Q2 The Filing Systemq


									

DWhat is a filing system?d

Almost all computer applications need some kind of access to an external
storage medium, for example a disk, to save information. The most common
reason for this is that the computer does not remember what is in its
memory after it has been switched off. Also, some programs may need to use
large amounts of data that will not all fit into memory at the same time.
To use this external data, some means of accessing it must be provided in
order to retrieve or modify existing items and to create new items. The
items are refered to as Efilese. A Efiling systeme provides a convenient way
of performing these tasks.


DWhat about DFS?d

Many BBC series computers are fitted with DFS to access disks with. DFS is
useful, but suffers from one major limitation. You are only allowed to
have 31 files on each disk (62 if you count both sides of a disk). You
could use ADFS, but that requires different hardware. HADFS allows you to
use the existing DFS hardware and uses a modified DFS-format disk to
provide a full hierarchial disk-based filing system. It also provides easy
access to cheap hard drives and other large storage devices.


DDirectories and subdirectoriesd

HADFS is a hierarchial filing system. What this means is that the disk can
contain files and directories. Directories are collections of files and
can also contain more directories, or subdirectories. The structure is
called a 'tree' because it looks like an upside-down tree. Everything
starts at the 'root', which, being upside-down, is at the top.

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The root directory is called I$i. This diagram, which shows part of the
HADFS System disk, shows the root directory containing several sub-
directories, IExtrasi, IManuali, ILibraryi and IUtilsi. ILibraryi and IUtilsi in turn
each have files and subdirectories of their own.

When HADFS is first used, or a B*MOUNTb command is used, the I$i directory
becomes the 'Currently Selected Directory', or CSD. When you type B*CATb or
B*.b a catalogue of the CSD is displayed. You can also show other directories
by giving the B*.b command a directory name, eg:

	B*CAT Libraryb  or
	B*.Utilsb


DMoving around the directory structured

For directories to be useful, you have to 'move around' and create new
directories. The B*DIRb command is used to change the CSD and move around
the directory tree, for example to make the root the CSD, use:

	B*DIR $b

You can use pathnames of any length, eg:

	B*DIR $.Library.lpb

If you want to move back 'up' the directory tree, you use the B^b symbol,
pronounced 'up'. Suppose you had set the CSD to I$.Library.lpi using the
above command, then 

	B*DIR ^b

would move you back to I$.Libraryi. You can also select another disk by
starting the pathname with B:b, so for example

	B*DIR :1b

would set the CSD to B$b on drive 1.


DCreating directoriesd

You can create new directories anywhere on the disk using the B*CDIRb
command. As an example

	B*CDIR $.Utils.Gamesb

would create the directory IGamesi inside the IUtilsi directory. If the CSD
was already set to I$.Utilsi, you could use

	B*CDIR Gamesb

for the same effect. Once you have created a directory, you can move into
it with the B*DIRb command and move back out of it with B*DIR ^b.


DWildcardsd

A means of abbreviating long names is provided by the 'wildcard' facility. 
The wildcards are B#b and B*b. The B*b character is used to ignore everything
after the position it is used in. For instance

	B*INFO Let*b

would show the information on the first object that started with the
letters 'Let'. If would match ILettersi, ILettacei and ILettingsi. The B#b
character matches to any character for one position only. For instance

	B*INFO F##Db

would match to IFoodi, IFindi, IFordi or IFredi. It would not match to IFindsi, IFlyi
or IFeedingi.


DAccess codesd

Files and directories have associated with them an access code which
descibes what operations can be done on them. These are:

D - Directory. This is a directory which may contain more files and
directories.

R - Read. If this letter is present, the file can be read using LOAD or
OPENIN, or can be run using B*RUNb or B*nameb.

W - Write. If this letter is absent, the file cannot be written to with
OPENOUT. The file can however be overwritten with SAVE.

L - Locked. A locked file cannot be deleted or renamed and cannot be
overwritten with SAVE. Directories are locked by default.

E - Execute. If this option is present, the file can only be run as
machine code with B*nameb, B*/nameb or B*RUNb. If the Execute option is set, the
Read option is masked out.

