The AOD Virtual Partition Specification - First Draft - 2016-04-04 -

From	"Mike Gonta" <mikegonta@gmail.com>
Newsgroups	alt.os.development
Subject	The AOD Virtual Partition Specification - First Draft - 2016-04-04 -
Date	2016-04-04 14:38 -0400
Organization	Aioe.org NNTP Server
Message-ID	<nducbn$lol$1@gioia.aioe.org> (permalink)

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The AOD Virtual Partition Specification
An alt.os.development Community Project
- First Draft - 2016-04-04 -

The Virtual Partition  makes  it  possible  for  a  hobbyist
system to manage  a  FAT32  volume  as  easily  as  a  FAT12
floppy  disk  image.   Restricting  access  to   the    root
directory combined with the  small  sizes  of  the  FAT  and
root directory tables, resulted in a  simple  FAT12  driver.

The USB flash drive  and  FAT32  have  replaced  the  floppy
disk, however increase capacity and differences  with  FAT32
have  prevented  the  use  of  that  same  simple    driver.

The Virtual Partition is a subset of the  three  FAT32  file
system components - FAT  table,   root  directory  and  data
space. It is these smaller, "fixed size"  tables  that  make
a simple driver simple.

One  of  the  differences  with  FAT32  is  that  the   root
directory is stored as a  file  and  not  as  a  table.   By
pre-allocating a small number of contiguous  root  directory
clusters, we  effectively  have  our  root  directory  table
once again. A "fence file" (an  empty  file  the  same  size
our Virtual Partition) is the  first  file  written  to  the
root  directory.   This  file  reserves  the  space  of  our
Virtual Partition for the hobbyist  system  so  that  "other
files" (written to a folder in the root  directory)  can  be
written to the drive. The "fence file" is  then  deleted  so
that all files now written - up to the end  of  our  Virtual
Partition - will  be  accessible  to  the  hobbyist  system.

Typically,  in  the  hobbyist  FAT12  driver  when  a  table
entry  is  modified  the  entire  small  table  was   simply
written back to the  drive.   A  simple  enhancement  is  to
calculate  the  sector  in  the  table  that  contains   the
modification and only write that sector back.   Table  sizes
of up to 64KB can be  easily  managed.  This  size  provides
up to 2048 root files and  (with  one  sector  per  cluster)
8MB of data space, more than  sufficient  for  the  hobbyist
system.


Mike Gonta
look and see - many look but few see

http://mikegonta.com
http://tawk.to/mikegonta

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The AOD Virtual Partition Specification - First Draft - 2016-04-04 - "Mike Gonta" <mikegonta@gmail.com> - 2016-04-04 14:38 -0400
  Re: The AOD Virtual Partition Specification - First Draft - 2016-04-04 - James Harris <james.harris.1@gmail.com> - 2016-04-04 22:35 +0100
    Re: The AOD Virtual Partition Specification - First Draft - 2016-04-04 - "Mike Gonta" <mikegonta@gmail.com> - 2016-04-04 18:51 -0400

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