INT 10 - VIDEO - SCREENR v1.55+ - API
        AX = 5049h ('PI')
        BX = function
            0000h installation check
            0001h lock mode
            0002h unlock mode
            0003h lock palette
            0004h unlock palette
Return: AX = 0000h if installed
            BX = TSR version (BH=major,BL=minor)
            CL = mode locking status
                00h mode not locked
                01h mode locked : INT 10/AH=00h disabled
            CH = palette locking status
                00h palette not locked
                01h palette locked, the following functions are disabled:
                        AX=1000h, AX=1001h, AX=1002h, AX=1010h, AX=1012h
Program: SCREENR is a TSR supplied with Patrick Ibbetson's SCREEN display
          utility.