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Amiga Collections: MegaDisc
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MegaDisc 38 (1994-02)(MegaDisc Digital Publishing)(AU)(Disk 2 of 2).zip
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MegaDisc 38 (1994-02)(MegaDisc Digital Publishing)(AU)(Disk 2 of 2).adf
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Tutes_&_CLI
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NonDOS_Disks
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NonDOS_Disks
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Text File
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1994-02-14
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7KB
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133 lines
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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