more work on garbage collector

src/basic/engine/varstr_garbage_collect.s

@@ -2,7 +2,133 @@
 // #LAYOUT# *   BASIC_0 #TAKE
 // #LAYOUT# *   *       #IGNORE
 
+// XXX test this routine
 
 varstr_garbage_collect:
 
-	rts // XXX implement this
+	// The routine utilizes the 'shift_mem_up' - which leaves INDEX+0 and INDEX+1
+	// free to be used, but INDEX+2 and beyond would be overwritten
+
+	// We will reuse TXTPTR as the moving pointer, so store it on the stack
+
+	lda TXTPTR+0
+	pha
+	lda TXTPTR+1
+	pha
+
+	// INDEX+0 and INDEX+1 will be our cumulative shift counter
+
+	lda #$00
+	sta INDEX+0
+	sta INDEX+1
+
+	// Start from the highest string and go downwards
+
+	lda MEMSIZ+0
+	sta TXTPTR+0
+	lda MEMSIZ+1
+	sta TXTPTR+1
+
+varstr_garbage_collect_loop:
+
+	// Check whether we reached FRETOP
+
+	lda TXTPTR+0
+	cmp FRETOP+0
+	bne varstr_garbage_collect_check_bptr
+	lda TXTPTR+1
+	cmp FRETOP+1
+	bne varstr_garbage_collect_check_bptr
+
+	// End of strings - nothing more to do
+
+	// FALLTROUGH
+
+varstr_garbage_collect_end:
+
+	// Restore TXTPTR and quit
+
+	pla
+	sta TXTPTR+1
+	pla
+	sta TXTPTR+0
+
+	rts
+
+varstr_garbage_collect_check_bptr:
+
+	// Decrement TXTPTR by 2, so that it points to the back-pointer
+
+#if !HAS_OPCODES_65CE02
+
+	lda #$02
+	jsr varstr_TXTPTR_down_A
+
+#else // HAS_OPCODES_65CE02
+
+	dew TXTPTR
+	dew TXTPTR
+
+#endif
+
+	// Check whether the back-pointer is NULL
+
+	ldy #$00
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<TXTPTR
+	jsr peek_under_roms
+	sta DSCPNT+2
+	iny
+	jsr peek_under_roms
+	ora DSCPNT+2 
+#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	// XXX consider optimized version without multiple JSRs
+	jsr peek_under_roms_via_TXTPTR
+	sta DSCPNT+2
+	iny
+	jsr peek_under_roms_via_TXTPTR
+	ora DSCPNT+2 
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (TXTPTR),y
+	iny
+	ora (TXTPTR),y 
+#endif
+
+	beq varstr_garbage_collect_unused 
+	
+	// The back-pointer is not NULL - string is used
+
+	// XXX implement this
+
+varstr_garbage_collect_unused:
+
+	// The back-pointer is NULL - string is unused
+
+	// XXX implement this
+
+#if HAS_OPCODES_65C02
+	bra varstr_garbage_collect_end
+#else
+	jmp varstr_garbage_collect_end
+#endif
+
+
+
+
+
+
+	// XXX move this routine to a separate file
+
+varstr_TXTPTR_down_A:                  // .A - bytes to lower TXTPTR, uses DSCPNT+2
+
+	sta DSCPNT+2 
+
+	sec
+	lda TXTPTR+0
+	sbc DSCPNT+2 
+	sta TXTPTR+0
+	bcs !+
+	dec TXTPTR+1
+!:
+	rts