Differences

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--- base:detect_pal_ntsc [2020-11-10 21:51] – [Short PAL/NTSC detection] copyfault
+++ base:detect_pal_ntsc [2020-11-10 21:55] – [Source Code] copyfault
@@ Line 853: / Line 853: @@
 <code>
 chk:
-              ldx #$aa   //TAX
+              ldx #$aa   //$aa = TAX
               lda $d012
               bne chk+1
@@ Line 861: / Line 861: @@
 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 $aa (=TAX as opcode) contributes to making the whole routine as short as possbile.
+Why that [''ldx #$aa'']? 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 in 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.