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

3 files changed, 8 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) {
diff --git a/listbas.1 b/listbas.1
index 146df50..ba293f2 100644
--- a/listbas.1
+++ b/listbas.1
@@ -220,6 +220,7 @@ looks very similar to how it would appear on the Atari.
 \fBlistbas\fP does color syntax highlighting.
 .IP \(bu 2
 \fBlistbas\fP only supports Atari BASIC, not Turbo BASIC or BASIC XL/XE.
+This is a bug, not a feature...
 .IP \(bu 2
 \fBlistbas\fP doesn\(aqt show information about the variables. Use \fBvxrefbas\fP(1)
 for that.
diff --git a/listbas.rst b/listbas.rst
index 46a44a4..63eb575 100644
--- a/listbas.rst
+++ b/listbas.rst
@@ -188,6 +188,7 @@ protection.
 - **listbas** does color syntax highlighting.
 
 - **listbas** only supports Atari BASIC, not Turbo BASIC or BASIC XL/XE.
+  This is a bug, not a feature...
 
 - **listbas** doesn't show information about the variables. Use **vxrefbas**\(1)
   for that.