Name: ZiLOG System 8000
Brand: ZiLOG
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HRM

Zilog

HRM

4.9.4.

Break

Registers:

Two

8-bit

hardware

registers,

the

System

Break

(SBR)

and

the

Normal

Break

(NBR),

are

accessable

I/O

ports

the

CPU

board.

During

any

memory

reference,

the

16-bit

logical

address

offset

gen-

erated

the

CPU

compared

the

break

value

given

the

contents

'of

either

the

SBR

theNBR.

The

SBR

referenced

for

the

break

value

the

segment

number

zero

one,

and

the

NBR

otherwise.

the

MMU

confi-

guration

specifies

separation

code,

data,

and

stack

areas,

and

the

CPU

status

indicates

nonprogram

reference

(status

10xx),

then

the

result

this

comparison

selects

between

data

and

stack

references.

the

logical

address

offset

less

than

the

break

value,

the

current

reference

for

data

(MMU

enabled);

otherwise,

stack

reference

(MMU

enabled).

The

following

paragraphs

describe

the

possible

configurations

operating

systems

(OS)

and

user

programs

(USER).

Nonsegmented as,

Nonsegmented

USER

This

configuration

intended

run

operating

systems

memory

segment

and

user

programs

any

segment,

63.

(Segment

recommended

for

running

user

programs.)

For

operating

systems

executing

this

configuration,

MMU

(code)

enabled

for

program

references

indicated

CPU

status

code

11xx,

instruction

space

access~

For

memory

references

other

than

program

references,

the

logical

address

offset

generated

the

CPU

compared

against

the

contents

the

SBR,

the

segment

number

zero

one;

however,

the

se~ment

number

2 -

63,

the

comparison

made

against

the

contents

the

NBR.

the

result

the

comparison

less

than

zero,

the

select

logic

enabll~s

MMU

(data);

otherwise,

MMU

(stack)

enabled.Addition-

ally,

logic

the

CPU

board

detects

memory

references

made

segment

0,1·

while

the

CPU

normal

operation

mode.

This

logic

generates

segment

trap

violation

the

Z8001

CPU,

disables

the

three

MMUs,

and

asserts

suppress

signal

that

prohibits

main

memory

references.

the

operating

system,

any

part

thereof,

executes

segment

mode,

the

separation

code,

data,

and

stack

spac~s

still

applies.

NOTE

The

separation

between

data

and

stack

spaces

based

the

contents

the

SBR

for

segment

and

the

NBR

for

references

all

other

segments.

4-68

Zi10g

4-68

Brand	ZiLOG
Model	System 8000
Category	Desktop
Language	English

ZiLOG System 8000 Hardware Reference Manual

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