IBM RT Series Hardware Reference Manual page 155

Input Status Register Zero Format
Bit 7 6 5
I
Bit 4
Bits 5 and 6
Bit 7
4 3 2
1
0
I
I I I I
:
Not Used
Switch Sense
Reserved
Reserved
CRT Interrupt
Switch Sense-When set to 1, this bit allows the
processor to read the four configuration switches
on the board. The setting of the CLKSEL field
determines which switch is being read. The
switch configuration can be determined by
reading byte 40:88H in RAM.
Bit 3: Switch 4 ; Logical 0
=
switch closed
Bit 2: Switch 3 ; Logical 0
=
switch closed
Bit 1: Switch 2 ; Logical 0
=
switch closed
Bit 0: Switch 1 ; Logical 0 = switch closed
Feature Code-These bits are input from the
Feat (0) and Feat (1) pins on the feature
connector.
CRT Interrupt-A logical 1 indicates video is
being displayed on the CRT screen; a logical 0
indicates that vertical retrace is occurring.
Input Status Register One
This is a read-only register. The processor port address is hex
3BA or hex 3DA.
IBM Enhanced Graphics Adapter 15