2. Codesets

The Tru64 UNIX operating system software supports the following Thai codeset:

TACTIS (Thai API Consortium/Thai Industrial Standard)

The TACTIS codeset, shown in Figure 2-1, is composed of the ASCII (ISO 646-1991) character set and the TIS 620-2533 character set. This is an 8-bit codeset with characters assigned values from 0x0 to 0xFF.

Figure 2-1: TACTIS Codeset

2.1 Character Classification

To facilitate the processing of characters encoded in the TACTIS codeset, such as displaying Thai characters and input-sequence checking, characters are classified into several categories:

Control Characters (CTRL). Nondisplayable control characters can be used to control output or for data communication. The 66 control characters are: 00-1F, 7F, 80-9F and FF.

Consonants (CONS). The 44 Thai consonants defined in TIS 620-2533.

Vowels (-V)

- Leading vowels (LV). The five leading vowels defined in TIS 620-2533.

- Following vowels (FV). The six following vowels defined in TIS 620-2533.

- Below vowels (BV). The two below vowels defined in TIS 620-2533.

- Above vowels (AV). The five above vowels defined in TIS 620-2533.

- Tone marks (TONE). The four tone marks defined in TIS 620-2533.

Diacritics (-D)

- Above diacritics (AD). The four above diacritics defined in TIS 620-2533.

- Below diacritic (BD). The below diacritic defined in TIS 620-2533.

Noncomposibles (NON). Those characters that do not fit into the other four character classes. This group of characters cannot be composed with above vowels, below vowels, tone marks, above diacritics, and below diacritic. There are 119 noncomposible characters divided into seven groups:

- Graphic characters. The 94 graphic characters defined in ISO 646-1991. They include:

* 52 English alphabetic characters (A-Z, a-z)

* 10 digits (0-9)

* 32 special characters: 21-2F, 3A-3F and 7B-7E

- Space. The character code is 20.

- No-Break space. The character code is A0.

- Thai digits. The ten Thai digits defined in TIS 620-2533.

- Word separator. The word separator defined in TIS 620-2533.

- Reserved code points. Six code points are reserved for future use.

To meet some special requirements of Thai input and output, some character classes, such as FV, BV, AV, and AD, are further divided into subclasses. For details, see Table 2–1.

Table 2-1: Thai Character Classification

Class

Number

Description

CTRL

66

ISO 646-1991 control codes: 00-1F, 7F, 80-9F, FF

NON

119

ISO 646-1983 character codes: 20-7E

TIS 620-2533 character codes: A0, CF, DC, DF, E6, EF, F0-F9, FA, FB.

Reserved code points: DB, DD, DE, FC, FD, FE.

CONS

44

A1-C3, C5, C7-CE

LV
5
E0, E1, E2, E3, E4

FV1

3

D0, D2, D3

FV2

2

E5

FV3

2

C4 and C6 (These two characters also behave as LV in the case of LV+CONS)

BV1

1

D8

BV2

1

D9

BD

1

DA

TONE

4

E8, E9, EA, EB

AD1

2

ED, EC

AD2

1

E7

AD3

1

EE

AV1

1

D4

AV2

2

D1, D6

AV3

2

D5, D7

Class	Number	Description
CTRL	66	ISO 646-1991 control codes: 00-1F, 7F, 80-9F, FF
NON	119	ISO 646-1983 character codes: 20-7E
		TIS 620-2533 character codes: A0, CF, DC, DF, E6, EF, F0-F9, FA, FB.
		Reserved code points: DB, DD, DE, FC, FD, FE.
CONS	44	A1-C3, C5, C7-CE
LV	5	E0, E1, E2, E3, E4
FV1	3	D0, D2, D3
FV2	2	E5
FV3	2	C4 and C6 (These two characters also behave as LV in the case of LV+CONS)
BV1	1	D8
BV2	1	D9
BD	1	DA
TONE	4	E8, E9, EA, EB
AD1	2	ED, EC
AD2	1	E7
AD3	1	EE
AV1	1	D4
AV2	2	D1, D6
AV3	2	D5, D7

2.2 Character Levels

Characters defined in the TACTIS codeset can also be classified according to character levels. There are five character levels:

Nondisplayable level includes all control characters in the CTRL category.

Base level includes all characters in the NON, CONS, FV and LV categories. Characters at this level are drawn on baseline.

Above level includes all characters in the AD3, AV1, AV2 and AV3 categories. Characters at this level are drawn directly above final consonants.

Below level includes all characters in the BV1, BV2 and BD categories. Characters at this level are drawn directly below final consonants.

Top level includes all characters in the TONE, AD1 and AD2 categories. Characters at this level are drawn on top of above characters. If above level characters do not exist, top level characters can be drawn at the above level. Characters at this level also indicate the end of character cells.