5. DEVICE CONCEPTS
The definitions of the control functions in this Standard are based on general assumptions about the architecture of character- imaging devices. Examples of devices conforming to these concepts are: an alpha-numeric display device, a printer or a microfilm output device.
5.1. The Received Data Stream
The received data stream is considered to be a continuous stream. It may be structured in messages, records and/or blocks, but this does not affect the operation of the device at the abstract level of description in this Standard; the logical or physical units of data are regarded as being concatenated to form a continuous stream.
The device may contain a buffer in which the received data are temporarily stored before they are used to produce the character image output, or in which the received data are permanently stored and continuously used to produce the character image output.
5.2. The Character Image Output
The character image output may consist of one or more pages of a pre-determined size.
A page is composed of a pre-determined number of lines, each being composed of a number of character positions.
The device may have the capability of varying the number of lines per page, the number of character positions per line, and the character spacing during the operation of the device.
If the character image output is not structured in pages, it is regarded as consisting of a single page of an unlimited number of lines.
The lines constituting a page as well as the character positions constituting a line are identified by the natural numbers 1, 2, 3,
Each character position either is erased or images SPACE or a graphic symbol. A graphic symbol represents a graphic character or one of the control functions for which a graphic representation is required.
The initial state of all character positions is "erased".
Depending on implementation, there may or may not be a distinction between an erased character position and a character position imaging SPACE.
Depending on the characteristics of the device, a character position may be capable of imaging a combination of two or more graphic symbols. This would permit the use of BACKSPACE to generate accented letters or other composite graphic symbols.
The width of a character position may be fixed or may depend on the character being imaged.
In this Standard, the character image output is regarded as being produced in the form of a continuous stream, but it may in actual fact be made available character-by-character, line-by-line, or page-by-page.
The character positions are numbered relative to the character image (page) output, not to the buffer (if any).
The character style and font design of the graphic symbols are not defined by this Standard, but their shapes and relative positioning to accomodate overlay of two or more symbols may be influenced by control functions in the received data stream.
5.3. The Active Position
At any time, there is a unique character position which is called the "active position".
The active position is the character position which is to image the graphic symbol representing the next graphic character of the received data stream or the next control function for which a graphic representation is required. The active position is also the reference position against which certain format effectors or editor functions are performed (see 5.4).
The line containing the active position is called the "active line".
Implicit movement
If the active position is not the last character position of a line, it is moved to the following character position of the active line.
An implicit movement is performed when SPACE or a graphic character is received or a control function, for which a graphic representation is required, is executed.
Explicit movement
The active position is moved to a specified character position.
An explicit movement is performed when a control function is executed which causes the active position to be moved to a specified position.
NOTE 3: In the case of an interactive display device it is common practice to mark the active position by means of a special indicator which is called the "cursor".
NOTE 4: In the following situations, the effect of an attempt to move the active position is not defined by this Standard:
- an attempt to perform an implicit movement when the active position is the last character position of a line,
- an attempt to perform an explicit movement to a non-existing character position, e.g. beyond the last character position of a line, or beyond the last line of a page.
Depending on implementation, an attempt to perform such an active position movement may:
- cause a wrap around movement,
- cause the active position to be blocked (a condition in which no graphic symbol can be entered until a valid explicit active position movement is performed),
- cause the active position to remain where it is but permit graphic symbols to be entered thereby replacing or overstriking the previously entered character,
- cause the cursor to disappear from the operator's view,
- cause the cursor to move to the opposite end of the display but one row or column offset,
- cause scrolling to occur,
- cause other implementation-dependent action.
5.4. Format Effectors and Editor Functions
Two classes of control functions have an action on the layout or positioning of information in character-imaging devices. They are format effectors and editor functions. Format effectors are intended to be used on all classes of imaging de- vices while editor functions are supplementary control functions required only in circumstances for a certain class of devices where an action is to be performed on previously entered data. The principal difference between editor functions and format effectors is that the latter are sensitive to the FORMAT EFFECTOR ACTION MODE, whereas the former are not (see Annex A).
5.4.1. Format effectors
Format effectors belong to the data stream and shall be treated as data which happen to have a format representation rather than a graphic representation. Format effectors describe how the originator of the data stream wishes the information to be formatted.
Therefore, if format effectors are not stored by the receiving device they shall be regenerated by the device for subsequent transmission to additional recipients in order to preserve data integrity.
Format effectors are processed as follows depending on the setting of the FORMAT EFFECTOR ACTION MODE (6.2.4) of the device.
If the FORMAT EFFECTOR ACTION MODE is set to EXECUTE, the action specified by the format effector (usually an active position movement) is immediately performed. Depending on implementation a format effector may be stored in addition to being performed.
