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Chapter 5 Input/Output Interfaces
Table 5-13. CPU Commands To The 8042 (Continued)
Value
Command Description
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. The contents are as follows:
Bit <7> Password Enable:
Bit <6> External Boot Enable:
Bit <5> Setup Enable:
Bit <4> VGA Enable:
Bit <3> Diskette Writes:
Bit <2> Reserved
Bit <1> Pointing Device Data Input Line
Bit <0> Keyboard Data Input Line
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. The contents are as follows:
Bit <7> Keyboard data stream
Bit <6> Keyboard clock
Bit <5> IRQ12 (pointing device interrupt)
Bit <4> IRQ1 (keyboard interrupt)
Bit <3> Pointing device clock
Bit <2> Pointing device data
Bit <1> A20 Control:
Bit <0> Reset Line Status;
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.
5-20 Compaq iPAQ Series of Desktop Personal Computers
0 = Disabled, 1 = Enabled
0 = Enabled, 1 = Disabled
0 = Enabled, 1 = Disabled
0 = Enabled, 1 = Disabled
0 = Disabled, 1 = Enabled
0 = Hold A20 low
1 = Enable A20
0 = Inactive
1 = Active
Second Edition – February 2001
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