BX flags: 0001H this is the only task switcher
0000H other task switchers exist
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 when they are
terminating. It gives you a chance perform any necessary cleanup
and disable any extra processing that you were doing to coexist
When called, interrupts are enabled you may use all DOS fns.
BX bit 0 will be set (BX=0001H) is this is the only active task
switcher. All other bits will be 0.
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 you must return 0. Tasks have not veto power here.