Acorn Filing Systems
Cassette Filing System
There are three Cassette Formats used by Acorn Computers.
The most basic is that used by the Acorn System 1, it is simply a
string of bytes preceded by a start and end address. In the System 2
Cassette Format the data is formatted into blocks of up to 256 data bytes each
with a Filename and other header information at the start of each block.
The integrity of the data is improved by inclusion of a checksum on each block.
The BBC Computer goes a step further by separating the header and data
information into separate blocks each with their own checksum.
The following diagrams which show the details of the
three Cassette Formats:
The Atom Computer supports both Acorn System 1 and 2 formats though is normally
used with the latter. There is a small difference between the System COS
and Atom COS implementations - the filename lengths differ. System COS
filenames can be up to 16 characters whilst Atom COS only allows 13 characters.
I'm not sure how this works, what happens if you load a System COS file with a
16 character filename on an Atom ? (something to try sometime). The BBC Computer is also capable of reading and
writing in the System 1 and 2 formats though I'm not sure how it handles long
filenames as it limits filenames to 10 characters.
The
System 1 Monitor program provides user commands for saving and loading
memory to/from cassette.
Acorn System COS and the Atom OS (which is essentially the same) takes this
further and provides user commands for saving and loading memory/programs using
named files, further it includes a set of standard OS calls to routines which
enable programs to interface with the cassette filing system:
- OSSAVE loads a file into a specified area of memory
- OSLOAD save a specified area of memory to a file
- OSBPUT outputs a byte to a file
- OSBGET reads a byte from a file
These OS calls are documented in Chapter 9 of the
System 2 Technical Manual.
Disk Filing System
The biggest hurdle with the cassettes tape is that tape
is serial, to get a
particular byte of data requires reading all the preceding data on the tape, the Floppy Disk
is a two dimensional device enabling random access to any location on the disk
by positioning of the read/write head.
A Floppy Disk is laid out as a series of Tracks
split into Sectors.
The System Disk Operating System (DOS) uses single sided, single density
disks with 40 or 80 tracks. Each track is split into 10 sectors
each holding 256 bytes.
The total capacity of a disk is therefore either 40 x 10 x 256 = 100K
(1K = 1024 bytes)
or 80 x 10 x 256 = 200K
Note: this is not the stored data capacity as some of this is
required for a catalogue.
Double sided disks are treated as two separate Volumes in DOS so giving
a total disk capacity of up to 400K.
A physical hole punched near the center of the disk is used as a
reference to determine Sector 0 as the disk rotates.
Originally the Acorn System was only provided with 5.25" Floppy Disks
(Mini Disks) as these were all that was available at the time however the
format can also be used with 3.5" Disks (Micro Disks), note though that
it may be necessary to modify the 8271 Disk Interface Controller to work
correctly with 3.5" drives,
see details here.
|
 |
Each disk or side of a disk, known as a Volume, can hold a total of 31 files.
File header information and the location of the file on the disk are stored in a
Catalogue which is held in Sectors 0 and 1 of Track 0:
|
Track 0 Sector 0 (256 bytes) |
|
Track 0 Sector 1 (256 bytes) |
| Byte |
File |
Contents |
|
Byte |
File |
Contents |
| 0 |
Volume |
First 8 characters of Volume Title |
|
0 |
Volume |
Last 4 characters of Volume Title |
| 1 |
|
1 |
| 2 |
|
2 |
| 3 |
|
3 |
| 4 |
|
4 |
BCD catalogue cycle number * |
| 5 |
|
5 |
Number of catalogue entries x 8 (i.e. 0 to &F8
in increments of 8 |
| 6 |
|
6 |
bits 0,1: MS
bits of number of (256 byte) sectors on disk (40t = 01, 80t = 11)
bits 4,5: disk boot option (00:no action, 01: load, 10: run, 11: exec)
bits 2,3,6,7 are unused and set to 0 |
| 7 |
|
7 |
low byte of number of (256 byte) sectors on
