Filesystem IO Interfaces

This document is based on FILEIO from The Psionics Files.

This document describes the use of the IO interface for accessing the filing system. For general discussion on IO, see Device Driver Interfaces.

The filing system is a special device driver, called FIL:. Whenever an open fails to locate a device, or if the name opened is not that of a device driver, the driver name FIL: is prefixed, and the open is retried. Thus the original name becomes the channel name. For file IO, the channel name is, or controls, the file name.

An addition filing system called TXT: also exists. This is identical to FIL:, and accesses the same files, except that all files are assumed to be text files (see below). There is no normal need to use TXT:; instead, use FIL: and open files with a format component of 2 (see below).

Filing system organisation

The filing system is divided into one or more nodes. A node has a name ending with a double colon. The nodes most commonly met are:

  • LOC:: files in the internal ram and on attached SSDs and 3-Link pods
  • REM:: files accessed via the 3-Link (not the 3-Link pod itself)
  • ROM:: files in the internal ROM

A node may be monolithic, or may be divided into devices. The devices available and the format of their names depends on the node. For example:

  • LOC:: devices are usually M:, A:, B:, and possibly C:.
  • REM:: device names depend on the remote system
  • ROM:: monolithic

Unless explicitly stated otherwise, the term device includes monolithic nodes. A node may be flat or hierarchical. In the former case, each device is a single directory which contains files. In the latter, each device has a directory hierarchy  starting with a root directory; each directory can hold files and other directories.

The format of file and directory names depends on the node. Various system calls (such as FilChangeDirectory) can be used to manipulate file names. The LOC:: node uses \ as a directory name terminator (the root directory is just called "\") and allows both file and directory names to consist of up to 8 characters, a dot, and then up to 3 characters. The ROM:: node is flat, and uses the same rules for file names.

The full pathname of a file consists of the node, the device (if any), the path of directories (if any), and the filename, all joined with no intervening spaces. It is limited to 128 characters.

The system has the concept of a current path. When any filename does not include a node, device, or directory, that of the current path is used. For example, if the current path is LOC::A:\APP\, then the names XXX\YYY and M:ZZZ actually refer to LOC::A:\APP\XXX\YYY and LOC::M:\APP\ZZZ. However, the default path will only be used for names which are on the same node (either explicitly or because no node is specified).

Opening files

There are two kinds of open: open for scanning, and open for access. These are both done with the IOOPEN keyword, but once the file is open, it is used in totally different ways.

Opening for scanning

A file is opened for scanning by specifying one of the following modes to IOOPEN:

Mode Description
0x0030 scan a directory
0x0050 scan a node
0x0060 scan all nodes
0x0040 format a device
0x8040 format a device in low density

In each case, once the file is opened, it is scanned or formatted by making several calls to function 1 until one fails with error -36 (end of file).

The IOREAD keyword should be equivalent to function 1. However, there have been reports of problems with using it.

Once a call fails, the handle should be closed. All these calls will ignore the length variable (argument 2).

Scanning generates a list of names. There are three different scans that can be done:

  • Scan all nodes; the name opened is ignored (and so would normally be FIL:) and a list of all the available nodes is returned.
  • Scan a node; the node is extracted from the name opened, and a list of all devices on that node is returned.
  • Scan a directory; the name opened should either identify a directory (on the LOC:: node, this is done by having it end with a backslash), or should contain wildcards in the filename part. [If neither of these is true, only the original name will normally be returned; A: counts as a directory for these purposes.] A list of all names in the directory, or all matching names, is returned.

One member of the list is returned, as a cstr, by each call to function 1. The buffer (argument 1) should be at least 128 bytes long.

Formatting is used to prepare a device to contain files and to erase any previous contents. If a device supports multiple densities, it can be formatted at normal or low density. The node and device part of the name identifies the device to format; the remainder, if any, specifies a new volume name for the device (otherwise the previous one is reused).

The first call to function 1 will write a count into the first word of the buffer (argument 1); subsequent calls will ignore the buffer. This first call will not alter the device, and if the handle is closed immediately afterwards, nothing will happen. Otherwise, the count indicates the number of subsequent calls required to format the device. If the handle is closed before that number of calls has been made, the device will be left in an unusable state; it can be recovered by reformatting, but any information normally transferred to a reformatted device, such as the previous volume name or the unique serial number, will be lost.

As an example, when formatting a 2Mb flash SSD, the format count was 513. This consisted of:

  • 256 calls which each wrote all zeroes to 8192 bytes of the card, starting at address 0.
  • 8 blocks of 32 calls; the first call in each block restored 256 kbytes to all $FF, and the other 31 did nothing.
  • A final call which restored the initial area of the card (serial number, volume name, empty root directory, etc.).

Opening for access

A file is opened for access in order to read or write the file. To open a file for access, the mode parameter to IOOPEN has the following value:

Bit(s) Value Description


(Service Component)

0 open an existing file
1 create a new file
2 replace an existing file
3 open a file for appending only
4 open a new file with a new, arbitrary, name


(Format component)

0 open a binary file for binary access
1 open a text file for binary access
2 open a text file for text access
8 1 allow writing
9 1 allow random access
10 1 allow shared access

These individual components are now described.

