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5-22 361834-002 Technical Reference Guide
Input/Output Interfaces
I/O Port 64h
I/O port 64h is used for reading the status register and for writing commands. A read of 64h by
the CPU will yield the status byte defined as follows:
A CPU write to I/O port 64h places a command value into the input buffer and sets the
CMD/DATA bit of the status register (bit <3>) to CMD.
Table 5-18 lists the commands that can be sent tothe 8042 by the CPU. The 8042 uses IRQ1 for
gaining the attention of the CPU.
Bit Function
7..4 General Purpose Flags.
3 CMD/DATA Flag (reflects the state of A2 during a CPU write).
0 = Data
1 = Command
2 General Purpose Flag.
1 Input Buffer Full. Set (to 1) upon a CPU write. Cleared by
IN A, DBB instruction.
0 Output Buffer Full (if set). Cleared by a CPU read of the buffer.
Table 5-18.
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.