Re: XML and ISO 10646 planes beyond the BMP

From: Misha Wolf (
Date: Mon Aug 11 1997 - 07:39:02 EDT

I've just sent the appended mail to the XML working group.



Background info

ISO 10646 supports a large number of 64K planes. Unicode 2.0 supports
the first seventeen planes: plane 0, aka the Basic Multilingual Plane
(BMP), and the next sixteen planes (planes 1-16). Encoding schemes
vary in the planes they support:

     UCS-4 All planes

     UTF-8 All planes

     UTF-16 Planes 0-16

     UCS-2 Plane 0

UTF-16 encodes all BMP characters identically to UCS-2. Thus the only
difference between UTF-16 and UCS-2 is that UCS-2 cannot encode planes


The XML spec is self-contradictory in regard to ISO 10646 planes beyond
the BMP. On the one hand:

L1. Section 4.3.3 includes the sentence: "It is recognised that ...
     the use of additional planes ... may be required.".

L2. Section 4.3.3 includes the sentence: "The values ... UTF-16 ...
     should be used for the various encodings ... of ... ISO/IEC 10646

On the other hand:

R1. The SGML Declaration for XML is based on UCS-2, ie admits only the

R2. UTF-16 is, I think, mentioned nowhere else in the XML spec.

Proposed solution

S1. Both in the "SGML Declaration for XML" and in the box labelled
     "Scope Document", amend the BASESET to reference IR-177 (ie UCS-4)
     rather than IR-176 (ie UCS-2). This will bring the XML spec into
     line with the HTML 4.0 spec.

S2. In both these places, amend the DESCSET accordingly: In the "SGML
     Declaration for XML", change the "65376" to "2147483488". In the
     box labelled "Scope Document", amend the "65536" to " 2147483648".
     The SGML Declaration for HTML 4.0 has "2147483486" rather than
     "2147483488" (see the least significant digit), but I'll seek to
     have that changed.

S3. Replace *all* other instances of "UCS-2" with "UTF-16".

S4. Add a note explaining that UCS-2 is a subset of UTF-16.

Misha Wolf Email: 85 Fleet Street
Standards Manager Voice: +44 171 542 6722 London EC4P 4AJ
Reuters Limited Fax : +44 171 542 8314 UK
Eleventh International Unicode Conference, Sep 2-5 1997,

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