disk, (40t = $90, 80t = $20) |
| 8 |
1 |
7 character Filename |
|
8 |
1 |
Load address |
| 9 |
|
9 |
| A |
|
A |
Execute address |
| B |
|
B |
| C |
|
C |
File Length in bytes |
| D |
|
D |
| E |
|
E |
bits 0,1 MS
bits of Start Sector
bits 2,3 MS bits of Load Address, set to 0 *
bits 4,5 MS bits of File Length
bits 6,7 MS bits of Execute Address, set to 0 * |
| F |
Qualifier |
|
F |
Start Sector (LS byte) |
| 10 |
2 |
7 character Filename |
|
10 |
2 |
Load Address |
| 11 |
|
11 |
| 12 |
|
12 |
Execute Address |
| 13 |
|
13 |
| 14 |
|
14 |
File Length in bytes |
| 15 |
|
15 |
| 16 |
|
16 |
bits 0,1 MS
bits of Start Sector
bits 2,3 MS bits of Load Address, set to 0 *
bits 4,5 MS bits of File Length
bits 6,7 MS bits of Execute Address, set to 0 * |
| 17 |
Qualifier |
|
17 |
Start Sector (LS byte) |
| 18-1F |
3 |
Filename & Qualifier |
|
18-1F |
3 |
File Attributes |
| 20-27 |
4 |
Filename & Qualifier |
|
20-27 |
4 |
File Attributes |
| 28-2F |
5 |
Filename & Qualifier |
|
28-2F |
5 |
File Attributes |
| 30-37 |
6 |
Filename & Qualifier |
|
30-37 |
6 |
File Attributes |
| 38-3F |
7 |
Filename & Qualifier |
|
38-3F |
7 |
File Attributes |
| 40-47 |
8 |
Filename & Qualifier |
|
40-47 |
8 |
File Attributes |
| 48-4F |
9 |
Filename & Qualifier |
|
48-4F |
9 |
File Attributes |
| 50-57 |
A |
Filename & Qualifier |
|
50-57 |
A |
File Attributes |
| 58-5F |
B |
Filename & Qualifier |
|
58-5F |
B |
File Attributes |
| 60-67 |
C |
Filename & Qualifier |
|
60-67 |
C |
File Attributes |
| 68-6F |
D |
Filename & Qualifier |
|
68-6F |
D |
File Attributes |
| 70-77 |
E |
Filename & Qualifier |
|
70-77 |
E |
File Attributes |
| 78-7F |
F |
Filename & Qualifier |
|
78-7F |
F |
File Attributes |
| 80-87 |
10 |
Filename & Qualifier |
|
80-87 |
10 |
File Attributes |
| 88-8F |
11 |
Filename & Qualifier |
|
88-8F |
11 |
File Attributes |
| 90-97 |
12 |
Filename & Qualifier |
|
90-97 |
12 |
File Attributes |
| 98-9F |
13 |
Filename & Qualifier |
|
98-9F |
13 |
File Attributes |
| A0-A7 |
14 |
Filename & Qualifier |
|
A0-A7 |
14 |
File Attributes |
| A8-AF |
15 |
Filename & Qualifier |
|
A8-AF |
15 |
File Attributes |
| B0-B7 |
16 |
Filename & Qualifier |
|
B0-B7 |
16 |
File Attributes |
| B8-BF |
17 |
Filename & Qualifier |
|
B8-BF |
17 |
File Attributes |
| C0-C7 |
18 |
Filename & Qualifier |
|
C0-C7 |
18 |
File Attributes |
| C8-CF |
19 |
Filename & Qualifier |
|
C8-CF |
19 |
File Attributes |
| D0-D7 |
1A |
Filename & Qualifier |
|
D0-D7 |
1A |
File Attributes |
| D8-DF |
1B |
Filename & Qualifier |
|
D8-DF |
1B |
File Attributes |
| E0-E7 |
1C |
Filename & Qualifier |
|
E0-E7 |
1C |
File Attributes |
| E8-EF |
1D |
Filename & Qualifier |
|
E8-EF |
1D |
File Attributes |
| F0-F7 |
1E |
Filename & Qualifier |
|
F0-F7 |
1E |
File Attributes |
| F8-FF |
1F |
Filename & Qualifier |
|
F8-FF |
1F |
File Attributes |
Volume Title is padded with NULs or spaces
Filenames are padded with spaces
Addresses and other 2-byte word length values are stored low byte first
* not used by System DOS
The files in the catalogue are sorted by Start Sector starting with the
highest Sector and then in descending order, empty files are given a Start
Sector of value 2. In this simple disk filing files cannot be fragmented
though there may be free sectors between files.
There is no concept of directories or folders, files are stored 'flat'
however each file has a Qualifier assigned which effectively provides a method
for separating file types e.g. a source file and object file.
The default Qualifier is '_' (space), note the BBC Computer uses '$'
System DOS (and Atom DOS which is the same) provide commands for saving and
loading files. The basic set of OS calls provided by COS are supplemented
by additional routines which make full use of the random access nature of disk
storage:
- OSRDAR returns the values of a files arguments (pointer, length and
region)
- OSSTAR sets the value of the files pointer
- OSFIND opens a file
- OSSHUT closes a file
These OS calls are documented from page 19 of the
DOS Manual.
Network Filing System
To follow.
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