fixed mising shift memory implementation for memory models 46K and 50K

c64/basic/memory/shift_mem_down_38k.s

@@ -0,0 +1,30 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+
+#if CONFIG_MEMORY_MODEL_38K
+
+shift_mem_down:
+
+	// Move __memmove_size bytes from __memmove_src to __memmove_dst,
+	// where __memmove_dst > __memmove_src
+	// This means we have to copy from the back end down.
+	// This routine assumes the pointers are already pointed
+	// to the end of the areas, and that Y is correctly initialised
+	// to allow the copy to begin.
+
+	php
+!:
+	lda (__memmove_src),y
+	sta (__memmove_dst),y
+	iny
+	bne !-
+	inc __memmove_src+1
+	inc __memmove_dst+1
+	dec __memmove_size+1
+	bne !-
+	plp
+	rts
+
+#endif