further IO stack refactoring

c64/kernal/time/rdtim.s

@@ -0,0 +1,27 @@
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 291
+// - [CM64] Compute's Mapping the Commodore 64 - page 232
+//
+// CPU registers that has to be preserved (see [RG64]): none
+//
+
+RDTIM:
+
+	// Disable interrupts for the duration of the routine, to prevent
+	// jiffy clock update while reeading it's values
+	sei
+
+	// Store the jiffy clock state
+	ldy TIME+0
+	ldx TIME+1
+	lda TIME+2
+
+	// Enable interrupts. More elegant solution would be pha + sei + ... + pla,
+	// but (checked by running a test program) original ROM routine always
+	// leaves IRQs enabled.
+	cli
+	rts
+