Several techniques are available to programmers, as follows:
1) The XL OS (not the 400/800 OS) provides a flag called PALNTS at decimal
memory location 98 (hex: $62). PALNTS indicates whether the CTIA/GTIA/FGTIA
has reported itself to be NTSC or PAL/SECAM, where 0 means NTSC, or 1 means
PAL/SECAM. In Atari BASIC, enter "? PEEK(98)" to determine the value of the
PALNTS flag.
2) An approach which works on all 400/800/XL/XE systems is to use the same
method used by the XL OS to set the value of the PALNTS flag described above.
That is, to read and interpret the "PAL" memory flag, decimal location 53268
(hex: $D014). The value of PAL is provided by the CTIA/GTIA/FGTIA chip
itself. Meanings are:
Bit 1-3 clear (xxxx000x) = PAL/SECAM
Bit 1-3 set (xxxx111x) = NTSC
(Proper interpretation of the value returned by PEEK(53268) in Atari BASIC
would thus be a bit of a programming challenge. This is left to the reader!)
3) Software may determine NTSC or PAL/SECAM by determining how many scan
lines are being generated by ANTIC. This is done by monitoring the VCOUNT
memory register. VCOUNT (54283 decimal, $D40B hex) is used by ANTIC to keep
track of which line is currently being generated on the screen. Values
reflect the line count divided by two. VCOUNT values range from zero to 130
for an NTSC ANTIC (131*2=262 scan lines), while VCOUNT values range from zero
to 155 for a PAL ANTIC (156*2=312 scan lines).
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