beginning of dummy string memory management

c64/basic/engine/alloc_temp_string.s

@@ -0,0 +1,81 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Allocates area for a temporary string
+//
+// Input:
+// - .A - string length
+//
+
+
+// XXX - this is a dummy implementation, until proper string handling is implemented
+// XXX - addresses are probably wrong, debug it
+
+
+alloc_temp_string:
+
+	// Check if temporary descriptor stack has a free slot
+
+	ldx LASTPT
+	cpx #$1F
+	bcs_16 do_FORMULA_TOO_COMPLEX_error
+
+	// Prepare data in index registers
+
+	tay                                // store length, we will need .A in a moment
+
+	// Increment temporary descriptor stack pointers by 3
+
+	lda LASTPT
+	adc #$03                           // Carry is already clear here
+	sta LASTPT
+	adc #$03
+	sta TEMPPT
+
+	// Fill in the new entry
+
+	cmp #$1C
+	bne !+
+
+	// Fill in the address - first entry
+
+	ldx TEMPPT
+	lda STREND+0
+	sta $01, x                    // address - lo byte
+	lda STREND+1
+	sta $02, x                    // address - hi byte
+
+	jmp alloc_temp_string_space_check
+!:
+	// Fill in the adress - use previous entry
+
+	ldx LASTPT
+	lda $01, x                   // address - lo byte
+	ldx TEMPPT
+	sta $01, x
+
+	ldx LASTPT
+	lda $02, x                   // address - hi byte
+	ldx TEMPPT
+	sta $02, x
+	inc $02, x
+
+	// FALLTROUGH
+
+alloc_temp_string_space_check:
+
+	// Fill in the length of the new string
+
+	ldx TEMPPT
+	tya
+	sta $00, x                    // length
+
+	// Check if we have enough space
+
+	lda $02, x
+	cmp FRETOP+1
+	beq_16 do_OUT_OF_MEMORY_error
+
+	rts