DOS:

        Set current date (INT 21h, 2Bh)

        Return
          AH = 2Bh
          DL = day
          DH = month
          CX = year
        Return:
          AL = 00h if no error
          AL = FFh if bad value sent to routine
               (DOS 3.3 also sets CMOS clock)


        ────────────────────────────────────────────────────────────



        DESQview:
        Installation check (INT 21h, 2Bh)

        Entry:
          AH = 2Bh
          AL = subfunction
                01h get version
                    Return: BX = version (v2.00 returns 2, v2.01
                              returns 0201h)
                02h get shadow buffer info, and start shadowing
                    Return: BH = rows in shadow buffer
                            BL = columns in shadow buffer
                            DX = segment of shadow buffer
                04h get shadow buffer info
                    Return: BH = rows in shadow buffer
                            BL = columns in shadow buffer
                            DX = segment of shadow buffer
                05h stop shadowing
          CX = 4445h ('DE')
          DX = 5351h ('SQ')
        Return:
          AL = 00h if DESQview installed, plus any other return values
             = FFh if DESQview not installed

        ────────────────────────────────────────────────────────────


        PC Tools v5.1 PC-CACHE - INSTALLATION CHECK: (INT 21h, 2Bh)

        Entry:
          AH = 2Bh
          CX = 4358h ('CX')
        Return:
          AL = FFh if PC-CACHE not installed
          AL = 00h if installed
            CX = 6378h ('cx')
            BX = ???
            DX = ???