INT 21 U - PenDOS PENDEV.SYS - GET ENTRY POINTS
        AX = 4402h
        BX = file handle for device "$$PENDOS" or "$$PD_REG"
        CX = size of buffer (4 for $$PENDOS and a 4,8,12, or 16 for $$PD_REG)
        DS:DX -> buffer for entry point record (see below)
Return: CF clear if successful
            buffer filled
        CF set on error
            AX = error code (01h,05h,06h,0Dh) (see AH=59h)
Program: A limited version of PenDOS by Communication Intelligence Corporation,
          which provides pen capability to keyboard-based programs, is bundled
          with IBM DOS 6.1
SeeAlso: AX=4403h"PENDEV.SYS"

Format of entry point record:
Offset  Size    Description
 00h    DWORD   -> array of jumps
 04h    WORD    offset of function to retrieve entry point
 06h  2 BYTEs   signature "Pe"
 08h    WORD    offset of function to set entry point
 0Ah  2 BYTEs   signature "nD"
 0Ch    WORD    offset of function to clear entry point
 0Eh    WORD    signature "OS"

Call function to retrieve entry point with:
        AX = index of entry point (0-9)
Return: CF clear if successful
            DX:AX -> desired entry point
        CF set on error (AX out of range)

Call function to set entry point with:
        AX = index of entry point (0-9)
        DX:SI -> new handler
Return: CF clear if successful
        CF set on error (AX out of range)

Call function to clear entry point with:
        AX = index of entry point (0-9)
Return: CF clear if successful
        CF set on error (AX out of range)
Note:   resets the jump at the specified entry point to its default target,
          which simply returns

.