WRITE byte - write the given data to the address given      (v40 sad)

 Command:       $AF $01
 Data:          $ww $xx $yy $zz $qq

 write the byte <$qq> to address <$wwxxyyzz>

 Command will be ACK'ed when received.
 Command ACK:   $00 $01
 Command DONE:  $1F $01

 BUGS
        This command does not exists in pre-V40 EXEC.
        This command can be emulated with the write_array command with
        a length of 1.