further IO stack refactoring

c64/kernal/iostack/f333.clrchn.s

@@ -0,0 +1,32 @@
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 282
+// - [CM64] Compute's Mapping the Commodore 64 - page 230
+//
+// CPU registers that has to be preserved (see [RG64]): .Y
+//
+
+
+CLRCHN:
+
+#if CONFIG_IEC
+	jsr clrchn_iec
+#else
+	nop // just to prevent double label
+#endif
+
+	// FALLTROUGH
+
+clrchn_reset: // entry needed by setup_vicii
+
+	// Set input device number to keyboard
+	lda #$00
+	sta DFLTN
+
+	// Set output device number to screen
+	lda #$03
+	sta DFLTO
+
+	rts

c64/kernal/iostack/f333.interop

@@ -0,0 +1,8 @@
+All the Kernal routines, for which there are vectors in $0314-$0333, have to be available at their original addresses.
+
+Reason: some autostart software (https://codebase64.org/doku.php?id=base:autostarting_disk_files) override these vectors, so that some point
+to new locations (custom IRQ routine, custom STOP routine, etc.), while others points the original Kernal routines. Different techniques modify
+different vectors (while resetting others to original C64 locations), some don't change the vectors at all, but might still put the original
+C64 Kernal addresses there (for example, if the file overrides keyboard input buffer and screeen memory).
+
+For various cartridges (like IDE64) this just means disabling their extensions; for us that would mean improperly functionning Kernal.