Z8

ZiLOG Z8 Technical Manual

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5.2.4

Overflow

flag

(V)

For

signed

arithmetic,

Rotate,

and

Shift

opera-

tions,

the

Overflow

flag

is

set

to

1

when

the

result

is

greater

than

the

maximum

possible

number

( ) 127)

or

less

than

the

minimum

possible

number

( <

-128)

that

can be

represented

in

two's

comple-

ment form.

The

flag

is

set

to

0

if

no

overflow

Instruction

Set

encoded

in

a

4-bit

field

called

the

condition

code

(CC), which forms

bits

4-7

of

the

conditional

instructions.

Section

5.4.2

lists

the

condition

codes and

the

flag

settings

they

represent.

occurs.

5.4

NOTATION

AND

BINARY

ENCODING

Following

logical

operations,

the

Overflow

flag

is

set

to

O.

REf

I changes

the

value

of

the

Overflow

flag

when

the

saved

Flag

register

is

restored.

5.2.5

Decimal-Adjust

flag

(D)

The

Decimal-adjust

flag

is

used

for

BCD

arith-

metic.

Since

the

algorithm

for

correcting

BCD

operations

is

different

for

addition

and

subtrac-

tion,

this

flag

specifies

what

type

of

instruction

was

last

executed

so

that

the

subsequent

Decimal

Adjust

(DA)

operation

can

function

properly.

Nor-

mally,

the

Decimal-adjust

flag

cannot be used

as

a

test

condition.

A

fter

a

subtraction,

the

Decimai-adjust

flag

is

set

to

1;

following

an

addition

it

is

cleared

to

O.

RETI

changes

the

value

of

the

Decimal-adjust

flag

when

the

saved

Flag

register

is

restored.

5.2.6

Half-Carry

flag

(H)

The

Half-carry

flag

is

set

to

1 whenever

an

addi-

tion

generates

a

carry

out

of

bit

3 (Overflow),

or

a

subtraction

generates

a borrow

into

bit

3.

The

Half-carry

flag

is

used

by

the

Decimal Adjust

(DA)

instruction

to

convert

the

binary

result

of

a

pre-

vious

addition

or

subtraction

into

the

correct

decimal

(BCD)

result.

As

in

the

case

of

the

Decimal-adjust

flag,

the

user

does

not

normally

access

this

flag.

RET!

changes

the

value

of

the

Half-carry

flag

when

the

saved

Flag

register

is

restored.

5.l

CONDITION

CODES

Flags

C,

Z,

S, and V

control

the

operation

of

the

"condit

ional"

Jump

instructions.

Sixteen

fre-

quently

useful

functions

of

the

flag

settings

are

In

the

detailed

instruction

descriptions

that

make

up

the

rest

of

this

chapter,

operands

and

status

flags

are

represented

by

a

notational

shorthand.

Operands

(condition

codes and

address

modes) and

their

notations

are

as

follows:

Notation

Address

Hode

cc

r

R

RR

Ir

IR

Irr

IRR

Condit ion

Code

Working

register

only

Register

or

working

register

Register

pair

or

working

register

pair

Indirect

working

register

only

Indirect

register

or

working

register

Indirect

working

register

pair

only

Indirect

register

pair

or working

register

pair

Actual

Operand/Range

See

condition

code

list

below

Rn:

where n

0-15

reg:

where

reg

repre-

sents

a number

in

the

range

0-127,

240-255

Rn:

where n = 0-15

reg:

where

reg

repre-

sents

an even number

in

the

range

0-126,

240-254

RRp:

where p

2,

•••

,14

0,

®

Rn:

where n 0-15

®

reg:

where

reg

re-

presents

a number

in

the

range

0-127,

240-255

®

Rn:

where n = 0-15

®

RRp:

where p =

0,

2,

•••

,14

®

reg:

where

reg

re-

sents

an even number

in

the

range

0-126,

240-254

®

RRp:

where p

0,

2,

•••

,14

5-3

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ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English

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