variable support cleanup

src/basic/engine_variables/varstr_free.s

@@ -0,0 +1,102 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Releases back memory used by string
+//
+// Input:
+// - DSCPNT+0           - string size
+// - DSCPNT+1, DSCPNT+2 - pointer to string
+//
+
+
+varstr_free:
+
+	// Check the string size - do not do anything if 0
+
+	lda DSCPNT+0
+	bne !+
+	rts
+!:
+	// Check if string is above FRETOP (in string area)
+	// XXX move this check out of the routine
+
+	jsr helper_cmp_fretop
+	bcs !+
+
+	// No, it does not belong to string area - quit
+
+	rts
+!:
+	// Check if this is the lowest string
+
+	bne varstr_free_inside
+
+	// This is the lowest string - so just increase FRETOP, this way
+	// the garbage collector will not be needed that quickly
+
+	jsr helper_FRETOP_up                         // free the string data
+
+#if !HAS_OPCODES_65CE02
+
+	lda #$02                                     // free the back-pointer
+	sta DSCPNT+0
+	jmp helper_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
+	// First preserve the string size, it will make it easier for the garbage collector
+
+	// Increase DSCPNT+1/+2 to point to the back-pointer minus 1
+
+	dec DSCPNT+0
+
+	clc
+	lda DSCPNT+1
+	adc DSCPNT+0
+	sta DSCPNT+1
+	bcc !+
+	inc DSCPNT+2
+!:
+	// Put the size-1, garbage collector will make use of this value
+
+	lda DSCPNT+0
+	ldy #$00
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<DSCPNT+1
+	jsr poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (DSCPNT+1), y
+#endif
+
+	// Now fill-in the back-pointer with 0's
+
+	tya
+	iny
+
+#if CONFIG_MEMORY_MODEL_60K
+	jsr poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (DSCPNT+1), y
+#endif
+
+	iny
+
+#if CONFIG_MEMORY_MODEL_60K
+	jmp poke_under_roms
+#else // CONFIG_MEMORY_MODEL_38K || CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	sta (DSCPNT+1), y
+	rts
+#endif