Int 7F Fn 0A  - Alloy Ntnx - Get System Flags                              [N]

   AH = 0Ah
   CL = 00h
   ES:DI -> buffer for system flags (see below)

Return: ES:DI buffer filled

Notes: on a slave, only the NX_Busy flag is returned
   all three flags are at fixed positions, so this function only needs to
     be called once
   an interrupt handler should only perform DOS or device accesses when
     all three flags are 00h

Format of system flags:
Offset Size    Description
 00h   DWORD   pointer to NX_Busy flag (nonzero when communicating with users)
 04h   DWORD   pointer to device driver busy flag
 08h   DWORD   pointer to InTimer flag