further IO stack refactoring

c64/kernal/time/settim.s

@@ -0,0 +1,27 @@
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 299/300
+// - [CM64] Compute's Mapping the Commodore 64 - page 232
+//
+// CPU registers that has to be preserved (see [RG64]): .A, .X, .Y
+//
+
+SETTIM:
+
+	// Disable interrupts for the duration of the routine, to prevent
+	// jiffy clock update while changing it's values
+	sei
+
+	// Store the jiffy clock state
+	sty TIME + 0
+	stx TIME + 1
+	sta 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
+