Chapter 2. System Board Features
Keyboard and Mouse Ports
The keyboard-and-mouse subsystem is controlled by a general purpose, 8-bit microcontroller. The
controller consists of 256 bytes of data memory and 2 KB of read-only memory (ROM).
The controller has two logical devices; one controls the keyboard, and the other controls the mouse. The
keyboard has two fixed I/O addresses and a fixed IRQ line (IRQ1). The keyboard can operate without a
companion mouse, but the mouse can only operate with its companion keyboard. The mouse has a fixed
IRQ line (IRQ12), but it does not have its own I/O address; it relies on the addresses used by the
keyboard.
The following table shows the typical system resource assignments for the keyboard and mouse.
Note: Keyboard and mouse is the default.
When the computer is started, the resource assignments are subject to change during POST.
For an external interface, the keyboard and mouse each have a 6-pin connector. For information on
connector pin assignments, see “Keyboard and Mouse Port Connectors” on page 50.
General-Purpose I/O Ports
The National Semiconductor PC87307 chip on the system board has up to 16 general-purpose
input/output (GPIO) pins which are supported by 2 GPIO ports. The port pins are used for specific
functions and are not configurable by the user.
The GPIO ports use I/O addresses 0078-007F. Each GPIO port occupies a 4-byte I/O address.
Real-Time Clock
The low-power, real-time clock provides a time-of-day clock and a calendar. The clock is accurate to
+/− 12 minutes per year. The clock settings are maintained by an external battery source at +2.4 volts.
The life expectancy of the battery is approximately 2.25 years.
An external crystal is used to drive the real-time clock, and the battery is used to maintain the state of the
CMOS RAM when the power to the computer is turned off. (The system has 242 bytes of battery-backed
CMOS RAM in two banks.) If the CMOS RAM becomes corrupted and the system will not boot, a jumper
is included on the system board to clear CMOS RAM so that POST can set CMOS RAM to factory default
values.
The following table shows the typical system resource assignments for the real-time clock.
Table 5. System Resource Assignments for the Keyboard and Mouse
Configuration ROM RAM I/O Address (Hex) IRQ DMA
Keyboard and mouse None None 0060, 0064 1 (keyboard)
12 (mouse)
None
Keyboard only None None 0060, 0064 1 None
Mouse only None None 0060, 0064 12 None
Table 6 (Page 1 of 2). System Resource Assignments for the Real-Time Clock
Resource Resource Assignment
ROM None
14 Technical Information Manual
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