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Keyboard input follows an event path beginning with the keyboard controller chip and ending with characters being placed in a 30-byte array called the keyboard typeahead buffer(predefined system data i.e. 0×001E to 0×003D in BIOS Data Area). Up to 15 keystrokes can be held there at any given moment, because each keystroke generates 2 bytes - one is ASCII code and other byte is Scan code(all extended keys will have scan codes only,but no ASCII codes). The following events occur when the user presses a key:

• The keyboard controller chip sends an 8-bit numeric scan code (sc) to the PC’s keyboard input port.
• The input port is designed so that it triggers an interrupt, which is a predefined signal to the CPU that an input-output needs attention. The CPU responds by executing the INT 9h service routine.
• The INT 9h service routine retrieves the keyboard Scan code (sc) from the input port and looks up the corresponding ASCII code (ac), if any. It inserts both the scan code and ASCII code into the keyboard typeahead buffer.If the Scan code has no matching ASCII code, then the ASCII code in the typeahead buffer is set to 0.
• Once the Scan code and ASCII code are kept safely in the typeahead buffer, they stay until the current running program retrieves them. Current program retrieves the typeahead buffer either by INT 16h or INT 21h service routine.

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This entry was written by Pratap Nirujogi and posted on February 14, 2007 at 2:12 pm and filed under Embedded Systems Programming, Miscellaneous. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.