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base:detect_pal_ntsc

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base:detect_pal_ntsc [2020-11-10 21:56] – [Source Code] copyfaultbase:detect_pal_ntsc [2020-11-10 21:59] – copyfault
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 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 $aa (=TAX as opcode) contributes to making the whole routine as short as possbile. Why that [''ldx #$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.
 +
 +
 +===== Very short full VIC-type detection =====
 +
 +Count cycles galore
 +
 +=== Source Code ===
 +<code>
 +yet
 +to
 +come
 +</code>
base/detect_pal_ntsc.txt · Last modified: 2020-11-11 01:49 by copyfault