[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System
Chipset and Controllers ]
Keyboard Controller Functions
Early PCs used an Intel 8042 keyboard controller to interface to the keyboard. The
controller on the motherboard actually interfaces with a similar controller inside the
keyboard using a specialized serial link. Many modern PCs emulate the 8042 for
compatibility instead of actually putting one on the motherboard. Also, some Super I/O chips are further integrated, incorporating into them the
keyboard control functions as well. This of course eliminates the need for a separate
keyboard controller chip.
The keyboard controller provides the following functions to the PC:
- Keyboard Control and Interpretation: Obviously, the
keyboard controller is the circuit that controls the keyboard. :^) When it receives a scan
code from the integrated controller in the keyboard itself, it signals the processor using
the hardware interrupt dedicated to the keyboard (IRQ1). The processor then interprets the
key pressed and deals with it appropriately. The controller also handles the keyboard's
typematic rate, the rate at which keys automatically repeat when a key is held down for a
period of time.
- PS/2 Mouse Support: Perhaps surprisingly, the keyboard controller
handles input from the integrated PS/2-style mouse found on many newer machines. This is
just the way IBM did the original design when they created this standard with their PS/2
line.
- Access to the High Memory Area: The so-called "high memory
area" is the first 64K or so of extended memory. Access to this particular part of
memory can be made by DOS despite the fact that it is not part of the traditional 640K of
conventional memory, due to a strange design flaw in the original IBM AT. When the 640K
DOS limit became constricting, using this area to save on conventional memory became
advantageous. Access to this area is via the "A20 line" (21st address line)
which is controlled using the keyboard controller on older systems. Many newer chipsets
now incorporate this support natively, to increase performance. This odd area of PC hardware is explored in some detail
here.
 Next: Super I/O
Controller Functions
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