optimizations in garbage collector

src/basic/engine_variables/helper_cmp_varnam_46K_50K.s

@@ -2,7 +2,7 @@
 // #LAYOUT# *   BASIC_0 #TAKE-HIGH
 // #LAYOUT# *   *       #IGNORE
 
-// This has to go $E000 or above - routines below banks out the main BASIC ROM!
+// This has to go $E000 or above - routine below banks out the main BASIC ROM!
 
 #if CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
 
@@ -26,13 +26,7 @@ helper_cmp_varnam:
 	cmp VARNAM+1
 !:
 	// Restore default memory mapping
-	// XXX consider deduplicating with 'is_line_pointer_null'
 
-	php
-	lda #$27
-	sta CPU_R6510
-	plp
-
-	rts
+	jmp remap_BASIC_preserve_P
 
 #endif

src/basic/engine_variables/helper_gc_fetch_backpointer_46K_50K.s

@@ -0,0 +1,32 @@
+// #LAYOUT# STD *       #TAKE-HIGH
+// #LAYOUT# *   BASIC_0 #TAKE-HIGH
+// #LAYOUT# *   *       #IGNORE
+
+// This has to go $E000 or above - routine below banks out the main BASIC ROM!
+
+#if CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+
+helper_gc_fetch_backpointer:
+
+	// Unmap BASIC lower ROM
+
+	lda #$26
+	sta CPU_R6510
+
+	// Copy the back-pointer to OLDTXT, check if it is NULL
+
+	ldy #$00
+	lda (TXTPTR),y
+	sta OLDTXT+0
+
+	iny
+	lda (TXTPTR),y
+	sta OLDTXT+1
+
+	ora OLDTXT+0
+
+	// Restore default memory mapping
+
+	jmp remap_BASIC_preserve_P
+
+#endif

src/basic/engine_variables/helper_gc_misc_46K_50K.s

@@ -0,0 +1,57 @@
+// #LAYOUT# STD *       #TAKE-HIGH
+// #LAYOUT# *   BASIC_0 #TAKE-HIGH
+// #LAYOUT# *   *       #IGNORE
+
+// This has to go $E000 or above - routines below bank out the main BASIC ROM!
+
+#if CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+
+helper_gc_set_memmove_src:
+
+	// Unmap BASIC lower ROM
+
+	lda #$26
+	sta CPU_R6510
+
+	// Set memmove__src
+
+	lda (OLDTXT),y
+	sta memmove__src+0
+	iny
+	lda (OLDTXT),y
+	sta memmove__src+1
+
+	// Restore default memory mapping
+
+	jmp_8 helper_gc_misc_end
+
+helper_gc_increase_oldtxt:
+
+	// Unmap BASIC lower ROM
+
+	lda #$26
+	sta CPU_R6510
+
+	// Increase OLDTXT accordingly
+
+	lda (OLDTXT),y
+	clc
+	adc INDEX+0
+	sta (OLDTXT),y
+	iny
+	lda (OLDTXT),y
+	adc INDEX+1
+	sta (OLDTXT),y
+
+	// FALLTROUGH
+
+helper_gc_misc_end:
+
+	// Restore default memory mapping
+
+	lda #$27
+	sta CPU_R6510
+
+	rts
+
+#endif

src/basic/engine_variables/varstr_garbage_collect.s

@@ -87,6 +87,12 @@ varstr_garbage_collect_check_bptr:
 
 	// Copy the back-pointer to OLDTXT, check if it is NULL
 
+#if CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+
+	jsr helper_gc_fetch_backpointer
+
+#else 
+
 	ldy #$00
 
 #if CONFIG_MEMORY_MODEL_60K
@@ -96,13 +102,6 @@ varstr_garbage_collect_check_bptr:
 	iny
 	jsr peek_under_roms
 	sta OLDTXT+1
-#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
-	// XXX consider optimized version without multiple JSRs
-	jsr peek_under_roms_via_TXTPTR
-	sta OLDTXT+0
-	iny
-	jsr peek_under_roms_via_TXTPTR
-	sta OLDTXT+1
 #else // CONFIG_MEMORY_MODEL_38K
 	lda (TXTPTR),y
 	sta OLDTXT+0
@@ -112,6 +111,9 @@ varstr_garbage_collect_check_bptr:
 #endif
 
 	ora OLDTXT+0
+
+#endif
+
 	beq_16 varstr_garbage_collect_unused 
 	
 	// The back-pointer is not NULL - string is used
@@ -159,12 +161,7 @@ varstr_garbage_collect_check_bptr:
 	jsr peek_under_roms
 	sta memmove__src+1
 #elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
-	// XXX consider optimized version without multiple JSRs
-	jsr peek_under_roms_via_OLDTXT
-	sta memmove__src+0
-	iny
-	jsr peek_under_roms_via_OLDTXT
-	sta memmove__src+1
+	jsr helper_gc_set_memmove_src
 #else // CONFIG_MEMORY_MODEL_38K
 	lda (OLDTXT),y
 	sta memmove__src+0
@@ -227,19 +224,8 @@ varstr_garbage_collect_check_bptr:
 	jsr poke_under_roms
 
 #elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
-	// XXX consider optimized version without multiple JSRs
-
-	jsr peek_under_roms_via_OLDTXT
-	clc
-	adc INDEX+0
-	sta (OLDTXT),y
-	iny
-
-	php
-	jsr peek_under_roms_via_OLDTXT
-	plp
-	adc INDEX+1
-	sta (OLDTXT),y
+	
+	jsr helper_gc_increase_oldtxt
 
 #else // CONFIG_MEMORY_MODEL_38K