further IO stack refactoring

c64/kernal/memory/vector_real.s

@@ -0,0 +1,42 @@
+
+
+vector_real:
+
+	// Use MEMUSS as temporary storage location - checked on real C64 that this is the
+	// address originally used; after calling VECTOR and checking zero page
+	// area afterwards, the address could be found there
+
+	// According to 'Compute's Mapping the Commodore 64' page 237,
+	// the CBM implementation does not disable IRQs - yet, the
+	// 'C64 Programmers Reference Guide' does not contain such
+	// warning and does not mention any preparations needed.
+
+	// Initial idea was to do php, cli, ..., plp to make the routine safer,
+	// but it seems it might cause problem with initialization on XEMU emulator
+	// XXX find out, why
+	
+	// Prepare the user data pointer - strange order to reduce risk
+	// of potential similarity to the original routine
+	sty MEMUSS + 1
+	stx MEMUSS + 0
+	
+	// Select routine variant - store or restore vectors
+	ldy #$1F
+	bcc vector_restore
+	
+	lda CINV, y
+	sta (MEMUSS), y
+
+	bcs vector_end_loop // branch always
+	
+vector_restore:
+	lda (MEMUSS), y
+	sta CINV, y
+
+	// FALLTROUGH
+
+vector_end_loop:
+	dey
+	bpl vector_restore
+
+	rts