string allocation routine complete, dealloc and garbage collection still to do

src/basic/engine/varstr_alloc_fretop.s

@@ -0,0 +1,31 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Helper routines for 'varstr_alloc'
+//
+
+varstr_FRETOP_down:                    // .A - bytes to lower FRETOP, uses memmove__tmp
+
+	sta memmove__tmp
+
+	sec
+	lda FRETOP+0
+	sbc memmove__tmp
+	sta FRETOP+0
+	bcs !+
+	dec FRETOP+1
+!:
+	// FALLTROUGH
+
+varstr_FRETOP_check:                   // check if FRETOP > STREND, Carry set if not
+
+	lda STREND+1
+	cmp FRETOP+1
+	beq !+
+	rts
+!:
+	lda STREND+0
+	cmp FRETOP+0
+	rts