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--- base:detect_pal_ntsc [2019-05-15 20:11] – [Detect NTSC/PAL] silverdr
+++ base:detect_pal_ntsc [2020-11-10 21:47] – copyfault
@@ Line 826: / Line 826: @@
 </code>
+===== When size really matters =====
+by Copyfault/The Solution/The Obsessed Maniacs
+In the following I'm going to present two routines for detecting PAL/NTSC: the first one is used for telling a EU-PAL-chip (with 63 cycles per line) apart from a "new NTSC"-chip (with 65cycles/line). This suffices in most cases since the mentioned systems were the most common in EU or US, resp.
+The 2nd routine is capable of detecting any of the four VIC-types that have been mentioned earlier on this page (EU-PAL, NTSC old _and_ new and the one for the Drean-PAL-system).
+==== Short PAL/NTSC detection ====
+The basic idea is to constantly read the raster beam position and keeping it in a backup register until rasterline==0 (or rasterline==$100) is reached. Then the backup register holds the value of the last line that was read before reaching line 0 (or $100 resp.).
+In case the last line was $ff, it does not tell much about the system at hand. Thus, the rasterline-read-and-backup-procedure is repeated until the backup value is not $ff afterwards.
+The well-known table of VIC-specs now reveals that the backup value must be one of the following:
+<code>
+#$37 -> 312 rasterlines -> PAL
+#$06 -> 263 rasterlines -> new NTSC
+#$05 -> 262 rasterlines -> old NTSC
+</code>
+It's even possible to tell new and old NTSC apart this way (not just PAL vs. NTSC as stated in the preface). The approach is NOT capable of detecting a Drean PAL-VIC-Chip since EU-PAL and PAL-N have the same number of rasterlines per frame.
+==== Source Code ====
+<code>
+chk:
+              ldx #$aa   //TAX
+              lda $d012
+              bne chk+1
+              txa
+              bmi chk
+</code>
+This routine continuously checks $D012==0 while saving the last read $D012-value in X and repeats the whole procedure when the value stored in X has the MSB set. This luckily suffices to distinguish the case "last line was $ff" from "last line was the last one of the frame", since the highest possible value is $37 (on PAL).
+Why that [ldx #TAX]? There's a small probability that the routine starts when we accidently *are* on line $0 (or $100). The [bne chk+1] would not branch, i.e. the check-for-line$0-loop would end promptly, without any backup value stored in X yet. Thus, we need an init value for X that forces the line$0-check to be repeated for this rare case. It could by any value with MSB set, but using $aa (=TAX as opcode) contributes to making the whole routine as short as possbile.