Service component

The service component indicates what to do with the existing contents of the file. There are five possibilities:

  • 0 and 3 open an existing file, leaving the content unchanged. They differ only in the setting of the current position (0 = start, 3 = end). If the file does not exist, the open fails.
  • 1 and 2 will create the file if it does not exist; 1 will fail if the file does already exist, while 2 will succeed. After opening, the file will be empty (length 0), and so the current position will be at both the start and end of the file.
  • 4 is equivalent to 1, but with a new file name. The file name passed is examined to determine the directory, and a new file is created in that directory; the resulting name overwrites the passed name. If the IOOPEN keyword is used, the name parameter is replaced by ADDR(qstr), as explained in the OPL manual. If the IOOPEN system call is used, the name (a cstr) is passed in the same way as for other modes, and will be overwritten; there should thus be room for 128 bytes.

Format component

There are two kinds of file: text and binary. A text file consists of a sequence of records, each ending with a newline character, while a binary file consists of arbitrary data, which is not broken into records.

Each node has its own way of storing text and binary files. On some nodes, the two types are completely distinct. On others, text files are merely a way of interpreting binary files. This is the case for the LOC:: and ROM:: nodes. In either case, a record in a text file is limited to 256 bytes, and may not contain 0x0A, 0x0D, or 0x1A.

In the LOC:: and ROM:: nodes, a text file obeys the following rules:

  • Each of the following sequences marks the end of a record (In each case, the longest applicable sequence is used. These bytes are not part of the record.):
    • 0x0A 0x0D 0x1A
    • 0x0A 0x1A
    • 0x0A
    • 0x0D 0x0A 0x1A
    • 0x0D 0x1A
    • 0x0D
    • 0x1A
  • Anything after a 0x1A byte (including one in a record terminator) is ignored.

Access Modes

The three access modes are used to determine the way the file is processed.

Mode Description
0 opens a binary file for binary access
1 opens a text file for binary access
2 opens a text file for text access

The meaning of each of these is discussed below, under reading and writing. If a node does not distinguish binary and text files, the contents of the file are treated in the way described above. If it does distinguish them, the open will fail if the file has the wrong type.

Other opening flags

If the "writing" flag is not set on opening, all attempts to alter the file will fail. This includes files created by the open function.

If the "random access" flag is not set on opening, all attempts to set the current position (as opposed to altering it by reading and writing) will fail.

If the "shared" flag is not set on opening, the file is locked, and no other process may open it until this one closes it. If the flag is set, the file is open for sharing; other processes may also open it for sharing, but none of the processes may alter to the file.

Reading and writing

When reading or writing a binary file, the requested data is transferred directly without change. Up to 16k can be read or written with one call. On a read, the length argument is changed to the actual number of bytes read. The data is read from or written at the current position, which is moved to the end of that data.

When reading a text file as binary, the file is treated exactly as if each record were followed by 0x0D 0x0A; partial records can be read into the buffer, and the remaining portion will be retained until the next read.

When writing a text file as binary, the following sequences of bytes are taken to mark the end of a record:

  • 0x0A 0x0D
  • 0x0A
  • 0x0D 0x0A
  • 0x0D

Again, in each case the longest applicable sequence is used, and these bytes are not written as part of the record.

In all other respects access to a text file as binary behaves the same as access to a binary file.

When reading a text file as text, exactly one record is read from the file into the buffer; no explicit end-of-record marker is added. If the record is larger than the buffer, the trailing data is ignored and error -43 is returned. The length argument is changed to the number of bytes placed in the buffer. The record is read from the current position, which is moved to the start of the next record. Since text records are limited to 256 bytes, no more than that can ever be read.

When writing a text file as text, the contents of the buffer are written as a single record at the end of the file, no matter what the current position. The current position is moved to the end of the file. Records are limited to 256
bytes, and must not contain 0x0A, 0x0D, or 0x1A.

When writing to a flash SSD, there is one special case: if the write is a single byte, and if every bit of that byte is left unchanged or changes from 1 to 0 [equivalently, if (old AND new) = new], then the byte will be modified in situ. In all other cases, the old data will remain in place, but the file structure will be modified to skip it and point to the new data instead.


Seeking in a file is done using IOSEEK. It is reported that mode 4 returns the position of the current record (which is the record just read) within a text file (*not* the pointer after the read), and mode 5 allows you to seek to that position.

This has not been verified.

Other IO functions

Function: 9

  • Argument 1: unused
  • Argument 2: unused

Any buffered data is written to the file, and the file's modification date is updated if necessary. On the LOC:: node, the only buffering done for binary files is of the modification date of files on flash. Text files are buffered.

Function: 11

  • Argument 1: (long) new file size
  • Argument 2: unused

If the file is shorter than the specified length, random data is appended to bring it to that length; the current position is unaltered. If it is longer, the file is truncated to that length, and the current position is set to the new end of file.