INT 16h,  00h (0)        Keyboard Read                                    all

    Returns the next character in the keyboard buffer; if no character is
    available, this service waits until one is available.

       On entry:      AH         00h

       Returns:       AL         ASCII character code
                      AH         Scan code

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       Notes:         The scan codes are the numbers representing the
                      location of the key on the keyboard. As new keys
                      have been added and the keyboard layout rearranged,
                      this numbering scheme has not been consistent with
                      its original purpose.

                      If the character is a special character, then AL
                      will be 0 and the value in AH will be the extended
                      scan code for the key.

                      Use the scan codes to differentiate between keys
                      representing the same ASCII code, such as the plus
                      key across the top of the keyboard and the gray plus
                      key.

                      After the character has been removed from the
                      keyboard buffer, the keyboard buffer start pointer
                      (at 0:041Ah) is increased by 2. If the start pointer
                      is beyond the end of the buffer, the start pointer
                      is reset to the start of the keyboard buffer.

                      If no character is available at the keyboard, then
                      the AT, XT-286, and PC Convertible issue an INT 15h,
                      Service 90h (Device Busy), for the keyboard,
                      informing the operating system that there is a
                      keyboard loop taking place and thereby allowing the
                      operating system to perform another task.

                      After every character is typed, the AT, XT-286, and
                      PC Convertible issue an INT 15h, Service 91h
                      (Interrupt Complete). This allows the operating
                      system to switch back to a task that is waiting for
                      a character at the keyboard.

                      See Service 10h for an equivalent service that
                      supports the enhanced (101/102-key) keyboard.