A critical component to our Operating System is Wait and Post.  The Wait function allows a task to wait for an event to occur while the Post function is used to signal an event has occurred.  An Event Control Block (ECB) is used for communication between Wait and Post.  The ECB is a fullword (4 bytes) that represents an event.  A task issues a Wait against and ECB and a Post is issued to an ECB.

The ECB status is represented using the first two bits of the first byte.  The first bit (x’80’) indicates an ECB is being waited on.  The second bit (x’40’) indicates the ECB has been posted.

Only one TCB/RB may wait on an ECB at any one time.  The wait status is associated with the RB and is proprigated from the active RB to the TCB.

[Next – SVC 1 – Wait]