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When Windows is ready to receive keyboard input, it enables the driver by
calling the Enable function. When Windows no longer requires keyboard input
(when terminating or switching to a full-screen MS-DOS application, for
example), it disables the driver by calling the Disable function.
 
The Enable function is responsible for:
 
♦  Saving the address of the keyboard-event callback function
 
♦  Initializing the Windows virtual-key state array
 
♦  Installing the keyboard-interrupt handler
 
♦  Installing language- and keyboard-specific translation tables
 
When Windows calls Enable, it passes the address of the keyboard-event
callback function. Enable saves this address──the keyboard-interrupt handler
uses it to pass scan and virtual-key codes back to Windows.
 
Windows also passes the address of its 256-element, virtual-key state array
(sometimes called the keyboard-state vector) to Enable. The elements in this
array specify the state of the virtual keys. Windows sets bit 7 in an
element if the corresponding virtual key is down, and clears the bit if the
key is up. Similarly, Windows toggles bit zero each time the virtual key is
pressed. Although Windows maintains the key-state array, Enable initializes
the array to reflect to current state of the SHIFT, CTRL, ALT, NUMLOCK,
CAPSLOCK, and SCROLLLOCK keys as specified by the keyboard flags in the RAM
area used by the ROM BIOS. (On most computers, the keyboard flags are at
address 40h:17h.)
 
Note:  If the keyboard driver reads data from the ROM BIOS, it must use the
       special segment selector, __ROMBIOS, to create ROM BIOS addresses.
       The selector (exported by the Windows kernel as KERNEL.173)
       corresponds to ROM BIOS address 0F0000H.
 
To install the driver's own keyboard-interrupt handler, the Enable function
first saves the existing keyboard-interrupt handler. (On most computers, the
keyboard interrupt is Interrupt 9.) The original handler address is saved to
allow the Disable function to restore it as well as to allow the driver to
call the original handler for certain actions. Enable should also save the
address of the Keyboard Service routine (Interrupt 16h). The Enable and
other functions use this address to set the keyboard lights.
 
Finally, Enable installs the proper language- and keyboard-specific
translation tables. To do so, the function reads the type, subtype,
KEYBOARD.DLL, and OEMANSI.BIN settings from the [Keyboard] section of the
SYSTEM.INI file. The Windows Setup program or Control Panel establish the
values for these settings. Enable uses the values to determine which type
and subtype of keyboard to support as well as with language-specific support
libraries to load. If the values are not given in the SYSTEM.INI file, the
keyboard must choose default values based on the computer-type identifier
and keyboard-type identifier (if the ROM BIOS provides one).
 
Note:  To support computers in which the keyboard and mouse use the same
       interrupt, Windows disables and enables both the keyboard and mouse
       drivers when switching to and from a full-screen MS-DOS application.
       Windows always enables the drivers in the reverse order in which it
       disables the drivers.
 
 
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