protected static long packId(long key, DBTraceSpaceKey spaceKey) {
		return spaceKey.getAddressSpace().isRegisterSpace() ? packRegId(key, spaceKey)
				: packMemId(key, spaceKey);
	}
For non-register space:

{@code
+---12----+----52----+
| SpaceID |    Key   |
+---------+----------+
}

For register space:

{@code
+---12----+----32----+----8----+--12--+
| SpaceID |  Thread  |  Frame  |  Key |
+---------+----------+---------+------+
}
Ghidra/Debug/Framework-TraceModeling/src/main/java/ghidra/trace/database/bookmark/DBTraceBookmarkManager.java