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Windows applications send characters to other Windows applications with the
WM_KEYDOWN and WM_KEYUP messages. These messages are expected to get the
appropriate virtual-key codes to send by using the VkKeyScan, OemKeyScan, or
MapVirtualKey function. The applications must not assume any fixed
translation of the VK_OEM_* keys. To support these functions, the keyboard
driver can scan existing translation tables to locate the given ANSI or OEM
character code and subsequently identify the scan code.
 
Key-Translation Tables
 
In the Microsoft retail keyboard driver, several of the translation tables
are associative tables that map a virtual-key code or a combination of a
virtual-key code and some other value (for example, shift state) into an
ANSI character. All such tables in these drivers are assembled using special
macros that arrange the tables so that the first column of the table is a
single vector, which may be searched with a single-string scan instruction.
 
Table         Description
────────────────────────────────────────────────────────────────────────────
keyTrTab      Maps scan codes to Windows virtual-key codes. Some entries in
              this table in TABS.ASM may be overlaid when a
              language-specific library is loaded.
 
USTransPatch  Maps selected scan codes to virtual-key codes. This table
              contains that part of the default keyTrTab table that can be
              overlaid by language-specific libraries. The driver uses this
              table to restore the default keyTrTab if an error occurs while
              changing translation tables.
 
Most of the remaining tables are either in a discardable segment in the
driver or in a language-specific library. If they are in the driver,
functions such as ToAscii that use the tables ensure that the table segment
is loaded by calling the GetTableSeg function.
 
The tables in the driver are for the U.S. keyboard. Several of the following
tables are blank for the U.S. keyboard since it has no dead keys or CTRL+ALT
keys.
 
In the language-specific versions of these tables, there is often padding
(with zeros) at the end of a table to allow for overlaying the tables.
 
Associative Table           Description
────────────────────────────────────────────────────────────────────────────
AscControlVK, AscControl    This table translates the virtual-key code +
                            CTRL to a control character, when the
                            virtual-key code is not one for a letter. For
                            letters, the table is not used.
 
AscCtlAltVK, AscCtlAlt      This table translates the virtual-key code +
                            CTRL + ALT to an ANSI character. This is empty
                            for the U.S. keyboard.
 
AscShCtlAltVK, AscShCtlAlt  This table translates the virtual-key code +
                            CTRL + ALT + SHIFT to an ANSI character. This
                            table is empty for the U.S. keyboard.
 
AscTranVK, AscTran          This table translates the virtual-key codes to
                            ANSI for unshifted and shifted key combinations.
                            AscTranVK is a byte array of virtual-key codes;
                            AscTran is a WORD array of their translations.
                            In each WORD, the low byte is the unshifted
                            translation, and the high byte is the shifted
                            translation. If the virtual-key code is one for
                            a letter (A..Z), this table is bypassed.
 
CapitalTable                This table is a list of the virtual keys for
                            which the CAPSLOCK key is effective, in addition
                            to the letter keys (VK_A .. VK_Z). This table is
                            empty for the U.S. keyboard.
 
DeadKeyCode, DeadChar       This table translates a combination of a dead
                            key and a letter into an accented letter. This
                            table is accessed when the key after a dead key
                            is pressed. If a translation is not found in
                            this table, ToAscii will return two characters
                            in its output buffer: the dead key plus the
                            second character. This table is empty for the
                            U.S. keyboard.
 
Morto, MortoCode            This table is searched to check if a particular
                            virtual-key code and shift combination is for a
                            dead key. If it is, the dead-key value is
                            returned by ToAscii, with a negative-character
                            count. This table is empty for the U.S.
                            keyboard.
 
SGCapsVK, SGTrans           This table has entries only in the DLL for
                            Swiss-German keyboards. It handles a special
                            case where keys with SHIFT LOCK are translated
                            differently from shifted keys.
 
Language-Specific Libraries
 
The translation tables for the language-specific libraries are in files that
contain data in two segments: the CODE segment, which is load-on-call and
discardable, and the DATA segment, which is fixed. Data from the CODE
segment of the DLL is used to overlay the tables in the DATA segment of the
main driver. Data in the DATA segment of the library is not copied; instead,
a pointer in the driver is set to the library's DATA segment address.
 
The initial values in the tables in the library's DATA segment are for
enhanced keyboards. For other keyboard types, the tables in the library's
DATA segment must be patched or overlaid from tables in the CODE segment.
 
The GetKbdTable function performs the copying or overlaying. The driver
calls the function after the driver loads the library. Once GetKbdTable is
called, the driver accesses the library's fixed DATA segment directly. After
initialization, the tables in the CODE segment of the library are no longer
used.
 
A header, containing offsets and sizes of the various tables in the
library's DATA segment, is always copied to the driver no matter what type
of keyboard is installed.
 
Some language-specific libraries contain the function GetKeyString. The
keyboard's GetKeyNameText function calls GetKeyString to obtain key name
strings in the language appropriate to the keyboard.
 
 
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