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  Address
  (hex)       Size       Description
 
  0000:0417   1 word     Current keyboard shift status (not always
                         related to the character at the top of the
                         buffer) in the following format:
 
                         Bit   Meaning (if set)
 
                         0     Right SHIFT key pressed
                         1     Left SHIFT key pressed
                         2     CTRL key pressed
                         3     ALT key pressed
                         4     SCROLL LOCK active
                         5     NUM LOCK active
                         6     CAPS LOCK active
                         7     INSERT active
                         8     Left CTRL key pressed
                         9     Left ALT key pressed
                         10    SYS REQ key pressed
                         11    Hold active (CTRL+NUM LOCK or PAUSE)
                         12    SCROLL LOCK key pressed
                         13    NUM LOCK key pressed
                         14    CAPS LOCK key pressed
                         15    INS key pressed
 
  0000:041A   1 word     Points to location of least recent keystroke at
                         head of circular keyboard buffer. When keyboard
                         service (Interrupt 16h) receives a request for a
                         request for a key, it retrieves the key codes
                         from the buffer head, then changes the value in
                         0000:041Ah to point to the next key code pair in
                         buffer.
 
  0000:041C   1 word     Points to tail of circular keyboard buffer. When
                         the keyboard handler (interrupt 09h) detects a
                         keystroke, it places a word identifying the key
                         into the buffer at this position. The value at
                         0000:041Ch is then reset to point to the next
                         open position in the buffer.
 
                         If no available space remains, the buffer is
                         full, and the handler beeps the speaker to alert
                         the user. Since the tail is not reset in this
                         case, the most recent keystroke is effectively
                         ignored.
 
  0000:041E   16 words   Default circular keyboard buffer. Holds up to 15
                         keystroke identifiers, each of which comprises a
                         1-byte scan code and a 1-byte ASCII code. The
                         keyboard handler (interrupt 09h) places the most
                         recent key identifier at buffer tail; interrupt 16h
                         reads the least recent key identifier from the
                         buffer head. When head = tail, buffer is empty.
 
  0000:0471   1 byte     Set to 80h when keyboard routine (interrupt 09h)
                         detects CTRL+BREAK key combination.
 
  0000:0480   1 word     Points to start of circular keyboard buffer.
 
  0000:0482   1 word     Points to end of circular keyboard buffer.
 
  0000:0496   1 word     If bit 4 set, indicates extended keyboard
                         installed.
 
  See also: Interrupt 09h (Keyboard Interrupt),
            Interrupt 16h (Keyboard Services),
            Interrupt 1Bh (CTRL+BREAK Handler)
                                    -♦-