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Dr. Jay
Account closed
Registered: Jan 2003
Posts: 32 |
PAL/NTSC detect
Just wanted to share a quick routine that detects PAL/NTSC WITHOUT using interrupts or latches.
;pal/NTSC detect - 0 = PAL, non-zero = NTSC
palntsc
sei ; disable interrupts
wait
lda $d012
bne wait ; wait for rasterline 0 or 256
wait1
lda $d011 ; Is rasterbeam in the area
bpl wait1 ; 0-255? if yes, wait
wait2
ldy #$00
synch1 lda $d012
cmp #$37 ; top PAL rasterline
bne synch1
lda $d012 ; if next is 0, then PAL
synch2 cmp $d012
beq synch2
lda $d012
cli ; enable interrupts
rts ; return
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Perff
Administrator
Posts: 1679 |
I know it's not in the 'hardcore' coding-spirit, but isn't it easier just to ask the system?
One thing you could do is check the IRQ-timer which is different for PAL & NTSC even though I can't remember the values.
This ofcourse only work is you come straight from basic, and/or nothing has tampered with the IRQ-timing.
Another thing is to check the PAL/NTSC variable.
The address $02A6 is 0 for NTSC and 1 for PAL set by the ROM but this again could be overwritten by something.
But the ROM can't. In the ROM somwhere I think it says something like:
lda #$01
sta $02A6
for PAL and lda #$00 for NTSC. Why not just check that? (yes. It might not be at the same location in each ROM-version)
I'm not 100% sure of this information though. Just something I think I remember. Please correct me if I'm wrong. :)
/Perff |
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Ninja
Registered: Jan 2002
Posts: 411 |
Perff: There is no PAL or NTSC-specific ROM. So any ROM checks for PAL/NTSC (counting rasterlines, see $FF5E...) and sets $02A6 depending on the result. And while we are just at it: Checking $02A6 is a poor way of detecting IMHO, because this location can easily be messed up. Those few bytes for a true PAL/NTSC-detection do not hurt, I think.
Dr Jay: What about this?
...
bne synch1
lda $d011
cli
rts
Bit 7 is set when PAL and cleared when NTSC. And I guess the LDY #$00 can be removed, too. |
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Dr. Jay
Account closed
Registered: Jan 2003
Posts: 32 |
LOL. I saw the LDY after I posted it ... artifact from an earlier attempt. Yes, I like the LDA $D011 ... much more elegant. And as for the ROM issue ... everything I've come to understand says that checking the $02a6 etc is unreliable as different versions, mods, carts, etc. may change this ... but I'm stuck in NTSC-land with a 64C so I haven't had the opportunity to try this on many other stock machines.
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Ninja
Registered: Jan 2002
Posts: 411 |
I think the main problem with $02A6 is, that it is set after a reset only. So, load some scene-production which doesn't care about KERNAL-variables and maybe exits with a plain RTS, and $02A6 is as reliable as your house-number :)
Also, the KERNAL-method does not work with accelerators.
False detections because of $02A6 are really annoying; especially as PAL/NTSC-checks are public and easy to implement.
So much for the preaching... ;) |
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Perff
Administrator
Posts: 1679 |
ninjadrm: Ok sorry for my "lame" idea. I just thought I remembered that somewhere in the ROM it said lda #$X sta $02A6 and thought of that as a 100% way to determin PAL/NTSC
Well.. I was wrong. You're right. Sorry. :) |
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Ninja
Registered: Jan 2002
Posts: 411 |
Perff: No need to be sorry or to excuse for a "lame" idea; it was just meant to be constructive criticism :) |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
do NOT use the pal/ntsc flag since it is unreliable, even without any kind of extensions. the best way is really to check if certain rasterlines exist or not.
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AlexC
Registered: Jan 2008
Posts: 298 |
Quote: do NOT use the pal/ntsc flag since it is unreliable, even without any kind of extensions. the best way is really to check if certain rasterlines exist or not.
Does it mean that original ROM can set this flag wrongly? Haven't studied this part of ROM but I remember that the detection routine used is quite short. |
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
yes, the rom routine has a flaw that sometimes makes it report ntsc when the machine is infact pal. (atleast thats how i remember it =D) |
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AlexC
Registered: Jan 2008
Posts: 298 |
Quote: yes, the rom routine has a flaw that sometimes makes it report ntsc when the machine is infact pal. (atleast thats how i remember it =D)
Thanks for clearing it up for me. I need to get NTSC machine to experiment a bit. |
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