Int 2F Fn 4B03  - DOS 5+ Task Switcher - Allocate Switcher Id              [T]

   AX = 4B03h
   ES:DI -> task switcher entry point (see AX=4B02h)

Return: AX = 0000h
   BX = switcher ID (0001h-000Fh), or 0000h if no more available

Notes: if a task switcher has determined that it is the first to be loaded, it
     must allocate an identifier for itself and provide this function to
     all subsequent task switchers; if it is not the first to be loaded,
     it must call this function to allocate an ID.  The switcher ID is
     used as the high four bits of all session identifiers to ensure
     unique session IDs.
   if no more switcher IDs are available, the new task switcher making the
     call must terminate or disable itself
   the task switcher providing the identifiers may call the new task
     switcher's entry point as needed
   this call is available from within DOSSHELL even if the task switcher
     is not installed
   this function is supported by PC Tools v8+ CPTASK, but appears to
     always return an ID of 0000h

See Also: AX=4B02h,AX=4B04h