INT 7F - IBM 8516 Touch Screen Device Driver - GET API ENTRY
        AX = ABCDh
        BX = 0000h
Return: AX = total number of functions available
        ES:BX -> entry point array
SeeAlso: AX=0104h,AX=0105h

Function numbers:
 00h    check initialization and reset
 14h    set user-defined subroutine
Notes:  each driver function takes two stack parameters using Pascal calling
          conventions: address of parameter block and address of results buffer
        all pointers are FAR pointers
        on return, AX contains the status of the call:
            AX = 0000h successful
                 0001h invalid input
                 0002h interface error
                 0003h unable to perform function

Format of parameter block for function 00h:
Offset  Size    Description
 00h    WORD    0000h (function number)
Note:   this function should be called before any other device driver functions

Format of results buffer for function 00h:
Offset  Size    Description
 00h    WORD    touch screen status
                0000h unavailable
                0001h uncalibrated
                FFFFh available
 02h    WORD    aux mouse status (0000h not present, FFFFh present)
Notes:  the following driver parameters will have been reset to zero:
          touchdown counter, liftoff counter, position at last touch, position
          at last lift, int call mask, select on count, select off count,
          pos select on count, pos select off count.
        the following driver parameters will have been reset as listed:
          mouse emulation mode: left on
          thresholds: 46 on screen, 96 push harder, 80 push release
          x, y hysteresis: 400
          data repeat rate: 40/sec
          select mechanism: push-harder - first-touch
          coordinate origin: upper left corner
          filter frequency: medium
          data block mask: all enabled
          click lock: on

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