process must be a full process (that is, not one created by *current-process*).
process-stop causes process to stop until some other process explicitly wakes it up. If it is called on the current process, the current process stops during the call, and returns from
process-stop after the process gets woken up.
In SMP LispWorks, if process is not the current process,
process-stop returns immediately and the execution of process stops at some point, possibly after
process-stop returned. In non-SMP LispWorks if process is not the current process, process stops before
process-interrupt does not wake up a stopped process.
There is a discussion of a typical use of
process-stop in the section Stopping and unstopping processes.
LispWorks User Guide and Reference Manual - 20 Sep 2017