Understanding Non-DOS disks
                        
                          by Paul Mclachlan
    


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    In my younger and less sensitive years, I was once amused by a
  friend of mine's shortcomings in Amiga literacy.  (We both had
  500's) While I have now moved on from such arrogant attitudes, you
  may find it interesting to have me recount a situation that once
  occurred.

    We had both received a copy of a European demo (similar to the
  ones in Tim's NDOS collection).  Back in those days (1.3) you could
  tell if a disk was non dos by whether a CLI opened on bootup or not.
  There were devious tricks that I have covered in a previous article
  (compilations, a while back), but basically, that was the story.

    Anyway, knowing my friend rather well, I specifically told him not
  to 'run' ANYTHING through disk-doctor without calling me first and
  checking that it would be OK.  Well, this one weekend I happened to
  be away.  (I'm sure you can see this coming).

    I got back to school the following monday to find my berated
  holding a disk and requesting another copy.  It seems that due to an
  unfortunate mishap with a virus, the disk ceased to work.  John (my
  friend) had another Amiga friend at his place at the time, (who had,
  probably, given him the virus) and who suggested that they run the
  unfortunate disk through the great disk-doctor.

    John replied that he didn't think that it was such a great idea,
  using words to the effect of:  A friend of mine knows a lot about
  this kinda stuff, and he said not to.  This didn't stand up to a
  "What harm can it do?".

    Now, as anyone who remembers diskdoctor could tell you, the great
  doctor could turn *anything* into a DOS disk, regardless of the
  consequences, without warnings, and without a second chance.
  DiskDoctor grinded a lot, complained, came out with a disk called
  'Lazarus" that was effectively empty.

    To finish the tale, I had, of course, deleted my copy, but
  retained a backup of the bootblock (that being the only thing that
  was ever damaged by a virus back then).  John never did get a look
  at that demo.  And, at the time, I just laughed at him:  "I told you
  so."


    A fascinating tale, I hope.  But it serves a purpose (besides
  showing that I used to be rather cruel).  The Non-Dos disk cannot be
  treated by the OS at all, and if anything happens to it, the only
  thing you can do is pray for a backup to fall from the heavens.

    I now aim to educated you, teach you to understand the Non-Dos
  disk, understand why it is used, and to underst...  I'll just tell
  you what I know about it, OK?

    Of course, the non-dos disks that don't work are really non-dos.
  They might be IBM or Atari format, or anything could be wrong with
  them.  I am only referring to Amiga Non-Dos disks in the rest of
  this article:

    When you 'format' a disk, you are simply structuring the disk so
  that files and so forth can be written on it by AmigaDOS.  Besides
  wiping over what used to be on the disk, this also writes two
  special blocks, the bootblock, and the directory block.  These two
  are on the lower side of the disk, found at the start and middle
  respectively.  Now, in the directory block of the disk is a listing
  of all the files on the disk, as well as where the data for those
  files is.  In the bootblock is just a really small program and a
  checksum, which makes the disk bootable.

    What the standard bootblock does (if anyone has ever looked at it)
  is open the 'dos.library', and then return.  You see, at the end of
  a warm (or cold) boot, the OS loads and runs the program on the
  bootblock.  Then, if this program returns, control goes back to the
  OS, and a CLI will be opened and the startup sequence executed.

    However, not all bootblocks do this.  Some are viruses, in which
  the virus loads itself when this bootblock is run, and then loads
  the dos library and returns control (so nobody is any the wiser).
  Some are virus protectors or bootblock utilities (and there are some
  really fabulous ones out there), and finally, are the non dos disk
  bootblocks.

    The idea of a non-dos disk is that the OS is never used.  The
  bootblock loads the program off the disk because it knows where it
  is on the disk, and there are no 'files' or 'directories' as such.
  Just data that has been put on the disk without a particular format.

    There are several advantages of creating disks in this format, the
  first of which is that data stored like this will load quicker (be
  it a game or a demo), since it does not need to load the startup
  sequence or search the directory to find where on the disk the data
  it wants is.  Another advantage is that there is no chance of
  someone trying to multitask in the background, and also you have all
  the memory and capabilities of the machine to yourself.  Non-DOS
  disks allow authors to add advanced copy protection to their disks
  as well.

    The disadvantages are that the OS does not, by itself do anything
  for you.  If you want a mouse pointer, you are going to have to
  write it yourself, same for windows and screens.  You cannot access
  any libraries, and even using normal DOS disks would be difficult.
  Of course, none of these disadvantages apply to games and demos.

    The major disadvantage of Non-Dos disks these days is the lack of
  hard drive installation.  This will annoy many users with hard
  drives who like your demo/game or whatever, and wish to install it
  on their hard drives for faster access.

    Other problems when dealing with non-dos disks are that most of
  the time DiskCopy will not copy them, and you need another more
  advanced copy program to do it (DCopy, XBackup etc).  This is
  because DiskCopy checks to ensure that a disk is DOS, and otherwise
  does some wierd things to the disk you are copying onto.

    While I can see the use of non-dos disks, I believe that authors
  would be best to leave them alone, due to the advent of hard drives.
  Before Hard drives were popular (became the rule rather than the
  exception), the use of non-dos disks was perfectly acceptable,
  especially due to the advantages they provided.  But now I believe
  that it is a section of Amiga life that should be laid to rest.



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