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Function 13     : Set User Function.

Entry          : BX = 13.
                 DX:AX = Address to user function.
Exit           : None.

This function is invoked to specify the address of an optional user
function. By default, the user function is not specified.

When the address of the user function is specified, CT-VOICE
driver calls the user function on encountering each voice data
block.

Upon entry to the user function, ES:BX points to the first byte of
the current data block, which is the block type.

User functions can carry out any task is desires and specify to the
driver whether current data block is to be precessed by the driver
or not.

The user function must observe the following conditions:

     A.    The function must perform a far RET.

     B.    The function must preserve the DS, DI, SI and the flag register
           except the carry flag.

     C.    The function must clear the carry flag, if current data block
           has to be processed by the driver, otherwise set the carry flag.

     D.    The function must not set the carry flag, if current data block
           is the terminate block, i.e. block type is 0.

To disable the call to the user function, invoke this function with
the contents of both DX and AX set to zero.