work towards string deallocation

src/basic/engine/varstr_free.s

@@ -0,0 +1,75 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Releases back memory used by string
+//
+// Input:
+// - DSCPNT+0, DSCPNT+1 - pointer to string descriptor
+//
+
+// XXX test this routine
+
+varstr_free:
+
+	// First copy the string descriptor data to DSCPNT
+	// XXX write optimized version for CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+
+	ldy #$00
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<DSCPNT
+	jsr peek_under_roms
+#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	jsr peek_under_roms_via_DSCPNT
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (DSCPNT),y
+#endif
+
+	sta DSCPNT+2                                 // length of the string
+	iny
+
+#if CONFIG_MEMORY_MODEL_60K
+	jsr peek_under_roms
+#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	jsr peek_under_roms_via_DSCPNT
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (DSCPNT),y
+#endif
+
+	pha
+	iny
+
+#if CONFIG_MEMORY_MODEL_60K
+	jsr peek_under_roms
+#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	jsr peek_under_roms_via_DSCPNT
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (DSCPNT),y
+#endif
+
+	sta DSCPNT+1
+	pla
+	sta DSCPNT+0
+
+	// Quick check - is the string the lowest one?
+
+	cmp FRETOP+0
+	bne varstr_free_inside
+	lda DSCPNT+1
+	cmp FRETOP+1
+	bne varstr_free_inside
+
+	// This is the lowest string - increase FRETOP, this way
+	// the garbage collector will not be needed that quickly
+
+	// XXX implement this
+
+varstr_free_inside:
+
+	// THis is not the lowest string; mark it as free
+
+	// XXX implement this
+
+	rts