Expect: AX    0003H
BX    ID of the session that is now active
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 has
been started or that a suspended session has been resumed.

When called, interrupts are enabled you may use all DOS fns.

BX will contain the session ID of the session that has been
activated.

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)

INT 2fH 4bxxH (switcher functions)
- -

SwNtfy Fn 0004H: Session Is Now Active