P - Private. This can make an entry invisible to a user that does not own
the directory. By default, the whole disk is owned.

Each file has two sets of access codes. The ones to the left of the '/'
are the owner's access code, the ones to the right are the public's access
codes. In general use, you will only need to be concerned with the owner
codes as the entire disk is owned by you. See Chapter 4 for more
information.

The B*ACCESSb comand allows a file's access code to be changed:

	B*ACCESS DATA LWRb	Allows the file to be read and written with
				OPEN, but cannot be renamed, deleted or
				overwritten with SAVE.

	B*ACCESS PROG WR/Rb	Allows the file to be read by anyone, but
				can be written to with OPEN only by its
				owner. It can be deleted, overwritten or
				renamed (only by its owner).

The default attributes are BWR/wrb for a file and BLb for a directory.


DDeleting, renaming and copyingd

When a file is no longer needed, it can be deleted from the disk. This
releases the space it used for use by other files. The B*DELETEb command
removes a single named entry from a directory; the filename must not have
any wildcards in it. For example:

	B*DELETE Rubbishb

You cannot delete a file which has been locked with the 'BLb' access code. 
To successfully delete such a file, the access must be removed, eg:

	B*ACCESS datab
	B*DELETE datab

Directories are created locked, and they must also be empty when you
delete them.

Being able to rename files is very useful, as it can be used not only to
change the name of a file, but to move it around the directory structure. 

The B*RENAMEb command is followed by the old filename, and then the new
filename. The command

	B*RENAME old newb

will change the file Ioldi to have the name Inewi. Furthermore, the command

	B*RENAME test $.Programs.testb

actually moves the file Itesti from the CSD to the directory I$.Programsi. 
You can also change the name of a file while moving it, so

	B*RENAME OldFile ^.NewFileb

will move IOldFilei into the directory above the CSD, and change its name to
INewFilei. Directories as well as files can be renamed like this. You cannot
use wildcards in the newname part of a B*RENAMEb command and you cannot
rename a directory into a subdirectory of itself.

The B*COPYb command makes a copy of a file whilst retaining the original. 
The command is followed by the source filename, and the the destination
filename. An example would be:

	B*COPY ThisFile $.Data.ThatFileb

B*COPYb uses as much unused shadow memory as possible to transfer the file
and avoids using the program memory. If you want to copy lots of files, or
copy files between different disks in the same drive or between different
filing systems, the ITreeCopyi program should be used, see Chapter 5. In
current versions of HADFS, the B*COPYb command is a transient command in the
system ILibraryi directory.


DLibrariesd

When a B*bcommand is issued, the MOS and sideways ROMs fitted in the
computer are searched to find a match. If the command cannot be found
there, then it is passed on to the filing system. HADFS looks in the CSD
for a file of the name. If it cannot find one, then it looks in the
currently selected library directory, the LIB. By default, the LIB is set
to a directory called ILibraryi on the default drive or the drive mounted
on. To set the CSD to the parent of the current directory. To specify a
new library, the B*LIBb command is used, eg:

	B*LIB $.Utilsb

You could set the LIB to be the same as the CSD by using B@b, which refers
to the CSD:

	B*LIB @b


DFile and disk informationd

HADFS knows more things about each file that is shown in the display given
by B*CATb. The B*INFOb command displays this extra information. The result of
a B*INFOb command might look like this:

     BChapter1   FFFFFF00 FFFFFFFF   001B78   WR/WR  28/12/1993  00078Bb

The numbers shown are the load address, the execution address and the
length, then the access code, date and sector address. There is also a
command B*EXb which does a B*INFOb on all the entries in the specified
directory.

If you use B*INFO $b information about the disk is displayed, for example:

     B$          00006400 00015556   000300   DL/    28/12/1993  000047b

The load address is the disk size, the execution address holds the disk
flags in the top 16 bits, and the disk ID in the bottom 16 bits, and the
date is the date the disk was *INSTALLed.

Disks have only a finite amount of space on them. To find out how much
space is available, the B*FREEb command is used. A typical display would be:

	B>*FREE
	0001A4 Sectors =   107,520 bytes free
	00049C Sectors =   302,808 bytes usedb

The first hexadecimal figures are the number of sectors free or used, the
decimal number is the space in bytes.


