SystemThread, a class within sqldk.dll, can be considered to be at the root of SQLOS’s scheduling capabilities. While it doesn’t expose much obviously exciting functionality, it encapsulates a lot of the state that is necessary to give a thread a sense of self in SQLOS, serving as the beacon for any code to find its way to an associated SQLOS scheduler etc. I won’t go into much of the SQLOS object hierarchy here, but suffice it to say that everything becomes derivable by knowing one’s SystemThread. As such, this class jumps the gap between a Windows thread and the object-oriented SQLOS.
The simplest way to conceptualise a thread in SQL Server is to think of a spid or connection busy executing a simple query, old skool sysprocesses style. It’s not hip, but it’s close enough to the truth to be useful. This conflates a few things that are separate entities, but it is a good starting point for teasing things apart as needed
This is another dive into internals; prepare your thinking caps.