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FILEIO

Filing System IO Interfaces

By Clive Feather. Last modified 1998-06-23. Original location: https://www.davros.org/psion/psionics/fileio

 

PSIONICS FILE - FILEIO
======================
Filing system IO interfaces
Last modified 1998-06-23
===========================

This document describes the use of the IO interface for accessing the filing
system. For general discussion on IO, see the Psionics file DEVICES.

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:
    $0030 = scan a directory
    $0050 = scan a node
    $0060 = scan all nodes
    $0040 = format a device
    $8040 = 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).
[Note: 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:
    Bits 0 to 3: (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
    Bits 4 to 7: (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
    Bit  8: allow writing
    Bit  9: allow random access
    Bit 10: 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 $0A, $0D, or $1A.

In the LOC:: and ROM:: nodes, a text file obeys the following rules:
* Each of the following sequences marks the end of a record:
    $0A $0D $1A
    $0A $1A
    $0A
    $0D $0A $1A
    $0D $1A
    $0D
    $1A
  In each case, the longest applicable sequence is used. These bytes are not
  part of the record.
* Anything after a $1A byte (including one in a record terminator) is ignored.

The three access modes are used to determine the way the file is processed.
* Mode 0 opens a binary file for binary access.
* Mode 1 opens a text file for binary access.
* Mode 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 $0D $0A; 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:
    $0A $0D
    $0A
    $0D $0A
    $0D
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 $0A, $0D, or $1A.

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.


Seek
----

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. I have not verified this. @@@@@


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 filesize
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.
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