Input/Output Interfaces

The keyboard interface configuration registers are listed in the following table:

Table 5-17.

Keyboard Interface Configuration Registers

Index

 

 

Address

Function

R/W

 

 

 

30h

Activate

R/W

 

 

 

70h

Primary Interrupt Select

R/W

 

 

 

72h

Secondary Interrupt Select

R/W

 

 

 

F0h

Reset and A20 Select

R/W

 

 

 

8042 Control

The BIOS function INT 16 is typically used for controlling interaction with the keyboard. Sub-functions of INT 16 conduct the basic routines of handling keyboard data (i.e., translating the keyboard's scan codes into ASCII codes). The keyboard/pointing device interface is accessed by the CPU through I/O mapped ports 60h and 64h, which provide the following functions:

■Output buffer reads

■Input buffer writes

■Status reads

■Command writes

Ports 60h and 64h can be accessed using the IN instruction for a read and the OUT instruction for a write. Prior to reading data from port 60h, the “Output Buffer Full” status bit (64h, bit <0>) should be checked to ensure data is available. Likewise, before writing a command or data, the “Input Buffer Empty” status bit (64h, bit <1>) should also be checked to ensure space is available.

I/O Port 60h

I/O port 60h is used for accessing the input and output buffers. This register is used to send and receive data from the keyboard and the pointing device. This register is also used to send the second byte of multi-byte commands to the 8042 and to receive responses from the 8042 for commands that require a response.

A read of 60h by the CPU yields the byte held in the output buffer. The output buffer holds data that has been received from the keyboard and is to be transferred to the system.

A CPU write to 60h places a data byte in the input byte buffer and sets the CMD/ DATA bit of the Status register to DATA. The input buffer is used for transferring data from the system to the keyboard. All data written to this port by the CPU will be transferred to the keyboard except bytes that follow a multibyte command that was written to 64h

Technical Reference Guide

361834-002

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