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I am drawing a line on VIC-20, but it blinks...
Posted: Tue Dec 14, 2021 4:25 pm
by stenlik
Hello,
I am so frustrated and exhausted
Hope some good developer can help me…
I have made a simple VIC-20 program (attached), which waits for the raster beam, changes the color once the line is detected and on the next line it changes it back, so it draws a line. And it should stay like that. It loops forever, but sometimes SOMETHING goes wrong, so the background is filled with line color and itn result it blinks.
I cannot find what can possibly go wrong. Or could that be caused by the VICE emulator (I have not tested it on real VIC-20)?
I have recorded a video:
https://youtu.be/Df-67EqRFkY
Here is the code:
Code: Select all
*=$1001
BYTE $0B, $10, $0A, $00, $9E, $34, $31, $30, $39, $00, $00, $00
SCREEN_COL_BORDER equ $900F ; Screen and border register
RASTER_BEAM_LINE equ $9004 ; Currently drawn raster beam line
program_init:
lda #%01011011 ; Black background and border
ldy #%00001011 ; Red background and border
loop:
ldx #50
red_line: ; Draw red line
cpx RASTER_BEAM_LINE ; Wait for raster line 50
bne red_line
sty SCREEN_COL_BORDER ; Set the red color
ldx #51
rest_black: ; Fill rest with the black color
cpx RASTER_BEAM_LINE ; Wait for raster line 51
bne rest_black
sta SCREEN_COL_BORDER ; Set the red color
jmp loop ; Go tot start
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Tue Dec 14, 2021 4:37 pm
by BigEd
Hi! I'm no expert, but I'll chip in with the observation that by waiting for the very next line, you might be in a race against time. Might an interrupt come in and take a lot of time, such that waiting for line 51 (or 31 according to your comment) is going to wait almost a whole frame, because the line is past?
I'd suggest trying for a 10 or 20 line band and see if that works any better.
(Or, change the branch so it's a relative comparison rather than a comparison for equality.)
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Tue Dec 14, 2021 5:03 pm
by stenlik
Hello BigEd,
Thanks for the answer. See my comments:
but I'll chip in with the observation that by waiting for the very next line, you might be in a race against time.
I have tried it, but it did not help... I am using the VIC-20 PAL emulator version with 1.108.405 Hz 6502 CPU. Given the PAL is 50Hz and there are 371 lines per screen it gives me 1.108.405/ (50*371) = 71 cycles per line, which even if the line start is detected with few cycle delays immediately should be sufficient.
or 31 according to your comment
My bad sorry - I have fixed the comment.
Thanks a lot
Petr
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Tue Dec 14, 2021 5:51 pm
by BigEd
Right, 71 cycles is a lot for your code, but what might an interrupt handler take? If you disable interrupts, is your program more reliable?
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Wed Dec 15, 2021 4:38 am
by stenlik
Hi BigEd,
Might an interrupt come in and take a lot of time, such that waiting for line 51 (or 31 according to your comment) is going to wait almost a whole frame, because the line is past?
Right, 71 cycles is a lot for your code, but what might an interrupt handler take? If you disable interrupts, is your program more reliable?
I should have read your comment more carefully in the first place !! You are right - interrupt routines from KERNAL are still set and handled. Calling SEI just helps to make my example wor.
You made my day.
Or, change the branch so it's a relative comparison rather than a comparison for equality.
Please -what do you mean by that, please? I am curious to learn something new.
Thanks a lot
Petr
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Wed Dec 15, 2021 5:48 am
by barrym95838
Or, change the branch so it's a relative comparison rather than a comparison for equality.
Please -what do you mean by that, please? I am curious to learn something new.
Thanks a lot
Petr
Hi, Petr. I think Ed is talking about possibly deleting the
ldx #51 and changing the
bne rest_black to
bcs rest_black to limit the damage to just a line or two instead of a whole frame if an interrupt causes you to miss your trigger. It's the assembly language way to change the wait loop from
while (RASTER_BEAM_LINE!=51) ; to
while (RASTER_BEAM_LINE<=50) ; in the C vernacular.
A
beq rest_black should also work, changing the wait loop to
while (RASTER_BEAM_LINE==50) ;.
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Wed Dec 15, 2021 8:18 am
by BigEd
Thanks Mike - indeed, I was thinking of BCS or BCC, but couldn't quite be bothered to figure out exactly which. The idea is to ask if we are at, or past, the target, rather than asking if we are exactly at the target.
Such a change will also give you a visual flicker which will allow you to estimate how long those interrupt routines run for!
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Wed Dec 15, 2021 3:46 pm
by barrym95838
Thanks Mike - indeed, I was thinking of BCS or BCC, but couldn't quite be bothered to figure out exactly which.
I know what you mean. It took me several edits to get it to where I felt confident enough to leave it and go to bed, and I must admit that I'm still only 90% sure I got it right. I think the
beq proposal burns fewer brain cells and still gets the job done.
Re: I am drawing a line on VIC-20, but it blinks...
Posted: Wed Dec 15, 2021 3:49 pm
by BigEd
Hey, it starts at 50/50, so 90% is pretty good!