some preparations for string variables, added BASIC command COLD

src/basic/engine/varstr_alloc.s

@@ -0,0 +1,106 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Allocates memory for string - lowers FRETOP (but not below STREND)
+//
+// Note: We are not going to use the C64-style memory management here, as it implies
+// a very slow garbage collector. Instead, we are using a scheme similar to used in
+// BASIC V3.5 and later, with 'back-pointers' after each string, telling about the
+// location of the string descriptor. 
+//
+// See:
+// - https://www.c64-wiki.com/wiki/Memory_(BASIC)
+// - https://sites.google.com/site/h2obsession/CBM/basic/variable-format
+//
+// Input:
+// - DSCPNT+0, DSCPNT+1 - pointer to string descriptor, first pointed byte should contain desired length
+//
+
+varstr_alloc:
+
+	// Mark garbage collector as not run yet
+
+	lda #$00
+
+	// FALLTROUGH
+
+varstr_alloc_retry:
+
+	sta GARBFL                                   // Computes Mapping the Commodore 64, page 10
+
+	// Backup FRETOP in case of alocation problems
+
+	lda FRETOP+0
+	sta INDEX+0
+	lda FRETOP+1
+	sta INDEX+1
+
+	// Lower FRETOP to make space for string descriptor
+
+#if !HAS_OPCODES_65CE02
+
+	lda #$02
+	jsr varstr_FRETOP_down 
+
+#else // HAS_OPCODES_65CE02
+
+	dew FRETOP
+	dew FRETOP
+
+#endif
+
+	jsr varstr_FRETOP_check
+	bcs varstr_alloc_fail
+
+	// XXX finish this
+
+
+
+
+
+varstr_alloc_fail:
+
+	// Unable to alocate memory - restore FRETOP value
+
+	lda INDEX+0
+	sta FRETOP+0
+	lda INDEX+1
+	sta FRETOP+1
+
+	// Check if it is worth to perform garbage collection and retry
+
+	bit GARBFL
+
+	// XXX finish this
+
+
+
+
+// XXX move these helper routines to a better place
+
+
+varstr_FRETOP_down: // memmove__tmp - length to lower FRETOP
+
+	sec
+	lda FRETOP+0
+	sbc memmove__tmp
+	sta FRETOP+0
+	bcs !+
+	dec FRETOP+1
+!:
+	rts
+
+varstr_FRETOP_check:
+
+	// Check if FRETOP > STREND, Carry set if not
+
+	lda STREND+1
+	cmp FRETOP+1
+	beq !+
+	rts
+!:
+	lda STREND+0
+	cmp FRETOP+0
+	rts