3 Disc organisation

Throughout this and the following sections, a distinction will be made
whenever there are differences between the two supported file structures;
the Acorn DNFS format, and the HDFS native format. In cases where no
distinction is made, the file structures are the same.

3.1 Files

Data is stored in named files by the disc filing system. These files have
certain features which are common across filing systems; they are
accessible in the same way by a set of operating systems commands. In both
the DNFS and HDFS files have certain features; they are held in contiguous
areas on the disc, have the same attributes, and the maximum length of
filenames is 7 characters. A file's attributes are its length, start
sector address, load address, execution address, and permissions. The
maximum length of a file is determined by the amount of free space
available in the directory; this can be up to 200K for the DNFS, and up to
400K for HDFS. The DNFS allocates 18 bits to the file length, whereas HDFS
allocates 19 bits. The start sector address determines where the file is
held, relative to the start of the directory. There are 10 bits allocated
to the start sector by DNFS and 11 bits by HDFS. The load address
determines whereabouts in memory the file is placed when it is loaded.
There are 18 bits allocated to the load address by both HDFS and DNFS; if
the high-order bits (above 16) are all 1s, the low-order 16 bits are used
to refer to an address in the I/O processor memory, if a second processor
is fitted. The execution address determines where the computer jumps to
start a program that is run from the disc. As with the load address, 18
bits are allocated to the execution address, and if the high-order bits of
the execution address are all 1s, the address referred to is in the I/O
processor. If there is no second processor fitted, the load and execution
addresses always refer to the I/O processor. A file's permissions
determine whether the file can be read, written, executed, or deleted.

3.2 Directories

Directories are used for partitioning files. In the DNFS, directory names
are a single character prefix which may be used to sort the files into
different types. The 'current directory' is a single prefix character
which is added to all filenames which do not specify a directory. There
may be no more than 31 files per disc in the DNFS. The directory
information for the DNFS takes two sectors (512 bytes). HDFS supports
hierarchical, nested directories. These directories are held in contiguous
blocks, exactly like files. Directory names must be between 2 and 7
characters long, and appear like normal filenames in the directory
listing. Each of these directories, as well as the disc's root directory,
may contain 31 files and sub-directories. HDFS directories have all of the
normal file attributes, and also a flag which marks them as directories
rather than files. The length of a directory determines how much space is
available for files and sub- directories inside the directory . It will
always be a whole number of sectors (a multiple of 256 bytes) and 2
sectors (512 bytes) will be used for the directory information. The load
and execution addresses are meaningless for directories. The readable and
writable permissions are meaningless for directories.

3.3 Permissions

The HDFS supports the full range of permissions for files; they may be
marked as readable, writable, executable, and/or locked (not deletable).
The characters R , W , X , and L are used to indicate these in directory
listings. The readable and writable permissions refer to the file-handling
operations OSFIND, OSBPUT , OSBGET and OSGBPB. Unwritable files can still
be saved over, and unreadable files can still be loaded. HDFS directory
permissions are the same as for files except that the readable and
writable flags are meaningless. The executable flag has a special meaning;
when a directory is executed, it is made the current directory, and the
directory options are used to determine whether to search for and load,
run, or *EXEC a file called !BOOT, in the same way as the auto-boot
facility . This facility allows applications to be organised within
directories, and set up and run by simply executing the directory. The
DNFS only supports the locked permission. In DNFS, locked files are
unwritable, and all files are readable and executable.

3.4 Auto-booting

Both the DNFS and HDFS support the auto-boot facility; if SHIFT-BREAK is
pressed, and BREAK is released first, the filing system will look at the
root directory's options to determine whether to search for and load, run,
or *exec a file named !BOOT. The values of the options for each of the
possibilities is given in section 11.7. To allow the HDFS and DNFS to
co-exist in a machine, each filing system checks whether certain keys are
held down before initialising on a BREAK. If there are no keys held down,
the filing system in the higher priority socket will take control. The
DNFS will not initialise if any other key than D is pressed with BREAK.
HDFS recognises two keys, H and I. The H key causes a normal
initialisation. The I key initialises the computer for I/O processor
operation only, if a second processor is present. This has the same effect
as turning the second processor off, in software. CTRL-BREAK must be
pressed to re-activate the second processor.