further IO stack refactoring

c64/kernal/time/udtim.s

@@ -0,0 +1,41 @@
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 303
+// - [CM64] Compute's Mapping the Commodore 64 - pages 27, 232
+//
+// CPU registers that has to be preserved (see [RG64]): .Y
+//
+
+UDTIM:
+
+	// Update jiffy clock. The whole day is $4F1A00 jiffies
+	// (24 hours * 60 minutes * 60 seconds * 60 jiffies),
+	// after this time we have to reset the clock.
+
+	inc TIME+0
+	bne udtim_time_done   // done with clock
+	inc TIME+1
+	bne udtim_clock_rollover
+	inc TIME+2
+
+	// FALLTROUGH
+
+udtim_time_done:
+
+	jmp udtim_keyboard
+
+udtim_clock_rollover:
+
+	// At this point TIME+0 contains 0
+	lda TIME+1
+	cmp #$1A
+	bne udtim_time_done   // done with clock
+	lda TIME+2
+	cmp #$4F
+	bne udtim_time_done   // done with clock
+	lda #$00
+	sta TIME+2
+	sta TIME+1
+	beq udtim_time_done   // done with clock - will always jump