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The keyboard-interrupt handler services hardware interrupts generated by the
keyboard. The handler retrieves scan codes from the keyboard, translates the
scan codes into virtual-key codes, and passes the scan and virtual-key codes
to Windows through the keyboard-event callback function. It also updates the
keyboard flags (usually at 40h:17h in the RAM area of the ROM BIOS) and
checks for special keys or key combinations.
 
When the interrupt handler receives control, such as when the user presses
or releases a key, the handler retrieves the current input from the keyboard
data port and determines whether the input is a scan code or a command byte.
If it is a command byte, it passes control to the original
keyboard-interrupt handler. Otherwise, it continues to process the scan
code.
 
When processing the scan code, the handler checks for key combinations, such
as CTRL+ALT+DELETE, and passes control to the original keyboard-interrupt
handler to carry out the corresponding actions. It also checks for the
ALT+PRINTSCREEN and SHIFT+PRINTSCREEN key combinations and translates these
into the VK_SNAPSHOT virtual-key code. When Windows receives this
virtual-key code, it takes a snapshot of the active window or the entire
screen. The handler indicates which snapshot to take by setting the BX
register to zero for the active window (ALT+PRINTSCREEN) and to 1 for the
entire screen (SHIFT+PRINTSCREEN).
 
To support debugging, the interrupt handler checks for the CTRL+ALT+SYSREQ
key and eithers simulates a nonmaskable interrupt (NMI) (to pass control to
a debugger) or jumps to the CVWBreak function (KERNEL.205) to carry out a
controlled break. If Windows is running with OS/2, the handler ignores the
CTRL+ALT+SYSREQ key combination.
 
To complete processing the scan code, the handler sets the interrupt flag
using the SetInterruptFlag (sti) instruction. Before doing so, it makes sure
a subsequent keyboard interrupt does not invoke the handler again as only
one interrupt can be handled at a time. If Windows is running in enhanced
mode, this may require masking out the keyboard interrupt (usually IRQ1) at
the interrupt controller.
 
If Windows is running with OS/2, the interrupt handler checks for the
ALT+ESC and CTRL+ESC key combinations. If either of these key combinations
is carried out, the interrupt handler passes the scan codes to the original
keyboard-interrupt handler so that OS/2 can switch from Windows to OS/2
programs. However, the Windows keyboard-interrupt handler only passes these
scan codes if screen-switching is enabled. The display driver specifies when
screen-switching is enabled or disabled by calling the ScreenSwitchEnable
function. The display driver disables switching whenever it enters a
critical section. If switching is disabled, the Windows keyboard-interrupt
handler ignores the ALT+ESC and CTRL+ESC key combinations.
 
 
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