IBM Personal System/2 65 SX User Manual

System-Control
Port
A
(Hex
0092)
Function
-5 Reserved
Watchdog-Timer Status
Security-Lock Latch
Reserved
Alternate Gate A20
Alternate Hot Reset
Figure
3-24.
System-Control
Port
A
Bits 7-5
Bit 4
Bit 3
Bit 2
Bit 1
Bito
These bits are reserved.
This
read-only
bit
indicates
the
watchdog
timer
status.
When
this
bit
is
set
to
1,
a
watchdog
time-out
has
occurred.
The
Hardware
Interface
Technical
Reference
manuals
contain
more
information
about
the
Watchdog
Timer.
This
bit
provides
the
security
lock
for
the
secured
area
of
RT/CMOS
RAM.
When
set
to
1,
this
bit
electrically
locks
the
8-byte,
power-on
password.
Once
this
bit
is
set
by
POST,
it
can
be
cleared
only
by
turning
the
system
off.
This bit is reserved.
This
bit
is
used
to
enable
the
‘address
20’
signal
(A20)
when
the
microprocessor
is
in
the
real-address
mode.
When
this
bit
is
set
to
0,
A20
cannot
be
used
in
real
mode
addressing.
This
bit
is
sét
to
0
during
a
system
reset.
This
bit
provides
an
alternative
method
of
resetting
the
system
microprocessor.
This
alternative
method
supports
operating
systems
requiring
faster
operation
than
was
provided
on
the
IBM
Personal
Computer
AT".
Resetting
the
system
microprocessor
switches
the
microprocessor
from
the
protected
mode
to
the
real-address
mode.
The
alternative
reset
takes
13.4
microseconds.
This
bit
is
set
to
0
by
either
a
system
reset
or
a
write
operation.
When
a
write
operation
changes
this
bit
from
0
to
1,
the
alternative-reset
pin
is
pulsed
high
for
100
to
125
nanoseconds.
The
reset
occurs
after
a
minimum
delay
of
6.72
microseconds.
While
the
reset
is
occurring,
the
latch
Personal
Computer
AT
is
a
trademark
of
the
International
Business
Machines
Corporation.
3-24
Model
65
System
Board
—
October
1990

IBM Personal System/2 65 SX Specifications

General

Brand	IBM
Model	Personal System/2 65 SX
Category	Desktop
Language	English