Commit 39787efd authored by Kenichi Handa's avatar Kenichi Handa

Comment for ISO 2022 encoding mechanism modified.

parent f805a125
......@@ -525,33 +525,37 @@ detect_coding_emacs_mule (src, src_end)
/*** 3. ISO2022 handlers ***/
/* The following note describes the coding system ISO2022 briefly.
Since the intention of this note is to help in understanding of
the programs in this file, some parts are NOT ACCURATE or OVERLY
SIMPLIFIED. For the thorough understanding, please refer to the
Since the intention of this note is to help understand the
functions in this file, some parts are NOT ACCURATE or OVERLY
SIMPLIFIED. For thorough understanding, please refer to the
original document of ISO2022.
ISO2022 provides many mechanisms to encode several character sets
in 7-bit and 8-bit environment. If one chooses 7-bite environment,
all text is encoded by codes of less than 128. This may make the
encoded text a little bit longer, but the text gets more stability
to pass through several gateways (some of them strip off the MSB).
There are two kinds of character set: control character set and
in 7-bit and 8-bit environments. For 7-bite environments, all text
is encoded using bytes less than 128. This may make the encoded
text a little bit longer, but the text passes more easily through
several gateways, some of which strip off MSB (Most Signigant Bit).
There are two kinds of character sets: control character set and
graphic character set. The former contains control characters such
as `newline' and `escape' to provide control functions (control
functions are provided also by escape sequences). The latter
contains graphic characters such as ' A' and '-'. Emacs recognizes
functions are also provided by escape sequences). The latter
contains graphic characters such as 'A' and '-'. Emacs recognizes
two control character sets and many graphic character sets.
Graphic character sets are classified into one of the following
four classes, DIMENSION1_CHARS94, DIMENSION1_CHARS96,
DIMENSION2_CHARS94, DIMENSION2_CHARS96 according to the number of
bytes (DIMENSION) and the number of characters in one dimension
(CHARS) of the set. In addition, each character set is assigned an
identification tag (called "final character" and denoted as <F>
here after) which is unique in each class. <F> of each character
set is decided by ECMA(*) when it is registered in ISO. Code range
of <F> is 0x30..0x7F (0x30..0x3F are for private use only).
four classes, according to the number of bytes (DIMENSION) and
number of characters in one dimension (CHARS) of the set:
- DIMENSION1_CHARS94
- DIMENSION1_CHARS96
- DIMENSION2_CHARS94
- DIMENSION2_CHARS96
In addition, each character set is assigned an identification tag,
unique for each set, called "final character" (denoted as <F>
hereafter). The <F> of each character set is decided by ECMA(*)
when it is registered in ISO. The code range of <F> is 0x30..0x7F
(0x30..0x3F are for private use only).
Note (*): ECMA = European Computer Manufacturers Association
......@@ -561,55 +565,61 @@ detect_coding_emacs_mule (src, src_end)
o DIMENSION2_CHARS94 -- GB2312('A'), JISX0208('B'), ...
o DIMENSION2_CHARS96 -- none for the moment
A code area (1byte=8bits) is divided into 4 areas, C0, GL, C1, and GR.
A code area (1 byte=8 bits) is divided into 4 areas, C0, GL, C1, and GR.
C0 [0x00..0x1F] -- control character plane 0
GL [0x20..0x7F] -- graphic character plane 0
C1 [0x80..0x9F] -- control character plane 1
GR [0xA0..0xFF] -- graphic character plane 1
A control character set is directly designated and invoked to C0 or
C1 by an escape sequence. The most common case is that ISO646's
control character set is designated/invoked to C0 and ISO6429's
control character set is designated/invoked to C1, and usually
these designations/invocations are omitted in a coded text. With
7-bit environment, only C0 can be used, and a control character for
C1 is encoded by an appropriate escape sequence to fit in the
environment. All control characters for C1 are defined the
corresponding escape sequences.
C1 by an escape sequence. The most common case is that:
- ISO646's control character set is designated/invoked to C0, and
- ISO6429's control character set is designated/invoked to C1,
and usually these designations/invocations are omitted in encoded
text. In a 7-bit environment, only C0 can be used, and a control
character for C1 is encoded by an appropriate escape sequence to
fit into the environment. All control characters for C1 are
defined to have corresponding escape sequences.
A graphic character set is at first designated to one of four
graphic registers (G0 through G3), then these graphic registers are
invoked to GL or GR. These designations and invocations can be
done independently. The most common case is that G0 is invoked to
GL, G1 is invoked to GR, and ASCII is designated to G0, and usually
these invocations and designations are omitted in a coded text.
With 7-bit environment, only GL can be used.
GL, G1 is invoked to GR, and ASCII is designated to G0. Usually
these invocations and designations are omitted in encoded text.
In a 7-bit environment, only GL can be used.
