set IRQ frequency depending on video system

c64/kernal/setup_pal_ntsc.s

@@ -0,0 +1,31 @@
+
+;;
+;; Detect video system (PAL/NTSC), use Graham's method, as it's short and reliable
+;; see here: https://codebase64.org/doku.php?id=base:detect_pal_ntsc
+;;
+
+setup_pal_ntsc:
+
+	lda VIC_RASTER
+setup_pal_ntsc_w1:
+	cmp VIC_RASTER
+	beq setup_pal_ntsc_w1
+	bmi setup_pal_ntsc
+
+	;; Result in A, if no interrupt happened during the test:
+	;; #$37 -> 312 rasterlines, PAL,  VIC 6569
+	;; #$06 -> 263 rasterlines, NTSC, VIC 6567R8
+	;; #$05 -> 262 rasterlines, NTSC, VIC 6567R56A
+
+	cmp #$07
+	bcs setup_pal
+
+setup_ntsc:
+	lda #$00
+	beq +
+
+setup_pal:
+	lda #$01
+*
+	sta PALNTSC
+	rts