No Fragments Archive 10: Diskmags/nf_archive_10.iso/MAGS/ST_NEWS/STN_01_C.MSA/DATA_DOC10

Text File | 1994-03-14 | 6KB | 138 lines

üDISKMANIPULATION ON THE ATARI SF DISK DRIVES PART IIÇ
by Richard Karsmakers

Originally  published  in üST NEWSÇ Volume 1 Issue  3,  launched  on
August 16th 1986.

In  the  previous issue of üST NEWSÇ,  I have told  you  some  basic
things you need to know when you start disk manipulating.  In this
issue,  I'll  talk about some specific data and things you'd  also
need to know if you start manipulating....

First,  let's  talk a bit about the File Allocation Table  or  FAT
(for people who owned a Commodore 64 before:  this can be compared
with  the Block Availability Map or BAM on the 1541  disk  drive).
Normally,  each  FAT  entry is 12 bits in  size.  It  can  contain
several values:

      $000       Cluster unused
      $FF7       Cluster unusable, mostly because of bad medium
      $FFF       Last cluster in a file
      other      Number of the next cluster in a file

Clusters are numbered starting at two,  and each cluster  normally
represents  two sectors (so 1024 bytes).  Now you know the  reason
why a 2-byte file takes up a whole kilobyte on disk....
Now the directory. Each directory entry is 32 bytes in size, which
are divided as follows:

       1- 8      Filename
       9-11      File extension
        12       File attribute
      13-22      Unused/Reserved
      23-24      Time
      25-26      Date
      27-28      Number of the first cluster of a file
      29-32      Size in bytes

The filename can have upto 8 characters in it, if which nearly all
letters,  etc.  are possible.  First Word has difficulties reading
filenames with german characters,  etc.  in it,  and the following
characters can (may) never be used in a filename:

[]()<>=*&,!|?/\$.:;+-

The filename extension cannot be carelessly manipulated,  since it
determines whether a file can be run or  only shown/printed.   The
following  extension  have special meaning (or  can  have  special
meanings):

      PRG        A regular program, that can use GEM. Can be run
      TOS        A program that doesn't use GEM (mouse pointer
                     disappears). Can be run
      TTP        A program where TOS Takes Parameters (TTP), e.g.
                     an assembler, where the source file has to be
                     specified. This can now be done in a box
      IMG        This is normally the extension of a TOS on disk,
                     but is also used in games like "X-TRON", a
                     database called "Zoomracks" and the digitized
                     music demo's from the Desaster Area (Oxygen &
                     Foreign Affair)
      SYS        This is the extension of a program called
                     "ASSIGN", usually present on drawing programs
                     like Easy Draw, Gem Paint, etc. te determine
                     whether the program runs in monochrome or co-
                     lor resolution

      ACC        (This is no advertisement) Provided that a pro-
                     gram has the format of a desk accesory, up
                     to five of these files can be loaded at sys-
                     tem BOOTing to become desk accesories
      DOC        The extension of a First Word text file, but
                     can be used otherwise as well
      TXT        Usually a text file
      ONE        The extension of a DB Master One data file
      ZRX        The extension of a Zoomracks data file
      APP        I don't know exactly what this does...
      ???        Only PRG, TOS, TTP and APP can be loaded....











Now the attributes. What exactly is a file attribute? It is a byte
that  uses  several  of  its bits  to  determine  certain  status-
qualities.  For  example,  there's a read-only  status,  a  hidden
status, and some more. The bits are located as follows:

      Bit 0:    Read only
      Bit 1:    Hidden
      Bit 2:    System
      Bit 3:    Volume
      Bit 4:    Subdirectory
      Bit 5:    Archive bit (hard disk only: written to & closed)

The read-only attribute seems quite clear:  if the specific bit is
set,  it is not possible to delete or rename the file.  The use of
both  hidden-and system attributes still have to be  explained  to
me,  but  they  take  care that a file isn't shown  an  a  regular
directory.  How  it is readable,  that's a mystery to me  (doesn't
that remember you to a certain song?!?).  The volume label is  set
if a file is no file but the name of a disk (the disk label).  The
subdirectory attribute is set if a file (again!) is no file, but a
subdirectory,  better known as a folder.  The TOS doesn't read it,

but  opens  a  subdirectory somewhere else on  the  disk  if  it's
clicked.
You'd better stay out of the unused/reserved bits;  you never know
what  people  might  do with these (may be even as  part  of  some
clever protection scheme!).
Both time and date are quite difficult to calculate,  since you'll
have to process individual bits. The two time-bytes are divided as
follows:

          Bit 0-4          Seconds
          Bit 5-10         Minutes
          Bit 11-15        Hours

You'll  have to multiply the seconds you get with two to  get  the
proper time!
The date bytes are divided as follows:

         Bit 0-4           Day
         Bit 5-8           Month
         Bit 9-15          Year

Here, you'll have to add 1980 to the value of "year"....
That's all folks,  for this issue of üST NEWSÇ.  Next time, we might
go and have a look at some specific disk manipulation. See ya!