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johncl
Registered: Aug 2007
Posts: 37 |
Screen copy CPU intensive
I am working on a little game for the C64 and have stumbled upon a slight problem. A smooth scroller is darn CPU intensive! Its a simple sidescroller where I want to scroll the upper 18 lines. The copy required to move the character and color ram is very expensive! I use kickassembler and at first I tried this:
.macro ScrollLines(screen,from,to) {
.var half = round([to-from]/2)
ldx #0
jmp !loop+
.align $100
!loop:
.for (var i=from;i<from+half;i++) {
lda screen+1+[i*40],x
sta screen+[i*40],x
lda SCREEN_COLOR+1+[i*40],x
sta SCREEN_COLOR+[i*40],x
}
inx
cpx #39
bne !loop-
ldx #0
jmp !loop+
.align $100
!loop:
.for (var i=from+half;i<=to;i++) {
lda screen+1+[i*40],x
sta screen+[i*40],x
lda SCREEN_COLOR+1+[i*40],x
sta SCREEN_COLOR+[i*40],x
}
inx
cpx #39
bne !loop-
}
So here I unroll a full column copy in two parts (because bne wont work if I unroll whole column). This even aligns the two loops so that the bne doesnt cross any page boundaries and end up costing another cycle. But still, this routine eats up my CPU big time.
I then tried a complete unroll of a pure copy:
.macro ScrollLines2(screen,from,to) {
.for (var i=from*40;i<=to*40;i++) {
lda screen+1+i
sta screen+i
lda SCREEN_COLOR+1+i
sta SCREEN_COLOR+i
}
}
This also copies the unecessary column too that I need to copy from my map data. But this complete unroll uses almost 2/3 of the screen raster time! Even with this I have to split up my copy so the top part is copied at the bottom redraw of the refresh and the bottom part on start of next screen refresh.
Am I doing something wrong here? |
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tlr
Registered: Sep 2003
Posts: 1723 |
You need to ask yourself how fast this has to scroll.
Usually this is double buffered, and the copy process is split into several frames. |
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chatGPZ
Registered: Dec 2001
Posts: 11136 |
for a simple sidescroller (always scrolling into the same direction with the same speed) might want to use doublebuffering, you only need to have an unrolled loop for the colorram then, for the screenram you'll have 7 frames (if you are scrolling with 1 pixel per frame).
and yes, scrolling is cpu intense, its one of the major problems with c64 game programming :=) |
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Oswald
Registered: Apr 2002
Posts: 5023 |
here is how to move the screen horizontally using HW only.
http://codebase64.org/doku.php?id=base:horizontal_screen_positi.. |
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johncl
Registered: Aug 2007
Posts: 37 |
Thanks for the tips. I managed to make it work in my game with 17 lines scrolling by splitting the copy in two, top half at the bottom of the raster redraw and the bottom half in the beginning of next screen. But it eats a lot of the available CPU time indeed.
Just before I read the answers here I also thought about what you have written here, to split up the copy in two halves and do double buffering by being a scroll screen ahead in time (depensing on direction player is moving). Since I'll maximum do 4 pixel scrolls (more likely 2-3 pixels) I should be fine.
Oswald, thanks for the code reference too, I must study that in detail to understand it as I am not very educated in the tricks that are used here. Does it do anything special to increase the number of available cycles to the CPU for the copy? |
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Oswald
Registered: Apr 2002
Posts: 5023 |
johncl, it is possible to move the screen by 40 chars without copying anything.
downsides:
- this process will move the screen also 1 char up (down? dont remember now) meaning you will need to work with 2 screens.
- needs stable raster, cycle exact timing
- doesnt works on some c64 revisions (this trick does change the memory refresh timing, and some rams does not tolerate that)
experiment with fungus' code if you're interested. turn all 65 cycle delays where commented into 63. |
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JackAsser
Registered: Jun 2002
Posts: 1990 |
Quote: Thanks for the tips. I managed to make it work in my game with 17 lines scrolling by splitting the copy in two, top half at the bottom of the raster redraw and the bottom half in the beginning of next screen. But it eats a lot of the available CPU time indeed.
Just before I read the answers here I also thought about what you have written here, to split up the copy in two halves and do double buffering by being a scroll screen ahead in time (depensing on direction player is moving). Since I'll maximum do 4 pixel scrolls (more likely 2-3 pixels) I should be fine.
Oswald, thanks for the code reference too, I must study that in detail to understand it as I am not very educated in the tricks that are used here. Does it do anything special to increase the number of available cycles to the CPU for the copy?
The trick is called VSP. Both VSP and normal double buffered scroll needs a double buffer.
But for normal scrolling you have 8 pixels to complete the other buffer, that is, you need to paste 1000/8 chars every frame (assuming 1 pixel / frame scrolling).
With VSP you only need to update one column per 8 pixels. So you only need to paste 25/8 chars. This has to be done twice because you must update the other screen which will replace the current one as soon as you have scrolled 40 chars. So all in all 25/8*2 chars per frame.
STILL! None of these techniques eliminates the update of the $d800 area. At some point all 1000 $d800 nybbles has to be updated in just one frame.
If you code for C128 you may double buffer $d800 aswell though, significantly increase the scrolling speed.
If you wanna learn more about VSP, read about the bad lines and the VSP trick in general in the VIC-article. It's quite good explained there and not as mysterious as many tries to make it.
/Andreas |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
VSP is a bad idea since it crashes on 50% of the machines. |
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JackAsser
Registered: Jun 2002
Posts: 1990 |
Quote: VSP is a bad idea since it crashes on 50% of the machines.
@Graham: U have to really beleive in it otherwise VSP will never work. It's like it knows u. |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
The standard approach for games is to define color attributes in large blocks instead of on a per-tile basis. So if you've got 4x4 blocks of equal color values for instance then you'd only have to update every fourth column when shifting the screen horizontally. Double buffered video matrix scrolling is important as well so you can split the work across several frames.
There are certain alternative hardware tricks too with better compatibility than VSP. I hacked together a charmode with 16-pixel tall tiles for a game project, which then only requires half as much effort to scroll.
All things considered though efficient scrolling is damned hard on the C64, especially when you want to combine both horizontal and vertical scrolling. |
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PopMilo
Registered: Mar 2004
Posts: 145 |
@doynax: "There are certain alternative hardware tricks..."
Could you be more specific? :)
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