The 3270 display system defines a one-byte Logical Character Set ID (LCID) for each character set loaded or installed in the device. In addition, some of the LCIDs are reserved for permanently installed character sets and loadable character sets.
Loadable character sets are control memory that can be programmed by the user application by the Load Programmed Symbols structured field. Also, whether the character set is defined for a one-byte or two-byte codepoint is reserved as well. When using two-byte codepoints, select an LCID that defines a two-byte character set.
The CA Gen runtime software does not load loadable character set buffers (LCIDs) and does not use these LCIDs if present. DBCS character sets are always assumed to be in LCID x'F8', and the SBCS character set LCID is the device default, x'00'.
The assigned character set buffers (LCIDs) are:
All 3270 character sets define 190 printable characters (glyphs). These characters are assigned codepoints x'41' - x'FE'. The first byte of a double byte character set also is restricted to the range x'41'-x'FE', and can be thought of as a selector of one of the 190 glyph sets. Therefore, DBCS characters can represent a maximum of 1+190X190 characters, or 36,101 characters (x'4040' is always a space, and is not part of the 190 groups of 190 characters).
The operation of double and single byte characters depends upon the character set for the field, the Input Control settings, the use of SO/SI characters on write to the device, and the use of SA order.
When a single byte or mixed field is created, the Start Field Extended (SFE) order sets the default character set LCID to x'00'. This selects the installed default for the target device. Although the default character set may differ from one device to another, x'00' always selects the installed default.
Typically, the default character set is a single byte character set, but it may not be English. This can be determined by querying the installed character sets of the terminal, by user definitions, or other mechanisms. The Character Set Descriptor self defining terms received on the inbound Query Reply (Character Sets) structured field contains the LCIDs of each installed character set, its characteristics (loadable, not loadable, number of panes, and so on), and its Coded Graphic Character Set Global ID (CGCSGID) and Coded Character Set ID (CCSID). The CGCSGID and CCSID identify the codepage number of the international standard codepage associated with that character set. Additionally, CICS queries each terminal and retains that information, allowing applications to query CICS to obtain it, rather than query the terminal. Under CICS, the standard API services should be used. Under TSO/CMS, the Query can be sent to the device to determine this information (as defined in the TIRDCX) exit.
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