Re: Coding Systems Different from ISO 2022

From: Erik van der Poel (
Date: Mon Oct 12 1998 - 18:59:24 EDT

Frank da Cruz wrote:
> Can anybody tell me where to find out what ISO means when it
> assigns an ISO 2022 escape sequence for a "coding system
> different from ISO 2022" (such as, for example, NAPLPS, or UCS-4,
> or UTF-8)?

ISO 2022 itself specifies what that means.

> Is the intention to identify the coding system to the
> recipient, so it can switch to it, and also disable ISO-2022
> character-set designation and invocation from that moment
> onwards, since we have now switched to a new coding system in
> which we will not necessarily be able to recognize escape
> sequences for further switching?

ISO 2022 allows you to switch to non-ISO-2022 coding systems with either
a standard return (ESC 2/5 4/0) or no standard return.

UCS-2 and UCS-4 are registered as coding systems without standard
return. Their escape sequences are:

UCS-2, level 1: ESC 2/5 2/15 4/0
UCS-4, level 1: ESC 2/5 2/15 4/1

(See registry below for others.)

> In particular, I'm curious about an environment in which the host
> switches the terminal to the UTF-8 coding system. Since Unicode
> includes ASCII as well as C0 and C1 controls (and so UTF-8 can
> include both sets of controls too), should it be possible to
> switch out of UTF-8 coding once having switched into it? (I
> know, why would anybody ever want to switch out of UTF-8? :-)

UTF-8 is also registered in several forms. See the registry:


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