cvhi4(input_buffer.i1a.r, start_index.i2.r, num_chars.i2.r, value.i4.r,
This routine will convert the number of hexadecimal characters
specified by "num_chars" from the user's byte buffer, "input_buffer",
to an value in "value". The conversion starts at the character
position specified by "start_index" (first character is position 1).
If a null character is encountered, the conversion will be terminated.
If a valid conversion has been performed, the value of "error" is
returned as zero. Otherwise, the position of the first 'bad'
character is returned in "error" as a negative value.