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The keyboard-interrupt handler calls the keyboard-event callback function
when it has scan and virtual-key codes to pass back to Windows. The
registers must be set as follows:
 
Register  Value
────────────────────────────────────────────────────────────────────────────
AL        Virtual key code. For more information about the specific values,
          see Section 6.1.7, "Virtual-Key Codes."
 
AH        80h if the key is up, or 00h if the key is down.
 
BL        Scan code. This is an OEM-dependent value.
 
BH        01h if the key is an enhanced key (that is, if the prefix 0E0h
          preceded this scan code), or 00h if the key is not enhanced.
 
DI:SI     Address of extra message information. If extra message information
          isn't used, set the DI and SI registers to 0.
 
An enhanced key is one of a set of duplicate keys, such as the ENTER,
INSERT, DELETE, and direction keys, found on some keyboards such as the IBM
enhanced keyboard. On the IBM keyboard, an enhanced key has the same scan
code as its corresponding nonenhanced key, but the keyboard inserts the 0E0h
prefix byte to indicate that the enhanced key generated the code. In this
case, the keyboard-interrupt handler sets the BH register to 1 before
calling the keyboard-event callback function. On the IBM enhanced keyboard,
the following are enhanced keys: Direction keys, INSERT, HOME, PAGE UP,
DELETE, END, PAGE DOWN, DIVIDE (on numeric keypad), and ENTER (on numeric
keypad).
 
For keyboards that have extended keys with special scan codes instead of
prefixes, Windows will not generate the proper WM_KEYUP or WM_KEYDOWN
message unless the interrupt handler converts the scan codes to the
corresponding IBM scan codes on the IBM enhanced keyboard and sets the BH
register to 1 before calling the keyboard-event callback function.
 
 
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