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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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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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Devia
Registered: Oct 2004
Posts: 401 |
So the shortest/fastes routine for reliable detection would be something like:
PALNTSC
sei
lda $01
pha
lda #<INT_NMI
sta $fffa
lda #>INT_NMI
sta $fffb
lda #$35
sta $01
;---important stuff start
: bit $d011
bpl :-
: lda $d012
cmp #<263
bcs :+ ;Result >= 0 then PAL
bit $d011
bmi :-
clc ;NTSC
:
;---important stuff end
pla
sta $01
rts
INT_NMI
rti
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tlr
Registered: Sep 2003
Posts: 1787 |
I came to basically the same conclusion:
http://codebase64.org/doku.php?id=bae:dtv_detect (scroll down) |
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Devia
Registered: Oct 2004
Posts: 401 |
ahh... The first lda $d011, bmi *-3 seems obsolete, though - unless I'm missing something?
Is there any particular reason for waiting for line 288?
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TNT
Account closed
Registered: Oct 2004
Posts: 189 |
Quote: ahh... The first lda $d011, bmi *-3 seems obsolete, though - unless I'm missing something?
Is there any particular reason for waiting for line 288?
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Frantic
Registered: Mar 2003
Posts: 1647 |
Just wanted to say that TLR missed one letter in his link to codebase... the link is:
http://codebase64.org/doku.php?id=base:dtv_detect
Also, don't hesitate to update the current NTSC/PAL detection routine if you come up something better:
http://codebase64.org/doku.php?id=base:detect_pal_ntsc |
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tlr
Registered: Sep 2003
Posts: 1787 |
Quote: ahh... The first lda $d011, bmi *-3 seems obsolete, though - unless I'm missing something?
Is there any particular reason for waiting for line 288?
So what happens if your version starts within 6 cycles of the raster counter wrapping to 0? ;)
288 is just about halfway between the max number of raster lines on PAL and NTSC respectively.
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Devia
Registered: Oct 2004
Posts: 401 |
Quote: So what happens if your version starts within 6 cycles of the raster counter wrapping to 0? ;)
288 is just about halfway between the max number of raster lines on PAL and NTSC respectively.
Well in that case it's apparently an NTSC machine ;-)
So, yes..
: bit $d011
bmi :-
: bit $d011
bpl :-
: lda $d012
cmp #<263
bcs :+ ;Result >= 0 then PAL
bit $d011
bmi :-
clc ;NTSC
:
Any more pitfalls then?
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