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rfc:rfc1843

Network Working Group F. Lee Request for Comments: 1843 Stanford University Category: Informational August 1995

             HZ - A Data Format for Exchanging Files of
           Arbitrarily Mixed Chinese and ASCII characters

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Abstract

 The content of this memo is identical to an article of the same title
 written by the author on September 4, 1989.  In this memo, GB stands
 for GB2312-80.  Note that the title is kept only for historical
 reasons.  HZ has been widely used for purposes other than "file
 exchange".

1. Introduction

 Most existing computer systems which can handle a text file of
 arbitrarily mixed Chinese and ASCII characters use 8-bit codes.  To
 exchange such text files through electronic mail on ASCII computer
 systems, it is necessary to encode them in a 7-bit format.  A generic
 binary to ASCII encoder is not sufficient, because there is currently
 no universal standard for such 8-bit codes. For example, CCDOS and
 Macintosh's Chinese OS use different internal codes.  Fortunately,
 there is a PRC national standard, GuoBiao (GB), for the encoding of
 Chinese characters, and Chinese characters encoded in the above
 systems can be easily converted to GB by a simple formula. (* The ROC
 standard BIG-5 is outside the scope of this article.)
 HZ is a 7-bit data format proposed for arbitrarily mixed GB and ASCII
 text file exchange.  HZ is also intended for the design of terminal
 emulators that display and edit mixed Chinese and ASCII text files in
 real time.

Lee Informational [Page 1] RFC 1843 HZ - A Data Format for Exchanging Files August 1995

2. Specification

 The format of HZ is described in the following.
 Without loss of generality, we assume that all Chinese characters
 (HanZi) have already been encoded in GB.  A GB (GB1 and GB2) code is
 a two byte code, where the first byte is in the range $21-$77
 (hexadecimal), and the second byte is in the range $21-$7E.
 A graphical ASCII character is a byte in the range $21-$7E. A non-
 graphical ASCII character is a byte in the range $0-$20 or of the
 value $7F.
 Since the range of a graphical ASCII character overlaps that of a GB
 byte, a byte in the range $21-$7E is interpreted according to the
 mode it is in.  There are two modes, namely ASCII mode and GB mode.
 By convention, a non-graphical ASCII character should only appear in
 ASCII mode.
 The default mode is ASCII mode.
 In ASCII mode, a byte is interpreted as an ASCII character, unless a
 '~' is encountered. The character '~' is an escape character. By
 convention, it must be immediately followed ONLY by '~', '{' or '\n'
 (<LF>), with the following special meaning.
 o The escape sequence '~~' is interpreted as a '~'.
 o The escape-to-GB sequence '~{' switches the mode from ASCII to
   GB.
 o The escape sequence '~\n' is a line-continuation marker to be
   consumed with no output produced.
 In GB mode, characters are interpreted two bytes at a time as (pure)
 GB codes until the escape-from-GB code '~}' is read. This code
 switches the mode from GB back to ASCII.  (Note that the escape-
 from-GB code '~}' ($7E7D) is outside the defined GB range.)
 The decoding process is clear from the above description.
 The encoding process is straightforward. Note that an (ASCII) '~' is
 always encoded as '~~'. A sequence of GB codes is enclosed in '~{'
 and '~}'.

Lee Informational [Page 2] RFC 1843 HZ - A Data Format for Exchanging Files August 1995