When a graphic character set of CHARS94 is invoked to GL, code 0x20
and 0x7F of GL area work as control characters SPACE and DEL
respectively, and code 0xA0 and 0xFF of GR area should not be used.
When a graphic character set of CHARS94 is invoked to GL, codes
0x20 and 0x7F of the GL area work as control characters SPACE and
DEL respectively, and codes 0xA0 and 0xFF of the GR area should not
be used.
There are two ways of invocation: locking-shift and single-shift.
With locking-shift, the invocation lasts until the next different
invocation, whereas with single-shift, the invocation works only
for the following character and doesn't affect locking-shift.
Invocations are done by the following control characters or escape
sequences.
invocation, whereas with single-shift, the invocation affects the
following character only and doesn't affect the locking-shift
state. Invocations are done by the following control characters or
escape sequences:
----------------------------------------------------------------------
function control char escape sequence description
abbrev function cntrl escape seq description
----------------------------------------------------------------------
SI (shift-in) 0x0F none invoke G0 to GL
SO (shift-out) 0x0E none invoke G1 to GL
LS2 (locking-shift-2) none ESC 'n' invoke G2 into GL
LS3 (locking-shift-3) none ESC 'o' invoke G3 into GL
SS2 (single-shift-2) 0x8E ESC 'N' invoke G2 into GL
SS3 (single-shift-3) 0x8F ESC 'O' invoke G3 into GL
SI/LS0 (shift-in) 0x0F none invoke G0 into GL
SO/LS1 (shift-out) 0x0E none invoke G1 into GL
LS2 (locking-shift-2) none ESC 'n' invoke G2 into GL
LS3 (locking-shift-3) none ESC 'o' invoke G3 into GL
LS1R (locking-shift-1 right) none ESC '~' invoke G1 into GR (*)
LS2R (locking-shift-2 right) none ESC '}' invoke G2 into GR (*)
LS3R (locking-shift 3 right) none ESC '|' invoke G3 into GR (*)
SS2 (single-shift-2) 0x8E ESC 'N' invoke G2 for one char
SS3 (single-shift-3) 0x8F ESC 'O' invoke G3 for one char
----------------------------------------------------------------------
The first four are for locking-shift. Control characters for these
functions are defined by macros ISO_CODE_XXX in `coding.h'.
(*) These are not used by any known coding system.
Control characters for these functions are defined by macros
ISO_CODE_XXX in `coding.h'.
Designations are done by the following escape sequences.
Designations are done by the following escape sequences:
----------------------------------------------------------------------
escape sequence description
----------------------------------------------------------------------
......@@ -632,40 +642,40 @@ detect_coding_emacs_mule (src, src_end)
----------------------------------------------------------------------
In this list, "DIMENSION1_CHARS94<F>" means a graphic character set
of dimension 1, chars 94, and final character <F>, and etc.
of dimension 1, chars 94, and final character <F>, etc...
Note (*): Although these designations are not allowed in ISO2022,
Emacs accepts them on decoding, and produces them on encoding
CHARS96 character set in a coding system which is characterized as
CHARS96 character sets in a coding system which is characterized as
7-bit environment, non-locking-shift, and non-single-shift.
Note (**): If <F> is '@', 'A', or 'B', the intermediate character
'(' can be omitted. We call this as "short-form" here after.
'(' can be omitted. We refer to this as "short-form" hereafter.
Now you may notice that there are a lot of ways for encoding the
same multilingual text in ISO2022. Actually, there exists many
coding systems such as Compound Text (used in X's inter client
communication, ISO-2022-JP (used in Japanese Internet), ISO-2022-KR
(used in Korean Internet), EUC (Extended UNIX Code, used in Asian
same multilingual text in ISO2022. Actually, there exist many
coding systems such as Compound Text (used in X11's inter client
communication, ISO-2022-JP (used in Japanese internet), ISO-2022-KR
(used in Korean internet), EUC (Extended UNIX Code, used in Asian
localized platforms), and all of these are variants of ISO2022.
In addition to the above, Emacs handles two more kinds of escape
sequences: ISO6429's direction specification and Emacs' private
sequence for specifying character composition.
ISO6429's direction specification takes the following format:
ISO6429's direction specification takes the following form:
o CSI ']' -- end of the current direction
o CSI '0' ']' -- end of the current direction
o CSI '1' ']' -- start of left-to-right text
o CSI '2' ']' -- start of right-to-left text
The control character CSI (0x9B: control sequence introducer) is
abbreviated to the escape sequence ESC '[' in 7-bit environment.
Character composition specification takes the following format:
abbreviated to the escape sequence ESC '[' in a 7-bit environment.
Character composition specification takes the following form:
o ESC '0' -- start character composition
o ESC '1' -- end character composition
Since these are not standard escape sequences of any ISO, the use
of them for these meaning is restricted to Emacs only. */
Since these are not standard escape sequences of any ISO standard,
the use of them for these meaning is restricted to Emacs only. */
enum iso_code_class_type iso_code_class[256];
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment