cvhi8(buffer.i1a.r, start.i2.r, nchars.i2.r, value.i8.r, ok.i2.r)
This routine will convert the number of hexadecimal characters
specified by "nchars" from the user's byte buffer, "buffer", to an
value in "value". The conversion starts at the character
position specified by "start" (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 "ok" is
returned as zero. Otherwise, the position of the first 'bad'
character is returned in "ok" as a negative value.