mov ah, UpDownFlag ; 00h for down stroke, 80h for up stroke
moval, VirtualKey ; Windows virtual-key code
mov bh, PrefixFlag ; 00h if no prefix, 1 if 0E0h prefix byte
mov bl, ScanCode ; hardware scan code
mov si, word ptr [ExtraInfo]
mov di, word ptr [ExtraInfo+2]
; extra information
call [Keyboard_Event] ; keyboard-event function
The Keyboard_Event function records a keystroke. The keystroke may subsequently be used to generate a WM_KEYUP or WM_KEYDOWN message.
The keyboard driver's interrupt handler calls this function.
UpDownFlag
Specifies whether the key is up or down. The parameter is 0x00h for a down stroke and 80h for an up stroke.
VirtualKey
Specifies a virtual-key code. The code must be a value in the range 1 to 254.
PrefixFlag
Specifies whether the scan code was preceded by the 0E0h prefix byte. The parameter is 00h if there was no prefix byte, and is 1 if there was a prefix byte.
ScanCode
Specifies the hardware-scan code for the key.
ExtraInfo
Specifies an additional 32-bit value associated with the key stroke.
This function has no return value.
Although the keyboard driver passes the hardware-scan code (which is OEM-dependent) to Windows, Windows does not use the code. Instead, Windows
subsequently passes the scan code to the ToAscii function, which may use it
for special purposes. The interrupt handler sets the up/down bit in the scan
code to 0.
This interrupt handler may process some keystrokes without passing them to Windows through the keyboard-event callback function. In particular, the handler processes CTRL+ALT+SYSREQ keytrokes and generates a nonmaskable interrupt (NMI). Under certain circumstances, the driver's interrupt handler passes keystrokes to the original keyboard-interrupt handler. This is the case for CTRL+ALT+ DELETE key combination and for the PAUSE key.
Use the PRINTSCREEN key to take a screen snapshot, which is saved in the Windows clipboard. This is handled in the interrupt routine by calling the event procedure with VK_SNAPSHOT in the AL register, with the value equal to 0 (for full-screen snapshot) or 1 (for active-window snapshot) in the BL register.