Re: DOS 3.3 Nibble Encoding

[mark@trillian.jsc.nasa.gov (Mark Manning)]

Ok, there has been a lot of confusion (IMHO) about what I am talking
about versus what others are talking about.  I'm going to try to clear
that up.

In order to do this, there are some terms I want to lay out so people
can see how I am looking at this.

Formatted - There are two kinds of formatting: Low Level and Normal.
  Low level formatting is the laying out of the bits onto the disk's
  surface.  Normal formatting pertains to how those bits are used.
  An example would be:

    Low level : 5 & 3 encoding, 6 & 2 encoding, etc....
    Normal    : DOS 3.3, ProDOS, CP/M, Apple Pascal, whatever.

  The term "Formatted" is generic.  People use it for both types of
  talking.  "I formatted a disk" could just as easily mean you formatted
  it for DOS 3.2.1 as it could mean for Unix or that you simply laid
  out the bits for further use down the line.

Special: From Webster - Uncommon, Noteworthy, Individual, Unique, Confined
  to or designed for a definite field of action, purpose, or occasion.

------------------------------------------------------------------------
Why is Track 0 Sector 0 special?

It is special because it fits the definition.  Without track 0 sector 0
you could not boot the Apple computer.  Without track 0 sector 0 being
low level formatted correctly, the Apple not only would not be able to
read the track and sector, but the Apple (again) would not boot.  Without
track 0 sector having the bootstrap code placed in it, in the proper
manner, your Apple computer (again) would not boot.  All of these things
together make track 0 sector 0 special.

As I have written to several of you, if you do not think that track 0
sector 0 is special, then swap it with another track and sector (on a
bootable disk) and try to boot the Apple.  It won't work.  It WOULD
work if track 0 sector 0 didn't hold any special meaning to the system.
Instead, the Apple would go seeking off across the disk to locate the
boot block and would then boot the system accordingly.  But it won't.
It won't because track 0 sector 0 MUST contain the code the Apple uses
to boot the rest of the disk.

This is why it is special.  Not because it is low level formatted
differently.  But because the format of that particular track and
sector has to be the same.  No matter what the rest of the disk
looks like.

------------------------------------------------------------------------
Several of you have written and talked to me about "Beneath Apple DOS"
or "Beneath Apple ProDOS".  I've got them both and have read them all the
way through.  I understand about nibble encoding and the layout of
the directories and such.  I've even had occasion to write programs, in
assembly language, to read such things as AWP files, Macintosh diskettes,
and even crashed diskettes of various kinds.  I understand that all of
the diskettes are readable at the assembly language level.  But what I
said, and what I meant was, that DOS 3.3 - without any help whatsoever -
can not read a ProDOS diskette.  And that is true.  You can not, under
DOS 3.3, put a ProDOS diskette into a disk drive, type CATALOG, and hope
to be able to see what is on that diskette.  This is because the
format (normal, not low level) of the diskette places such things as the
directory information in a different location than it does under DOS 3.3.

This (again) is what separates track 0 sector 0 from the rest of the
disk.  That no matter what the formatting (normal) of the disk is, that
track 0 sector 0 is going to have the same basic information in it if the
disk is bootable.  And that no matter what OS you have, the boot track
will remain the same.

------------------------------------------------------------------------
I hope this clears up why I say track 0 sector 0 is special.  True, I
don't think it would be considered special on a data disk, but on a
bootable disk it is.


----------------------------------------------------------------------------
I saw a mountain rising.  Rising above the sea.  I heard the wind calling. 
Calling unto me.  Oh, who has seen the sunset?  Who has seen it rise?  Who
could ever be so blind?  As to never open their eyes? - Mark Manning

mark@trillian.jsc.nasa.gov