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TC 304 This standard contains the specification of a G0 set of 94 graphic characters for use in the GL area of a 7-bit code. Use of the code requires in addition a C0 set of control functions to be invoked in the CL area. The standard requires this set of control functions to be a subset of the C0 set of ISO/IEC 6429.
The specification contains a number of options. Of the 94 code positions for graphic characters, 10 have no specific character allocated to them. These positions are available for national or application-oriented use. A further two positions have two alternative allocations.
This standard introduces the concept of a version of ISO/IEC 646. A version is obtained by:
The standard specifies one version itself. This is known as the International Reference Version (IRV) of ISO/IEC 646. Its C0 set is the C0 set of ISO/IEC 6429 and its G0 set is that registered as ISO-IR 6 in the ISO 2375 Register. This is the set commonly known as ASCII. In positions 02/03 and 02/04 it specifies the NUMBER SIGN and DOLLAR SIGN respectively. It is important to note that this is a change from the IRV of the second edition ISO 646:1983, which specified the CURRENCY SIGN in position 02/04 and was registered as ISO-IR 2.
The G0 sets of a number of other versions of ISO/IEC 646 are also registered in accordance with ISO 2375. The register entries of a selection of these, together with the escape sequences that designate them as a G0, G1, G2 or G3 code element, are as follows:
In these escape sequences, replacement of "gg" by 02/08, 02/09, 02/10 or 02/11 specifies designation as a G0, G1, G2 or G3 code element respectively. These Intermediate Bytes specify designation of a 94-position single-byte character set as the code element concerned; see designation of graphic character sets in the section of this guide on control functions.
For reasons of backward compatibility with previous versions, ISO/IEC 646 permits the use of BACKSPACE (BS) and CARRIAGE RETURN (CR) control functions to create composite characters. In particular, it specifies that the sequence of a letter character, BACKSPACE and one of QUOTATION MARK, APOSTROPHE or COMMA should be interpreted as that letter bearing a diaeresis, acute accent or cedilla respectively. The character set separately includes GRAVE ACCENT, CIRCUMFLEX ACCENT and TILDE which may be combined similarly to produce letters with other diacritical marks. More recent character set standards that permit characters to be combined, such as ISO/IEC 6937, make use of specific combining characters as described in the section of this guide on graphic characters, so avoiding the use of control functions.