3. Remarks & Recommendations

 We choose to encode any ASCII character except '~' as it is, rather
 than as a two byte code, and we choose ASCII as the default mode for
 the following reasons. The computer systems we use is ASCII based.  A
 HZ file containing pure ASCII characters (i.e. no Chinese characters)
 except '~' is precisely a pure ASCII file. In general, the English
 (ASCII) portion of a HZ file is directly readable.
 The escape character '~' is chosen not only because it is commonly
 used in the ASCII world, but also because '~' ($7E) is outside the
 defined range ($21-$77) of the first byte of a GB code.
 In ASCII mode, other potential escape sequences, i.e., two byte
 sequences beginning with '~' (other than '~~', '~{', '~\n') are
 currently invalid HZ sequences. Hence, they can be used for future
 extension of HZ with total upward compatibility.
 The line-continuation marker '~\n' is useful if one wants to encode
 long lines in the original text into short lines in this data format
 without introducing extra newline characters in the decoding process.
 There is no limit on the length of a line. In fact, the whole file
 could be one long line or even contain no newline characters. Any
 DECODER of this HZ data format should not and has no need to operate
 on the concept of a line.
 It is easy to write encoders and decoders for HZ. An encoder or
 decoder needs to lookahead at most one character in the input data
 stream.
 Given the current mode, it is also possible and easy to decode a HZ
 data stream by scanning backward. One of the implication is that
 "backspaces" can be handled correctly by a terminal emulator.
 To facilitate the effective use of programs supporting line/page
 skips such as "more" on UNIX with a terminal emulator understanding
 the HZ format, it is RECOMMENDED that the ENCODER (which outputs in
 HZ) sets a maximum line size of less than 80 characters.  Since '\n'
 is an ASCII character, the syntax of HZ then automatically implies
 that GB codes appearing at the end of a line must be terminated with
 the escape-from-GB code '~}', and the line-continuation marker '~\n'
 should be inserted appropriately. The price to paid is that the
 encoded file size is slightly larger.
 It is important to understand the following distinction.  Note that
 the above recommendation does NOT change the HZ format.  It is simply
 an encoding "style" which follows the syntax of HZ. Note that this

Lee Informational [Page 3] RFC 1843 HZ - A Data Format for Exchanging Files August 1995

 "style" is not built into HZ. It is an additional convention built
 "on top of" HZ.  Other applications may require different "styles",
 but the same basic HZ DECODER will always work. The essence of HZ is
 to provide such a flexible basic data format for files of arbitrarily
 mixed Chinese and ASCII characters.

4. Examples

 To illustrate the "stylistic" issue of HZ encoding, we give the
 following four examples of encoded text, which should produce the
 same decoded output. (The recommendation in the last section refers
 to Example 2.)
 Example 1:  (Suppose there is no line size limit.)
 This sentence is in ASCII.
 The next sentence is in GB.~{<:Ky2;S{#,NpJ)l6HK!#~}Bye.
 Example 2:  (Suppose the maximum line size is 42.)
 This sentence is in ASCII.
 The next sentence is in GB.~{<:Ky2;S{#,~}~
 ~{NpJ)l6HK!#~}Bye.
 Example 3: (Suppose a new line is started whenever there is a mode
            switch.)
 This sentence is in ASCII.
 The next sentence is in GB.~
 ~{<:Ky2;S{#,NpJ)l6HK!#~}~
 Bye.

Acknowledgement

 Edmund Lai was the first one who brought my attention to this topic.
 Discussions with Ed, Tin-Fook Ngai, Yagui Wei and Ricky Yeung were
 very helpful in shaping the ideas in this article. Thanks to Tin-Fook
 for his careful review of the draft and numerous interesting
 suggestions.

References

 [1] Fung Fung Lee, "HZ - A Data Format for Exchanging Files of
     Arbitrarily Mixed Chinese and ASCII Characters," September 4,
     1989.
     As part of //ftp.ifcss.org/software/unix/convert/HZ-2.0.tar.gz

Security Considerations

 Security issues are not addressed in this memo.

Lee Informational [Page 4] RFC 1843 HZ - A Data Format for Exchanging Files August 1995

Author's Address

 Fung Fung Lee
 Computer Systems Laboratory
 Stanford University
 Stanford, CA 94309
 Phone: +1 415 723 1450
 EMail: lee@csl.stanford.edu

Lee Informational [Page 5]

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