You don't make the border smaller, you make it bigger)
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Yes, you need assembler. It's an interrupt timing trick. The VIC is able to show sprites in the border, but the frame is just hiding them, so the sprites can slide behind it. It's connected to scan lines displayed by the VIC. For lower/upper borders it's quite simple:
- Programm an interrupt, synced to start at a certain scan line, 7 pixel or something like that before the lower border.
- Set the register in VIC to make the border smaller. (There is a register that can do that.)
- VIC now believes that the border already started and does not start to paint it.
- -> No border at the bottom.
- Programm another interrupt after the real border to set it back to original.
For sprites in the left/right borders, it's more complicated because the process has to be repeated for every scan line:
- Programm an interrupt, synced to start at a certain scan line.
- Then do some NOPs until you are 7 pixels before the right border.
- Set the register in VIC to make the border smaller.
- -> No border at the right side.
- Do some NOPs until you are after the real border and set the register back to original value.
- Again do some NOPs until step 2.
The problem is that all these NOPs are busy waits and steal the cycles you have for your stuff.
I was able to find some code for you, from an sprite scroller in the lower border. Here is the code. (It was ripped from some demo.)
C198 78 SEI C199 20 2E C1 JSR C12E # clear sprite area C19C 20 48 C1 JSR C148 # init VIC C19F A9 BF LDA #BF # set up IRQ in C1BF C1A1 A2 C1 LDX #C1 C1A3 8D 14 03 STA 0314 C1A6 8E 15 03 STX 0315 C1A9 A9 1B LDA #1B C1AB 8D 11 D0 STA D011 C1AE A9 F7 LDA #F7 C1B0 8D 12 D0 STA D012 C1B3 A9 01 LDA #01 C1B5 8D 1A D0 STA D01A C1B8 A9 7F LDA #7F C1BA 8D 0D DC STA DC0D C1BD 58 CLI C1BE 60 RTS ---------------------------------- # init VIC C148 A2 00 LDX #00 C14A BD 88 C1 LDA C188,X C14D 9D 00 D0 STA D000,X # set first 16 values from table C150 E8 INX C151 E0 10 CPX #10 C153 D0 F5 BNE C14A C155 A9 FF LDA #FF C157 8D 15 D0 STA D015 C15A A9 00 LDA #00 C15C 8D 1C D0 STA D01C C15F A9 FF LDA #FF C161 8D 17 D0 STA D017 C164 8D 1D D0 STA D01D C167 A9 C0 LDA #C0 C169 8D 10 D0 STA D010 C16C A9 F8 LDA #F8 C16E A2 00 LDX #00 C170 9D F8 07 STA 07F8,X C173 18 CLC C174 69 01 ADC #01 C176 E8 INX C177 E0 08 CPX #08 C179 D0 F5 BNE C170 C17B A9 0E LDA #0E C17D A2 00 LDX #00 C17F 9D 27 D0 STA D027,X C182 E8 INX C183 E0 08 CPX #08 C185 D0 F8 BNE C17F C187 60 RTS ---------------------------------- # data set into VIC registers C188 00 F7 30 F7 60 F7 90 F7 C190 C0 F7 F0 F7 20 F7 50 F7 ---------------------------------- # main IRQ routine C1BF A2 08 LDX #08 C1C1 CA DEX C1C2 D0 FD BNE C1C1 C1C4 A2 28 LDX #28 # 40 or so lines C1C6 EA NOP # "timing" C1C7 EA NOP C1C8 EA NOP C1C9 EA NOP C1CA CE 16 D0 DEC D016 # fiddle register C1CD EE 16 D0 INC D016 C1D0 AC 12 D0 LDY D012 C1D3 88 DEY C1D4 EA NOP C1D5 98 TYA C1D6 29 07 AND #07 C1D8 09 18 ORA #18 C1DA 8D 11 D0 STA D011 C1DD 24 EA BIT EA C1DF EA NOP C1E0 EA NOP C1E1 CA DEX C1E2 10 E4 BPL C1C8 # repeat next line C1E4 A9 1B LDA #1B C1E6 8D 11 D0 STA D011 C1E9 A9 01 LDA #01 C1EB 8D 19 D0 STA D019 C1EE 20 00 C0 JSR C000 # call main code C1F1 4C 31 EA JMP EA31 # finish IRQ讨论(0)
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