Int 15 Fn DE21  - Desqview V2.26+ - "assertvir" - Control 386 Screen Virt  [Q]

   AX = DE21h
   BX = new state
       0000h turn off
       nonzero turn on

Return: BX = old state of virtualization

Notes: this API call may be made from within a hardware interrupt handler
   under DV 2.40 and 2.42, this call appears to have no effect and always
     returns a nonzero value in BX which appears to be the offset within
     the DV common memory segment of the caller's task object; it may
     only have an effect within a hardware interrupt handler

See Also: AX=1117h,AX=DE17h