IBM AT 5170 Technical Reference page 71

Bit 7 Keyboard inhibit switch
o
=
Keyboard inhibited
1
=
Keyboard not inhibited
Bit 6 Display switch - Primary display attached to:
o
=
Color/Graphics adapter
1
=
Monochrome adapter
Bit 5 Manufacturing Jumper
o
=
Manufacturing jumper installed
1
=
Jumper not installed
Bit 4
RAM on the system board
o
=
Enable 512K of system board RAM
1
=
Enable 256K of system board RAM
Bit 3
Reserved
Bit 2 Reserved
Bit 1 Reserved
Bit 0 Reserved
Input-Port Bit Definitions
Bit 7 Keyboard data (output)
Bit 6 Keyboard clock (output)
Bit 5 Input buffer empty
Bit 4
Output buffer full
Bit 3 Reserved
Bit 2
Reserved
Bit 1 Gate A20
Bit 0
System reset
Output-Port Bit De"finitions
Keyboard data (input)
Keyboard clock (input)
Test-Input Bit Definitions
System Board 1-55