The Cassette Operating System, or COS, saves and loads data to and from tape using the Computer Users Tape Standard (CUTS), which is also known as Kansas City Standard. Data is coded as audio tones on the tape. A logic 0 consists of 4 cycles of a 1.2 kHz tone, and a logic 1 consists of 8 cycles of a 2.4 kHz tone. Each byte of data is preceeded by a logic zero start bit, and is terminated by a logic 1 stop bit. Each bit lasts for 3.33 mS, giving an operating speed of 300 bits/second.
Named files are stored as a number of blocks, each of which is 256 bytes or less, and includes a checksum over all the bytes in the block. Each block is identified by a name header, and includes the start address for loading that block, the execution address for that block, and the number of bytes in that block minus one.
Unnamed files are stored as a two-byte start address, a two-byte end address, and end minus start bytes of data. An unnamed file could have no name at all (when using *LOAD and *SAVE), or it may have a zero length name denoted by "". Unnamed files may thus be used anywhere that named files could be used. The format of an unnamed file on tape corresponds to the format of an Acorn System One computer.
All COS commands start with an asterisk to distinguish them from BASIC commands. Note the difference between SAVE and *SAVE, and LOAD and *LOAD:
SAVE creates text files from the ATOM's text space. No start address is specified. The execution address is automatically set to #C2B2, the entry point to BASIC.
*SAVE saves a block of memory whose start and end addresses must be specified.
LOAD loads text files to the current text space.
*LOAD loads a block of memory to a fixed address, or to an address specified in the command.
| *CAT Catalogue tape | *. |
The *CAT command gives a catalogue of a tape. Each block of a named file will appear in the catalogue as follows:
FILENAME SSSS EEEE NNNN BB
| where | FILENAME is the name of the file SSSS is the start address of the block EEEE is the execution address of the file (used by RUN) NNNN is the block number, startinq at zero |
| and | BB is the number of data bytes in the block, minus one. |
All the numbers are in hexadecimal.
Unnamed files will appear in the catalogue as:
SSSS LLLL
where SSSS is the start address
and LLLL is the last address, plus one. Again, both numbers are
in hexadecimal.
| *LOAD Load file | *L. |
To load a named file the syntax is:
*LOAD "FILENAME" XXXX
where XXXX is a hexadecimal address specifying where the data is to be loaded. If XXXX is omitted the data will be loaded back to the address from which it was originally saved. On pressing RETURN the system will reply:
PLAY TAPE
The cassette recorder should be played, and the ATOM's space bar pressed to indicate that this has been done.
The COS will display the names of any files that are encountered on the tape before the specified file is found. When the file is found it will be loaded and on completion the '>' prompt will reappear.
If the file to be loaded is part way past the tape heads the COS will display:
REWIND TAPE
The tape should then be rewound and the space bar pressed again, to which the COS will reply:
PLAY TAPE
and the loading process can be repeated.
To load an unnamed file the syntax is:
*LOAD "" XXXX or
*LOAD XXXX
where XXXX is again the optional, hexadecimal, start address. Since there is no name search the space bar should only be pressed during the hiqh-tone leader, and the first file encountered will be loaded. Unnamed files consist of a single block, a R there is no error checking; however they provide the fastest way of having and loading data or programs.
| CTRL and SHIFT |
During loading and *CAT:
CTRL will cause a return to the ATOM '>' prompt. If pressed during loading an error message will be given to indicate that part of the file being loaded was lost.
SHIFT will override the search for the high-tone leader, and can thus be used to load and catalogue files with very short periods of high-tone leader.
Note that there is no way to exit from SAVE or *SAVE except by BREAK.
| *SAVE Save file | *S. |
To save a named file on tape the syntax is:
*SAVE "FILENAME" SSSS LLLL EEEE
where FILENAME is the filename of up to 16 characters
SSSS is the start address
LLLL is the end address plus one
EEEE is the optional execution address
The execution address is used by the RUN command, and if omitted will default to the start address.
On pressing return the COS will respond with:
RECORD TAPE
The tape recorder should now be started in record mode, and the space bar pressed to indicate that this has been done. Once started, SAVE cannot be aborted except by BREAK.
To save an unnamed file the syntax is:
*SAVE "" SSSS LLLL or
*SAVE SSSS LLLL
where SSSS and LLLL are as above, and the data will be saved as one continuous block.
| *MON Enable messages | *M. |
The usual condition after switch-on and BREAK is for the messages:
PLAY/RECORD/REWIND TAPE
to be produced. The MON command may be used to enable messages if they have been disabled.
| *NOMON Disable messages | *N. |
This command turns off messages produced by the COS.
| *PLOAD Finish loading | *F. |
The normal LOAD command demands that files are loaded from the start of the first block, and will request that the tape be rewound if started in the middle of the file. FLOAD allows loading to commence from the start of any block in the file, and the syntax of the command is:
FLOAD "FILENAME" SSSS
where SSSS is an optional start address specifying the address to which the start of the first block is loaded if relocation is required.
FLOAD is useful after a checksum error has been encountered. The tape may be stopped and rewound to any point before the block that produced the error. FLOAD is then used to allow loadinq to continue, and the block headers will ensure that the blocks are being loaded in the correct place.
| *RUN Load and execute machine code file | *R. |
The syntax of this command is:
RUN "FILENAME" SSSS
The function is exactly as for LOAD, but on completion of loading execution is transferred to the execution address specified when the file was created. The optional start address SSSS may be used to relocate the file. The execution address is not affected by relocation.
| *DOS Link to Disk Operating System | *D. |
This command initialises the Disk Operating System, if present, by linking to #E000.
The following error messages are given for errors in commands to the COS; i.e. for commands starting with '*':
SUM ERROR 6 Checksum error
COM? ERROR 48 Command error
NAME ERROR 118 Name error
SYN? ERROR 135 Syntax error
ERROR 165 Premature exit from loading
A BASIC or Assembler subroutine may often be required for several different programs. In this case it is possible to store the subroutine text on a separate file, and append this text to the text in memory every time the subroutine is needed in a program.
The subroutine text should be entered in memory on its own, and should be written with fairly high line numbers, such as 9000-9999. The subroutine is then saved as usual; e.g.:
SAVE "SUB9"
A later date a program is written which needs a copy of this subroutine. First check that the program does not use any line numbers above the first line of the program. Then find the address of the end of the program by typing:
PRINT &TOP-2
Remember that this address will be in hexadecimal. Now, using *LOAD, load the subroutine into memory starting at the address printed out in the above step:
*LOAD "SUB9" XXXX
where XXXX is the address that was printed out. Finally, to reset TOP to the end of the subroutine, type:
END
Any number of text files can be appended in this way, but note that, unless the resulting text is to be renumbered, the parts appended should use line numbers which are larger than any line number in the text file already in memory.