If the FORMAT EFFECTOR ACTION MODE is set to STORE, the format effector is treated as a graphic and stored in the buffer. In this case, the specified action is intended to be performed by an auxiliary input/output device when the associated data are transferred.
5.4.2. Composite characters
Composite characters not already available shall be obtained using the format effector BACKSPACE rather than editor functions (see Annex A.3). For example small letter e with accent may be obtained as follows:
Wez's note: this section is intended to show the use of BS to implement what we refer to today as combining character display by printing accent characters and then backspacing and printing latin characters, but it didn't translate though the OCR. This section needs fixing up
| Data stream | Displayed |
|---|---|
| ~ BS e | |
| “~ BS e | é |
| “ BS ~ BS e | é |
5.4.3. Editor functions
The main purpose of editor functions is to edit, alter, or transpose the visual arrangement of data.
In most cases, editor functions will be perfomed immediately by the first receiver and then removed from the data stream.
Typical use of editor functions are:
- Coding of local functions for example encoding keyboard functions when the keyboard is logically uncoupled from the output imaging mechanism of a device.
- Transposing intended representation to an alternate representation in those cases where the receiving device is unable to display the intended image.
5.5. Editing Operations
This section is applicable primarily to buffered input/output devices. Editing operations (erasure, deletion, and insertion) are performed either in execution of control functions in the received data stream, or under control of a keyboard or another manual entry device.
5.5.1. Erasure
The state of one or more character positions is changed to "erased". Other character positions remain unaffected.
5.5.2. Deletion
The data contained in one or more character positions are removed by shifting the contents of an adjacent string of character positions so that the data to be removed are overwritten. As a result a string of character positions, equal in length to the deleted string, is erased at the beginning or the end of the shifted part.
5.5.3. Insertion
One or more erased positions are inserted by shifting the contents of a string of character positions adjacent to the position where the insertion is to be made. As a result a string of data, equal in length to the inserted string, is removed at the beginning or the end of the shifted part.
5.5.4. Editing modes and insertion/deletion
In case of a deletion or insertion, the string which is shifted either precedes or follows the position where the deletion or insertion is made, depending on the setting of the HORIZONTAL EDITING MODE (6.2.7) and of the VERTICAL EDITING MODE (6.2.17).
5.6. Selected and Qualified Areas
This section is applicable primarily to buffered input/output devices. It may be also applicable to unbuffered input/output devices when the SEND/RECEIVE MODE (6.2.13) is set to SIMULTANEOUS.
5.6.1. Selected areas
A selected area is a string of character positions, the contents of which may be eligible (see 6.3) to be transmitted in the form of a data stream or to be transferred to an auxiliary input/output device (see 5.7).
The beginning of a selected area is established by START OF SELECTED AREA (SSA). The character position which is the active position after receipt of SSA is the first character position of the selected area.
The end of a selected area is established by END OF SELECTED AREA (ESA). The character position which is the active position before receipt of ESA is the last character position of the selected area.
5.6.2. Qualified areas
A qualified area is a string of character positions with which certain characteristics are associated, such as one or a combination of the following:
- the contents of the character positions are protected against manual alteration;
- the set of characters which are permitted to be entered is restricted (e.g. to numeric or alphabetic characters only);
- the character positions are protected against erasure;
- a tabulation stop is associated with the first character position;
- the character positions are to be excluded, i.e. guarded (see 5.6.2.2) from transmission of a data stream, or from transfer to an auxiliary input/output device (see 5.7).
The beginning of a qualified area is established by DEFINE AREA QUALIFICATION (DAQ). The character position which is the active position after receipt of DAQ is the first character position of the qualified area. The type of area qualification is specified by the parameter of DAQ. The end of a qualified area is established by the beginning of the following qualified area.
5.6.2.1. Protected area
A protected area is a special case of a qualified area. It is a string of character positions, the contents of which are protected against manual alteration. A protected area may, in general, be either guarded or unguarded.
5.6.2.2. Guarded area
A guarded area is a special case of a qualified area. It is a protected area which is to be excluded from transmission of a data stream and from transfer to an auxiliary input/output device.
An alternative way to establish the beginning and end of a guarded protected area is by means of START OF PROTECTED AREA (SPA) and END OF PROTECTED AREA (EPA).
5.7. Auxiliary Input/Output Devices
This section is applicable primarily to buffered input/output devices. It may be also applicable to unbuffered input/output devices when the SEND/RECEIVE MODE is set to SIMULTANEOUS.
Data transfer from or to an auxiliary input/output device is initiated either by the operation of an appropriate button on a keyboard or by the control function MEDIA COPY (MC) appearing in the received data stream.
If there are more than one auxiliary input/output devices, the relevant device is specified by the parameter of MC.
An input data stream which is received from an auxiliary device is processed in the same way as any other received data stream. The method of terminating the input from the auxiliary device depends on implementation.