BINSCII(5)                FILE FORMATS                 BINSCII(5)



NAME
     bsc - format of a BinSCII file

CAVEAT
     The information below is based almost  entirely  on  my  own
     analysis  of  Marcel J. E. Mol's sciibin source code.  I can
     make no guarantees about its  accuracy.   Use  at  your  own
     risk.

DESCRIPTION
     Files encoded in the BinSCII  format  are  broken  into  12K
     (12288-byte)   segments,   and   each   segment  is  encoded
     separately.  All segments except the last are 12K long;  the
     final  segment  may be shorter than 12K if the length of the
     original file was not an exact multiple of 12K.

     Each segment of a BinSCII-encoded  file  contains  3  parts:
     the header, the body, and the checksum.

     The header consists of three lines.  The  first  line  is  a
     "sentinal" line used to locate the start of the segment.  It
     consists of the following text:

          FiLeStArTfIlEsTaRt

     The second line is the alphabet used in  the  encoding.   In
     all current incarnations of BinSCII, it looks like this:

     ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789()

     The third line contains the file name  and  characteristics,
     and  information  that  tells the decoder which piece of the
     file this segment represents.  The first 16  characters  are
     the  ProDOS  filename  -  the  first character indicates the
     length of the  filename  (A=1,  B=2,  C=3,  etc.),  and  the
     remaining  15  characters are the actual filename, padded on
     the right with blanks.  The final 36 chracters of the  third
     line  are  an  encoded  representation of the following data
     structure:

          struct binscii header {
                        -
               byte filesize[3]; /* Total size of original file */
               byte segstart[3]; /* Offset into original file of start of this seg */
               byte acmode;      /* ProDOS file access mode */
               byte filetype;    /* ProDOS file type */
               byte auxtype[2];  /* ProDOS auxiliary file type */
               byte storetype;   /* ProDOS file storage type */
               byte blksize[2];  /* Number of 512-byte blocks in original file */
               byte credate[2];  /* File creation date, in ProDOS 8 format */
               byte cretime[2];  /* File creation time, in ProDOS 8 format */
               byte moddate[2];  /* File modification date */
               byte modtime[2];  /* File modification time */



Sun Release 4.1    Last change: Dec. 18, 1992                   1






BINSCII(5)                FILE FORMATS                 BINSCII(5)



               byte seglen[3];   /* Length in bytes of this segment */
               byte crc[2];      /* CRC checksum of preceding fields */
               byte filler;      /* Unused filler byte */
          };

     All multibyte fields of this structure are stored  in  Apple
     II  byte  order  (low-order  byte first).  This structure is
     encoded in the same manner as the data lines,  as  described
     below.

     Immediately following the header lines come the data  lines.
     Each  data  line is exactly 64 characters long, representing
     48 characters of the original file (if the  file  length  is
     not  an  exact multiple of 48 bytes, the last line is padded
     with enough binary zeroes to fill out the line).

     Immediately following the last  data  line  of  the  segment
     comes  the  checksum.   The  checksum is a 2-byte CRC of the
     data in the current segment.  It is encoded by  attaching  a
     binary  zero  to the end to increase the length of the field
     to three bytes, which are then encoded in the  same  fashion
     as the data lines.

     The encoding scheme used by BinSCII takes data  three  bytes
     at  a  time,  and converts them to four 6-bit bytes, each of
     which is translated into an ASCII character.  If the bits of
     the three input bytes are as follows,

          abcdefgh ijklmnop qrstuvwx,

     then the four output bytes will be as follows:

          00stuvwx 00mnopqr 00ghijkl 00abcdef.

     Each of these bytes is translated into an ASCII character by
     indexing into the encoding alphabet (from the second line of
     the header of the BinSCII segment).

     The CRC checksums in BinSCII are calculated  using  the  CRC
     polynomial X^16 + X^12 + X^5 + 1.

SEE ALSO
     bsc(1), uuencode(1), uuencode(5)