DOS Fn 3fH: Read from File via Handle

                                                         Compatibility: 2.0+ 
 Expects: AH    3fH
          BX    file handle
          CX    number of bytes to read
          DS:DX address of buffer to receive data
 Returns: AX    error code if CF is set to CY
                if no error, AX=number of bytes actually read
    Info: Reads CX bytes of data (or fewer) from the file or device with
          handle number BX.  The data is read from the current position of
          the file's read/write pointer and is placed into the caller's
          buffer pointed to by DS:DX.

          This updates the file's read/write pointer to set up for a
          subsequent sequential-access read or write.  To access a specific
          part of a file, use Fn 42H (Lseek) before this call.

   Notes: ■ You should always compare the return value of AX (number of
            bytes read) to CX (number of bytes requested):

            • if AX = CX, (and CF=NC) the read was correct with no error
            • if AX = 0, the end of the file (EOF) was reached
            • if AX < CX (but not zero):
                if reading from a device, the input line is AX bytes long
                if reading from a file, the call has read up to the EOF

          ■ It is handy to use this function for reading default handles
            such as the Standard I/O handles, instead of the buffered input
            or character-by-character input functions.

          ■ When you read from a device, AX returns the length of the line
            up to and including the terminating CR (ASCII 13H).

See Also: Handle-Oriented File I/O
          DOS Functions