4 Disc management

The following sections contain some tips about disc and file management.

4.1 Formatting and verifying

New discs need to be formatted before they can store data. This lays down
the track and sector structure on the disc, so that data can be located
and written or read. The tracks on the disc are arranged in concentric
bands, each containing 10 sectors of 256 bytes. The sectors are numbered
from 0 within each track, but the sectors numbers along a radius on the
disc are not the same; this is because the drive takes a fixed amount of
time to step between tracks, during which the disc keeps turning. A few
sectors are missed while the step occurs, so the disc would have to do
another revolution to get back to the first sector. The discs are arranged
with a skew to avoid this, so that the first sector will be next under the
drive head after the drive has stepped. The skew value is normally 3, but
disc drives which step from track to track very slowly may need higher
skews to perform best, and fast disc drives may be able to use a smaller
skew. The built-in formatter in HDFS allows discs to be formatted double
or single sided, 40 or 80 tracks, and with an HDFS catalogue or an Acorn
catalogue. Double sided and 80 tracks can be used on all but a few old
disc drives. The drive characteristics can be set using the keyboard links
described in appendix B. Note that HDFS assumes that 80 track drives are
being used, and single stepping mode should be enabled if 40 track drives
are being used (see 6.11). The choice of Acorn or HDFS format catalogues
depends on whether the data must be readable on a computer which is not
running HDFS. Most disc filing systems support the Acorn catalogue format,
and so it should be used for discs which will be used to interchange data
between machines. The HDFS format should be used when a large number of
files may be put on the disc, or when a larger contiguous free space is
desirable, i.e. for all other discs. Never try to format a disc with more
tracks or sides than it is rated for. Discs are all made by the same
manufacturing process, and low density discs are simply discs which failed
tests for high density storage. It is likely that discs formatted with
more tracks or sides than they are rated for will lose their data. After
formatting a disc, it is a good idea to verify it to check that there are
no errors. A small percentage of discs will not format correctly, and may
need re-formatting. If a disc fails to format after a few tries, it should
be discarded.

4.2 Loading and saving

All of the normal loading and saving commands used by language ROMs are
supported by HDFS, as well as the operating system *SAVE and *LOAD
commands. A file name must be given when using these operating system
commands; no default name is assumed.

4.3 Copying and renaming

Files can be copied between discs and directories or duplicated using the
*COPY command. The *RENAME command is almost identical, except that it
deletes the original copy of the file. These commands use the I/O
processor memory as a buffer while copying files, and so should not be
used if you have valuable data in memory which has not been saved. It is
possible to copy files from disc to disc with one disc drive. In this case
the computer will prompt for the source or destination disc each time it
has finished reading or writing data. If the wrong disc is inserted, the
transfer will be stopped with an error message. If this happens, or if the
transfer is aborted, the disc catalogues will not be changed. It is
possible to copy or move files larger than the memory buffer with *COPY
and *RENAME. They load as much as possible into the memory at a time, and
so require the least number of disc changes possible. There is no specific
command for making backups of discs, but *COPY and *RENAME can be used to
copy a set of files into a directory to achieve this end.

4.4 File handling

All of the normal file opening, closing, and get and put byte commands
used by language ROMs are supported, as well as the operating system
OSGBPB routine for transferring blocks of data to and from files. Full
random access to files is possible, using the sequential pointer. Files
can be opened for input, output, or update (input and output). Files
opened for input cannot be used for output, and files opened for output
cannot be used for input. The HDFS only allows access for input or output
if the appropriate read or write permissions are set. Acorn format discs
always have read and write permission on files. The HDFS allows up to six
files open simultaneously. When a new file is opened for output, it is
located in the largest gap in the directory to allow it space to grow. A
maximum of &4000 bytes are allocated to that file immediately, and more
space can be claimed if it is available later. If more than one new file
is opened, the second file will probably be allocated immediately after
the first file, limiting the first file's length to &4000. If several
large files are to be created by a program, the *CREATE command or OSFILE
entry &7 can be used to make sure that enough space is allocated for each
of them. When a file is opened for output with the same name as an
existing file, an attempt is made to delete the existing file, and its
space is re-used. Files which are opened for update or input must exist
already. If the sequential pointer of a file is moved past the end of the
file, the file will be extended as necessary by padding with zero bytes.
The OSGBPB call should be used whenever possible, to reduce the overheads
associated with accessing files.

4.5 Compacting and extending

After a while, discs tend to get fragmented, with lots of short gaps
separated by files. The space can be recovered into one contiguous block
using the *COMPACT command. This shuffles all of the files and directories
down so that there are no gaps between them, leaving all of the space at
the end. Directories can be shrunk to fit the files that they contain
using the *COMPACT command. This is especially useful when the files in a
directory are not being changed at all, e.g. for program directories which
are finished, or games directories. The executable directory feature of
HDFS can be used to make these into self-contained applications which can
be *RUN. If a file has run out of space to grow, the *EXTEND command can
be used to move it into the largest gap on the disc, where it may have
more space to grow. This command can be combined with *COMPACT for maximum
effect; the sequence *EXTEND, *COMPACT, and then *EXTEND will ensure that
the maximum amount of space left in the directory is available for the
file to grow into. Both the *COMPACT and *EXTEND commands overwrite the
I/O processor memory, and so should not be used if you have valuable data
in memory which has not been saved.