Chapter 5 Input/Output Interfaces
5-20
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition – December 2000
Table 5-15 lists the commands that can be sent to the 8042 by the CPU. The 8042 uses IRQ1 for
gaining the attention of the CPU.
Table 5–15. CPU Commands To The 8042
Table 5-15.
CPU Commands To The 8042
Value
Command Description
20h
Put current command byte in port 60h.
60h
Load new command byte.
A4h
Test password installed. Tests whether or not a password is installed in the 8042:
If FAh is returned, password is installed.
If F1h is returned, no password is installed.
A5h
Load password. This multi-byte operation places a password in the 8042 using the following manner:
1. Write A5h to port 64h.
2. Write each character of the password in 9-bit scan code (translated) format to port 60h.
3. Write 00h to port 60h.
A6h
Enable security. This command places the 8042 in password lock mode following the A5h
command. The correct password must then be entered before further communication with the 8042
is allowed.
A7h
Disable pointing device. This command sets bit <5> of the 8042 command byte, pulling the clock
line of the pointing device interface low.
A8h
Enable pointing device. This command clears bit <5> of the 8042 command byte, activating the
clock line of the pointing device interface.
A9h
Test the clock and data lines of the pointing device interface and place test results in the output
buffer.
00h = No error detected
01h = Clock line stuck low
02h = Clock line stuck high
03h = Data line stuck low
04h = Data line stuck high
AAh
Initialization. This command causes the 8042 to inhibit the keyboard and pointing device and places
55h into the output buffer.
ABh
Test the clock and data lines of the keyboard interface and place test results in the output buffer.
00h = No error detected
01h = Clock line stuck low
02h = Clock line stuck high
03h = Data line stuck low
04h = Data line stuck high
ADh
Disable keyboard command (sets bit <4> of the 8042 command byte).
AEh
Enable keyboard command (clears bit <4> of the 8042 command byte).
C0h
Read input port of the 8042. This command directs the 8042 to transfer the contents of the input port
to the output buffer so that they can be read at port 60h.
C2h
Poll Input Port High. This command directs the 8042 to place bits <7..4> of the input port into the
upper half of the status byte on a continous basis until another command is received.
C3h
Poll Input Port Low. This command directs the 8042 to place bits <3..0> of the input port into the
lower half of the status byte on a continous basis until another command is received.
D0h
Read output port. This command directs the 8042 to transfer the contents of the output port to the
output buffer so that they can be read at port 60h.
D1h
Write output port. This command directs the 8042 to place the next byte written to port 60h into the
output port (only bit <1> can be changed).
D2h
Echo keyboard data. Directs the 8042 to send back to the CPU the next byte written to port 60h as if
it originated from the keyboard. No 11-to-9 bit translation takes place but an interrupt (IRQ1) is
generated if enabled.
D3h
Echo pointing device data. Directs the 8042 to send back to the CPU the next byte written to port
60h as if it originated from the pointing device. An interrupt (IRQ12) is generated if enabled.
D4h
Write to pointing device. Directs the 8042 to send the next byte written to 60h to the pointing device.
E0h
Read test inputs. Directs the 8042 to transfer the test bits 1 and 0 into bits <1,0> of the output buffer.
F0h-
FFh
Pulse output port. Controls the pulsing of bits <3..0> of the output port (0 = pulse, 1 = don’t pulse).
Note that pulsing bit <0> will reset the system.
