BS 6856:1987 pdf free download.British Standard Specification for Code extension techniques for United Kingdom 7-bit and 8-bit coded character sets [ISO title: Information processing — ISO 7•bit and 8-bit coded character sets — Code extension techniques].
6.1.6 Cod. extension characters of ISO 646
In ISO 646 the following control characters are provided for the purpose of code extension:
— ESCAPE ESC
— SHIFT-OUT SO
— SHIFT-IN SI
— DATA LINK ESCAPE DLE
BS 6856 does not describe the use of the control character DATA LINK ESCAPE which is reserved for the provision of additional transmission control functions. The use of this character is specified in other International Standards.
6.1.7 Other code extension characters
For use within a 7-bit environment, BS 6856 includes provision of some additional shift functions which are
not incbjded in ISO 646:
— LOCKING-SHIFT TWO LS2
— LOCKING-SHIFT THREE LS3
— SINGLE-SHIFT TWO SS2
— SINGLE-SHIFT THREE SS3
See annex B for the coded representation of these functions.
Three additional locking-shift functions LS1R, LS2R. LS3R are specified in 8.2.1; they are used in a 7-bit environment only when ii is necessary to preserve their use for transformation between 7-bit and 8-bit environments (see 10.2 and 10.4). When used in a 7-bit code, LS1R. LS2R and LS3R have the same effects as SO, LS2 and LS3, respectively.
6.1.8 CombInation of graphic characters
Some graphic sets may allow for the representation of additional graphic characters such as accented characters by the combination of two or more graphic characters in the same character position. Two methods of combining graphic characters in a single character position are provided for
a) graphic characters having implicit forward motion (spacing characters) used in conjunction with BACKSPACE or CARRIAGE RETURN;
b) graphic characters having no implicit forward motion (non-spacing characters) used in combination with spacing graphic characters.
ISO 646 allows for the first of these two methods to represent accented characters. Sponsors of graphic sets applying for registration under the provision of ISO 2375 are expected to identify any characters in the set that are non-spacing.
NOTE — A standard defining a character set should specify any restriction on combining characters, as this is not part of registration,
6.3.2 Structure of escape sequences
An escape sequence shall consist of two or more 7-bit combinations. The first shall always be the bit combination representing ESCAPE and the last shall always be that representing the Final character. An escape sequence may also contain any number of 7-bit combinations representing lnter mediate characters.
The meaning of an escape sequence shall be determined by the 7-bit combination representing its Intermediate character(sl, if any, and by the 7-bit combination representing its Final character.
Intermediate characters are the 16 characters of column 2 of the 7-bit code table; they are denoted by the symbol I.
Final characters are the 79 characters of columns 3 to 7 of the 7-bit code table excluding position 7/15: they are denoted by the symbol F.
NOTE – Although, in BS 6856, escape sequences are described in terms of characters or of positions in the code table. the meaning of an escape sequence is determined only by its bit combinations and it is unaffected by any meaning assigned to these bit combinations taken individually
The control characters in columns 0 and 1 and the character in position 7115 shall not be used as either Intermediate or Final characters to construct an escape sequence.
NOTE — As these prohibited characters may appear in an escape sequence in error, it may be necessary within an application to provide methods of identifying such a situation and of recovering from it. but this is not covered by this International Standard.
6.3.3 Categories of escape sequences
The use of escape sequences is specified in BS 6856. However, escape sequences with Final characters from column 3 are reserved for private use subject to the categorization outlined below. Escape sequences for private use are not subject to registration under ISO 2315.
NOTE – The implementors of any private escape sequence described as such in BS 6856 are alerted to the fact that other implementors may give different meanings to the same escape sequence or may use different escape sequences to mean the same thing. Furthermore, such meanings may subsequently be ass.gned to reg.stered escape sequences. Interchanging parties are warned that the use of such private escape sequences may reduce their capability to interchange data subsequently.
6.3.3.1 Two-character escape sequences
A two-character escape sequence shall be of the form
ESCF
Such escape sequences are used to represent additional control functions.
NOTE — The combined use of the “revision number” escape sequence and of the original designating escape sequence facilitates the recognition by older devices or systems of newer versions of character sets.
Escape sequence with 2/6 as the first Intermediate character and any further Intermediate characters are reserved for future standardization.
6.3.14 Three-character escape sequences without assigned m•anings
The escape sequences ESC 2/7 F and ESC 2/12 F have not been assigned meanings and are reserved for future standardization.
8.3.16 Summary of assignments of Intermediate characters
Table 1 summarizes the assignments of the Intermediate characters in the escape sequences. The shaded area denotes the combinations reserved for future standardization.
6.4 Initial designation and invocation
At the beginning of any information interchange, except where interchanging parties have agreed otherwise, all designations shall be defined by use of the appropriate escape sequences. and the shift status shall be defined by the use of the appropriate locking-shift functions. Interchanging parties who agree not to use such designators are warned that they may thereby reduce their capability to interchange data sub. sequently.
6.5 PIctorial representation of code extension in a 7-bit environment
Figure 7 summarizes, in a schematic form, the standard means of code extension available in a 7-bit environment.
7 Structure of a family of 8-bit codes
The family of 8-bit codes specified in BS 6856 is obtained by the addition of one bit to each of the bit combinations of the 7-bit code, thus producing a set of 256 8-bit combinations. The characters of the 7-bit set are assigned to the 128 bit combinations the eighth bit of which is set to ZERO. In this way, the set as defined in 6.1 forms a defined and integral part of an 8-bit code that is structured in accordance with this International Standard. The 128 additional bit combinations, the eighth bit of which is set to ONE, are available for further assignment.
7.1 The 8-bit code table
A 16-by-16 array of columns numbered 00 to 15 and rows numbered 0 to 15 contains 256 code positions (see figure 8).
Columns 00 to 07 of this array contain 128 character positions which are in one-to-one correspondence with the characters of the 7-bit set. Their coded representation is the same as in the 7-bit environment with the addition of an eighth, most signifi. cant bit, which is ZERO.