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  When you press the Sys Req key on an 84-key keyboard or Alt-Sys Req on a
  101/102-key keyboard, the ROM BIOS keyboard interrupt handler executes
  interrupt 15H with AH = 85H. You can detect when this key is pressed by
  hooking interrupt 15H and inspecting the value in AH.

  When the Sys Req key is first pressed, the ROM BIOS issues interrupt 15H
  with AH = 85H and AL = 00H. When the key is released, the BIOS executes
  interrupt 15H with AH = 85H and AL = 01H. Thus the structure of an
  interrupt 15H handler that detects Sys Req keystrokes would be as follows:

  IF (AH<>85H)
                  jump to previous interrupt 15H handler
  ELSE IF (AL = 00H)
                  process Sys Req keystroke
       ELSE
                  process Sys Req key release
       exit from interrupt handler