Expect: AX    0003H
BX    ID of the session that is about to be activated
CX    flags: 0001H=this is a new session
0000H=resuming a previously-suspended session
ES:DI addr of switcher's service-entry address
                                                                  
Return: AX    0000H (you must return 0 in all cases)
                                                                  
Info: Task switchers call this to let you know that a new session is
about to be started or that a suspended session is about to be
resumed.  When a session is resumed, this is called only after
all instance data and the session's interrupt table has been
reinstalled.

When called, interrupts are disabled you should NOT enable
interrupts or use any DOS fns.

BX will contain the session ID of the session that is about to be
activated.  Global TSRs may wish to keep track of new sessions if
they take special actions for each session.  You can identify a
new session by bit 0 of the value in CX.

ES:SI will contain the task switcher's service entry address.  Use this
address if you need to call any task-switcher service fns.

Return: AX=0000H (must be 0)

Notes: For new sessions, this will be preceded by a SwNtfyFn 0005H
(create session).

INT 2fH 4bxxH (switcher functions)
- -

SwNtfy Fn 0003H: Switcher Is Activating Your Session