DFormatting disksd

Before a new disk can be used, it must be formatted. Blank disks are like
a blank piece of paper, formatting it is like drawing lines on it. Note
that formatting completely erases everything that may be on a disk, so if
you accidently format a disk that already has information on it, there is
no way to recover its contents.

To format an HADFS disk, you use the B*FORMb command, followed by the number
of tracks. The most common size is double-sided 80 tracks, making 160
tracks in total, so you would use:

	B*FORM160 0b

to format the disk in drive 0. Before formatting, you are asked to confirm
the operation:

	BFormat drive 0
	Go? (Y/N)b

You must respond by pressing BYb. Anything else will abort the process. 
Once a disk has been formatted, it can be used immediately. Freshly
formatted disks are Data disks with the title '(Untitled)'. This can be
changed using B*TITLEb or B*INSTALLb. If you want the disk to be a system
disk, you must copy the IHADFSROMi file into the root directory, or use
B*INSTALLb, for example:

	B*INSTALL 0 $ NewDisk 400Kb

This installs the system files onto the disk in drive 0, setting its title
to 'NewDisk'.

Non-floppy disks, such as hard drives and RAM disks, usually do not need
formatting or will be supplied with a program to format them. You then
just need to initialise the drive with the B*INSTALLb command. For instance,
to install HADFS on a 16M hard disk in drive 4, use:

	B*INSTALL 4 $ IDEDisk4 16384Kb


DAutostartd

Sometimes it is useful to make a program automatically start when you
first use a disk. This can be done using a file called I!Booti and the B*OPT4b
command. This file can be in any directory, but the most common place for
it is in the B$b directory. When you log on to a directory using B*I AMb, then
the boot option set with B*OPT4b is looked at and the I!Booti file is loaded,
run or executed. Pressing Shift-Break is the same as doing

	B*I AM BOOTb

If there is no I$.BOOTi directory, then the I$i directory is logged into. 
Doing B*I AMb is similar to doing B*MOUNT <drv>b then B*DIR <dir>b.


DChanging disksd

An important fact to realise is that HADFS stores some disk information in
memory, and that information will stay there even if the disk is removed
from the drive. For example, if you take a disk out, and put a different
one in the same drive and try to do

	B*CATb

you will get a listing from the previous disk. If you try to write or save
anything, or select another directory, a BDisk changedb error will occur.
You have to inform the computer that you have changed the disk.  You do
this using B*BYEb and B*MOUNTb. B*BYEb closes all open files, and then goes into
a 'dismounted' state. After using B*BYEb, all disks can be removed and
replaced. B*MOUNTb forgets all information, and selects a new drive. If no
parameter is given, then the default drive (usually drive 0) is selected.
If a drive parameter is given, then that drive is selected. B*BYEb is the
same as B*CLOSEb followed by B*MOUNTb.


DSpecial charactersd

B$b means 'root directory' and points to the top of the directory tree.

B^b means 'parent directory' and points to the directory that is the
	parent of the current one. The parent of the I$i directory is I$i.

B%b means 'library directory' and points to the directory set with B*LIBb
	or the one selected after doing B*MOUNTb, B*BYEb or B*I AMb.

B@b means 'current directory', the one selected with B*DIRb. If a command
	needs a directory and you need to select the CSD, you use this, eg
	using B*LIB @b makes the library directory the same as the
	currently selected directory.

B:b means 'drive', and is followed by a drive number. Drive numbers can
	be from 0 to 9, and A to V.


DCommands on other systemsd

MSDOS disks have a similar directory structure, but the commands to move
around it are slightly different. In summary:

Operation:			BBC		MSDOS
Show this directory:	B*CATb or B*CAT @b		BDIRb or BDIR .b
Move down a directory:	B*DIRb Inamei		BCDb Inamei
Move up a directory:	B*DIR ^			CD ..b
Move to another drive:	B*DIR :bIdi			BCD bIdiB:b
To change a disk:	B*MOUNTb or B*MOUNTb Idi	<Ctrl>-C