Cleaner assembly formatting string, syscall length checking

user/usys.py

@@ -1,13 +1,18 @@
 #!/usr/bin/env python3
 
-template = """.global {name}
+template = """
+.global {name}
 {name}:
     li a7, SYS_{name}
     ecall
-    ret"""
+    ret
+""".strip()
 
 
 def genstub(syscall: str):
+    if not 3 >= len(syscall) <= 15:
+        raise ValueError("Syscall {} is too short/long!".format(syscall[:35]))
+
     return template.format(name=syscall)