simplify pyc header format

pep-0552.txt

@@ -61,19 +61,26 @@ address here that can make pycs non-deterministic.)
 Specification
 =============
 
-Python will begin to recognize two magic number variants for every pyc
-version. One magic number will correspond to the current pyc format and the
-other to "hash-based" pycs introduced by this PEP.
+The pyc header currently consists of 3 32-bit words. We will expand it to 4. The
+first word will continue to be the magic number, versioning the bytecode and pyc
+format. The second word, conceptually the new word, will be a bit field. The
+interpretation of the rest of the header and invalidation behavior of the pyc
+depends on the contents of the bit field.
 
-In hash-based pycs, the second field in the pyc header (currently the
-"timestamp" field) will become a bitset of flags. We define the lowest flag in
-this bitset called ``check_source``. Following the bitset is a 64-bit hash of
-the source file. We will use a SipHash_ with a hardcoded key of the contents of
-the source file. Another a fast hash like MD5 or BLAKE2_ would also work. We
-choose SipHash because Python already has a builtin implementation of it from
-:pep:`456`, although an interface that allows picking the SipHash key must be
-exposed to Python. Security of the hash is not a concern, though we pass over
-red-flag hashes like MD5 to ease auditing of Python in controlled environments.
+If the bit field is 0, the pyc is a traditional timestamp-based pyc. I.e., the
+third and forth words will be the timestamp and file size respectively, and
+invalidation will be done by comparing the timestamp and file size of the source
+file with that in the header.
+
+If the lowest bit of the bit field is set, the pyc is a hash-based pyc. We call
+the second lowest bit the ``check_source`` flag. Following the bitset is a
+64-bit hash of the source file. We will use a SipHash_ with a hardcoded key of
+the contents of the source file. Another a fast hash like MD5 or BLAKE2_ would
+also work. We choose SipHash because Python already has a builtin implementation
+of it from :pep:`456`, although an interface that allows picking the SipHash key
+must be exposed to Python. Security of the hash is not a concern, though we pass
+over red-flag hashes like MD5 to ease auditing of Python in controlled
+environments.
 
 When Python encounters a hash-based pyc, its behavior depends on the setting of
 the ``check_source`` flag. If the ``check_source`` flag is set, Python will