DeviceFS Technical Details (Rev #3)

DeviceFS

» DeviceFS Technical Details

DeviceFS Technical Details

Special Fields

Special Fields within the DeviceFS are commonly used to specify parameters to the device. ie what buffers to be used, if the device should be flushing when a stream is closed and so on.

A validation string can be specified to parse the special field when the stream is being opened. DeviceFS will parse the string and return a block of data that relates to the string contents. The data will remain intact for as long as the stream remains open. If no validation string is specified then it is the duty of the device to take and manage a copy of the data and filter out any unwanted information.

Validation Strings

The syntax for validation strings is as follows:

keyword [, keyword ]/ escape_seq [/ escape_seq ]

Keywords

Keywords associate each command with an escape sequence. More than once keyword can be associated with a particular escape field. The reason for this is twofold

• When a different word has the same meaning. e.g. Colour or color
• When defining the various states for a switch

The escape sequence describes how preceding data should be validated and what to do with the rest of the special field string (up to the next separator).

The Characters below are valid within in escape sequences:

/N number /S switch

Parameters

Within the special field string each parameter is separated by a comma or a character which is out of place, ie a non-numeric in a numerical field. Each keyword within the special field string is separated by a semi-colon.

The buffer passed to the device contains 1 word per escape character, set to &DEADDEAD if the corresponding keyword is not present in the special field string.

Numbers are simply stored into the word; they are decoded using OS_ReadUnsigned and stored away. Switches store the state of the keywords placed, ie:

mike,dennis/S

This yields 0 if ‘mike’ is present within the string, 1 if ‘dennis’ is present within the string.

The order of commands within the validation string and the special field string need not match; the commands within the validation string control how the values are returned back to the caller.

Device Description

Devices are specified as a list of 24 byte blocks, terminated by a null word.

The device name is used:

• in the DeviceFS directory structure
• to create a DeviceFS$Device$options variable if this does not exist
• a path variable (if the flag bit is set)

See also

• Buffer Flags
• DeviceFS_Deregister
• DeviceFS_Register
• OS_ReadUnsigned