Section APPENDIX I PAGE STRUCTURE


   The need for FTP to support page structure derives principally from
   the  need to support efficient transmission of files between TOPS-20
   systems, particularly the files used by NLS.

   The file system of TOPS-20 is based on the concept of pages.  The
   operating system is most efficient at manipulating files as pages.
   The operating system provides an interface to the file system so that
   many applications view files as sequential streams of characters.
   However, a few applications use the underlying page structures
   directly, and some of these create holey files.

   A TOPS-20 disk file consists of four things: a pathname, a page
   table, a (possibly empty) set of pages, and a set of attributes.

   The pathname is specified in the RETR or STOR command.  It includes
   the directory name, file name, file name extension, and generation
   number.

   The page table contains up to 2**18 entries.  Each entry may be
   EMPTY, or may point to a page.  If it is not empty, there are also
   some page-specific access bits; not all pages of a file need have the
   same access protection.

      A page is a contiguous set of 512 words of 36 bits each.

   The attributes of the file, in the File Descriptor Block (FDB),
   contain such things as creation time, write time, read time, writer's
   byte-size, end-of-file pointer, count of reads and writes, backup
   system tape numbers, etc.

   Note that there is NO requirement that entries in the page table be
   contiguous.  There may be empty page table slots between occupied
   ones.  Also, the end of file pointer is simply a number.  There is no
   requirement that it in fact point at the "last" datum in the file.
   Ordinary sequential I/O calls in TOPS-20 will cause the end of file
   pointer to be left after the last datum written, but other operations
   may cause it not to be so, if a particular programming system so
   requires.

   In fact, in both of these special cases, "holey" files and
   end-of-file pointers NOT at the end of the file, occur with NLS data
   files.






   The TOPS-20 paged files can be sent with the FTP transfer parameters:
   TYPE L 36, STRU P, and MODE S (in fact, any mode could be used).

   Each page of information has a header.  Each header field, which is a
   logical byte, is a TOPS-20 word, since the TYPE is L 36.

   The header fields are:

      Word 0: Header Length.

         The header length is 5.

      Word 1: Page Index.

         If the data is a disk file page, this is the number of that
         page in the file's page map.  Empty pages (holes) in the file
         are simply not sent.  Note that a hole is NOT the same as a
         page of zeros.

      Word 2: Data Length.

         The number of data words in this page, following the header.
         Thus, the total length of the transmission unit is the Header
         Length plus the Data Length.

      Word 3: Page Type.

         A code for what type of chunk this is.  A data page is type 3,
         the FDB page is type 2.

      Word 4: Page Access Control.

         The access bits associated with the page in the file's page
         map.  (This full word quantity is put into AC2 of an SPACS by
         the program reading from net to disk.)

   After the header are Data Length data words.  Data Length is
   currently either 512 for a data page or 31 for an FDB.  Trailing
   zeros in a disk file page may be discarded, making Data Length less
   than 512 in that case.