several features are now optional, added legacy BASIC memory model

c64/basic/peek_line_pointer_null_check.s

@@ -0,0 +1,34 @@
+// Return C=1 if a pointer in BASIC memory space is NULL, else C=0
+// X = ZP pointer to check
+
+peek_line_pointer_null_check:
+
+	ldy #$00
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<basic_current_line_ptr+0
+	jsr peek_under_roms
+	cmp #$00
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (basic_current_line_ptr),y
+#endif
+
+	bne ptr_not_null
+	iny
+
+#if CONFIG_MEMORY_MODEL_60K
+	jsr peek_under_roms
+	cmp #$00
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (basic_current_line_ptr),y
+#endif
+
+	bne ptr_not_null
+
+	// Pointer is NULL
+	clc
+	rts
+
+ptr_not_null:
+	sec
+	rts