--- nono/util/sramedit/params.cpp	2026/04/29 17:04:55	1.1.1.1
+++ nono/util/sramedit/params.cpp	2026/04/29 17:05:16	1.1.1.2
@@ -252,13 +252,21 @@ w_bootdev(const sraminfo_t& si, const st
 			warnx("%s='%s': bad SASI ID", si.name, input.c_str());
 			return -1;
 		}
+		if (id < 0 || id > 16) {
+			warnx("%s='%s': out of range", si.name, input.c_str());
+			return -1;
+		}
 		newval = 0x8000 + (id << 8);
 	} else if (starts_with_ignorecase(input, "fd")) {
 		errno = 0;
-		start = input.c_str() + 4;
+		start = input.c_str() + 2;
 		int id = strtoul(start, &end, 10);
 		if (end == start || *end != '\0' || errno == ERANGE) {
-			warnx("%s='%s': bad SASI ID", si.name, input.c_str());
+			warnx("%s='%s': bad unit number", si.name, input.c_str());
+			return -1;
+		}
+		if (id < 0 || id > 4) {
+			warnx("%s='%s': out of range", si.name, input.c_str());
 			return -1;
 		}
 		newval = 0x9070 + (id << 8);