added routine to free string memory

src/basic/engine/varstr_free.s

@@ -61,15 +61,68 @@ varstr_free:
 	cmp FRETOP+1
 	bne varstr_free_inside
 
-	// This is the lowest string - increase FRETOP, this way
+	// This is the lowest string - so just increase FRETOP, this way
 	// the garbage collector will not be needed that quickly
 
-	// XXX implement this
+	jsr varstr_FRETOP_up                         // free the string data
+
+#if !HAS_OPCODES_65CE02
+
+	lda #$02                                     // free the back-pointer
+	sta DSCPNT+2
+	jmp varstr_FRETOP_up
+
+#else // HAS_OPCODES_65CE02
+
+	inw FRETOP                                   // free the back-pointer
+	inw FRETOP
+
+	rts
+
+#endif
 
 varstr_free_inside:
 
-	// THis is not the lowest string; mark it as free
+	// This is not the lowest string; mark it as free
+	// First preserve the string size, it will make it easier for the garbage collector
 
-	// XXX implement this
+	lda DSCPNT+2
+	tay
+	dey
 
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<FRETOP
+	jsr poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (FRETOP), y
+#endif
+
+	// Now increase DSCPNT+0/+1 to point to the back-pointer
+
+	clc
+	lda DSCPNT+0
+	adc DSCPNT+2
+	sta DSCPNT+0
+	bcc !+
+	inc DSCPNT+1
+!:
+	// Fill the back-pointer with 0
+
+	lda #$00
+	ldy #$00
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<DSCPNT
+	jsr poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (DSCPNT), y
+#endif
+
+	iny                                          // $00 -> $01
+
+#if CONFIG_MEMORY_MODEL_60K
+	jmp poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (DSCPNT), y
 	rts
+#endif