Compatibility: 5.0+
Expects: AX    4B03H  (INT 2fH)
BX    0000H
ES:DI Service-function address of task switcher to allocate
                                                                  
Returns: AX    0000H if successful (if a switcher ID is available
BX    switcher ID for caller (if successful)
                                                                  
Info: Programs which wish to act as a task switcher (or session
manager) must obtain a switcher ID.  That ID will be in the range
0000h-000fH; it will used as the high four bits in the task IDs
it creates for tasks it handles.

The first task switcher loaded must intercept this fn and provide
this service (you know you're the first when INT 2fH 4B02H
returns AX=0000h).

This in NOT called by normal application programs--only by task
switchers.

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INT 2fH 4B03H: Allocate Switcher ID