IBM 770 manual Port A Hex, System Control, 0092, System Board, Bits

System Control	Port A (Hex		0092)
Bit	Function
7–4	Reserved
3	Security	lock	latch
2	Reserved (must be set to 0)
1	Alternate	gate	A20
0	Alternate	hot	reset

Figure

2-30. System

Control

Port

A

(Hex

0092)

Bits

7–4

These

bits

are

reserved.

Bit

3

This

bit

provides

a

security

lock for the secured area of

RT/CMOS. If this bit is set to 1, the 8-byte power-on

password

is

locked

by

the

software. After

this

bit

is

set

by

POST,

it

can

be

cleared

only

by

turning

the

system

off.

Bit

2

This

bit

is

reserved.

Bit

1

This

bit

is used to enable the ‘addressA20 )

20’ signal

(

when

the

microprocessor

is

in

the real address mode. If

this bit is A20set cannotto0, be used in real mode

addressing. This

bit

is

set

to 0 during a system reset.

Bit

0

This

bit

provides an alternative method of resetting the

system microprocessor. This alternative method

supports

operating systems requiring faster operation

than that provided on the IBM Personal Computer AT.

Resetting the system microprocessor switches the

microprocessor from protected mode to real address

mode.

This

bit

is set to 0 by either a system reset or a write

operation. If

a

write

operation

changes

this bit from 0

to

1,

the

‘processor

reset’

signal

is

pulsed

after

the

reset

has occurred. While the reset is occurring, the latch

remains

set

so

that

POST

can

read

this

bit. If

the

bit

is

set

to

0, POST

assumes

that

the

system

was

just

powered

on. If the bit is

set to 1, POST assumes that

the

microprocessor

has

been

switched

from

protected

mode

to

real

mode.

If

bit

0 is used to reset

the system microprocessor to the

real

mode,

use

the

following

procedure:

1. Disable all maskable and nonmaskable interrupts.

2-30 System Board