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System Service Calls - Introduction

1. Using the operating system.
2. List of services.
3. List of errors
4. File types and Extensions
5. Control Codes

1. Using the operating system.

The Psion organiser has many features built into its ROM, and some of these can be called from machine code using the SWI instruction. The SWI instruction is followed by a byte indicating which of the system services you want. The SWI instruction calls a routine in the ROM which performs the task required, and after that has finished the program execution continues at the instruction which follows immediately after the service byte.

Most services need some parameters. Mostly the registers (A,B and X) will contain the information required by the service, but sometimes further information is needed which is often put in the scratch registers UTW_S0 and UTW_S1, at address $41/42 and $43/44 respectively. Sometimes the runtime buffer RTT_BF ($2188-$2287 with length byte RTT_BL $2187) is needed too. Information is passed back from the system services in the same way. Unless otherwise noted, assume the contents of the registers A,B,X and the scratch registers UTW_S0-5 are trashed by the system services.

Some services can not always perform their tasks, and an error can be returned. A service shows an error has occurred by returning with the carry flag set, and with B containing the error code. The error codes are the same as those used in OPL. Some services that can never return an error may not always clear the carry flag, so the carry flag should only be tested after services that can cause errors.

The scratch registers UTW_S0 to UTW_S5 (which lie between $41 and $4C) are used by most services, so any values stored there will most likely be trashed. You can freely use these addresses yourself as long as you keep in mind that the information stored there is lost by calling the operating system. See also BT$PPRG if more scratch variables are needed.

Note that strings are usually leading byte-count strings, which will from now on be abbreviated by 'lbc string'. An lbc string is a string of (ascii) characters which is preceded by a length byte. Thus the word 'hello' would be stored in lbc form as 05,68,65,6C,6C,6F in hex. The address of an lbc string is the address of the length byte, not the address of the first character.

2. List of services.

Here is an alphabetic list of all the operating system services. Each service has a short name. This allows us to use a slightly more descriptive way of denoting the SWI instruction. Instead of SWI 0 for example, we now write OS AL$FREE instead. A very short description is also given. Those marked with an asterisk are available on the LZ only, and those with a plus sign only on the LZ and later versions of the CM/XP.

3. List of Errors

4. File types and Extensions

Here are the various file type used by the Psion, and the extensions used in filenames on the LZ. Note that these are not the extensions used by the comms link for PC filenames. The comms link uses only ODB, OPL (text only procedure) and OBx where x is in the range 2-F.

The file type 82-8F are block files, which means that the record containing the filename is immediately followed by a single data block (which internally uses file type 80). Records of type 81 are used to store the names of ordinary data files but no further data other than the file type of its data records which lies in the range 90-FE. Since each ordinary file must have a unique data record type in that range, there can be only 111 ordinary files on a pack. The MAIN file is always assigned type 90 for its data. Note that the filetypes 00-7F are used to signify deleted records on a pack, and that FF is used for error correcting purposes.

5. Control codes

The following character codes will, when printed, not produce a character but control the screen or cursor in some way.