From aa101f79c63fda046a2494311012d477061f7a1c Mon Sep 17 00:00:00 2001
From: FeralChild64 <you@example.com>
Date: Fri, 3 Jul 2020 16:47:23 +0200
Subject: [PATCH] added routine to free string memory

---
 src/basic/engine/varstr_alloc.s        |  4 +-
 src/basic/engine/varstr_alloc_fretop.s |  6 +--
 src/basic/engine/varstr_free.s         | 61 ++++++++++++++++++++++++--
 src/basic/engine/varstr_free_fretop.s  | 18 ++++++++
 4 files changed, 80 insertions(+), 9 deletions(-)
 create mode 100644 src/basic/engine/varstr_free_fretop.s

diff --git a/src/basic/engine/varstr_alloc.s b/src/basic/engine/varstr_alloc.s
index 8a24fec7..dd1e0d88 100644
--- a/src/basic/engine/varstr_alloc.s
+++ b/src/basic/engine/varstr_alloc.s
@@ -45,7 +45,7 @@ varstr_alloc_retry:
 #if !HAS_OPCODES_65CE02
 
 	lda #$02
-	jsr varstr_FRETOP_down 
+	jsr varstr_FRETOP_down_A
 
 #else // HAS_OPCODES_65CE02 - this time code is slightly longer, but faster
 
@@ -92,7 +92,7 @@ varstr_alloc_retry:
 	lda (DSCPNT),y
 #endif
 
-	jsr varstr_FRETOP_down
+	jsr varstr_FRETOP_down_A
 	bcs varstr_alloc_fail
 
 	// Success - fill in the string descriptor
diff --git a/src/basic/engine/varstr_alloc_fretop.s b/src/basic/engine/varstr_alloc_fretop.s
index 09f033bc..b00d2cb3 100644
--- a/src/basic/engine/varstr_alloc_fretop.s
+++ b/src/basic/engine/varstr_alloc_fretop.s
@@ -6,13 +6,13 @@
 // Helper routines for 'varstr_alloc'
 //
 
-varstr_FRETOP_down:                    // .A - bytes to lower FRETOP, uses memmove__tmp
+varstr_FRETOP_down_A:                  // .A - bytes to lower FRETOP, uses DSCPNT+2
 
-	sta memmove__tmp
+	sta DSCPNT+2 
 
 	sec
 	lda FRETOP+0
-	sbc memmove__tmp
+	sbc DSCPNT+2 
 	sta FRETOP+0
 	bcs !+
 	dec FRETOP+1
diff --git a/src/basic/engine/varstr_free.s b/src/basic/engine/varstr_free.s
index 448c161f..c61aae3a 100644
--- a/src/basic/engine/varstr_free.s
+++ b/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
diff --git a/src/basic/engine/varstr_free_fretop.s b/src/basic/engine/varstr_free_fretop.s
new file mode 100644
index 00000000..a9fec660
--- /dev/null
+++ b/src/basic/engine/varstr_free_fretop.s
@@ -0,0 +1,18 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Helper routines for 'varstr_free'
+//
+
+varstr_FRETOP_up:                    // DSCPNT+2 - bytes to increment FRETO, uses DSCPNT+2
+
+	clc
+	lda FRETOP+0
+	adc DSCPNT+2
+	sta FRETOP+0
+	bcc !+
+	inc FRETOP+1
+!:
+	rts