add sanity checking to BASIC header, in parse_header().

1 files changed, 6 insertions, 2 deletions

diff --git a/bas.c b/bas.c
index cf73fef..24508e4 100644
--- a/bas.c
+++ b/bas.c
@@ -115,13 +115,17 @@ void parse_header(void) {
 	vvstart = vvtp - TBL_OFFSET;
 	code_end = starp - TBL_OFFSET;
 
-	if(lomem) die("This doesn't look like an Atari BASIC program (no $0000 signature).");
-
 	if(filelen < code_end) {
 		fprintf(stderr, "Warning: file is truncated: %d bytes, should be %d.\n", filelen, code_end);
 	}
 
 	if(verbose) dump_header_vars();
+
+	/* these checks are actually kind of conservative. */
+	if(lomem) die("Not an Atari BASIC program (no $0000 signature).");
+	if(vntp < 0x100) die("Not an Atari BASIC program (invalid VNTP).");
+	if(vvtp < vntd) die("Not an Atari BASIC program (invalid VVTP).");
+	if(starp < vvtp) die("Not an Atari BASIC program (invalid STARP).");
 }
 
 void update_header(void) {