Commodore Plus 4 - Page 293

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The

6802

Microprocessor

281

Example:

This

program

places

the

processor

into

decimal

mode

using

the

SED

instruction

and

adds

the

contents

$2100

and

$2101.

The

result

placed

$2102,

and

decimal

mode

terminated.

2000

2001

2002

2005

2008

200B

200C

SED

CLC

LDA

ADC

STA

OLD

BRK

$2100

$2101

$2102

Put

the

processor

into

decimal

mode.

Prepare

add.

Get

the

first

decimal

value.

Add

the

second

decimal

value.

Store

the

result.

Exit

decimal

mode.

Stop

processing.

Use

the

(Memory)

command

examine

and

change

the

contents of

$2100-$2102

experiment

with

decimal

mode

addition.

SEI—Set

Interrupt

Disable

Flag

The

interrupt

disable

flag

set

prevent

interrupts

from

occurring.

Operation:

Flags

Affected:

Addressing

Mode

Syntax

Opcode

Bytes

Cycles

Implied

SEI

This

instruction

sets

the

flag

the

microprocessor

status

When

this

flag

set,

maskable

interrupts

not

occur.

The

interrupt

caused

the

BRK

instruction

cannot

prevented

this

way.

Example:

This

example

alters

the

IRQ

vector.

Before

executing

it,

check

the

default

contents

$0314-$0315

with

the

(Memory)

command.

The

low

byte of

the

default

address

for

IRQ

processing

located

$0314

and

the

high

byte

$0315.

this

address

not

$CE0E,

replace

the

$CE0E

the

program

with

it.

2000

SEI

Disable

maskable

interrupts.

2001

LDA

#$0D

Get

the

low

byte

the

address

the

new

IRQ

processor.

2003

STA

$0314

Store

the

vector.

2006

LDA

#$20

Get

the

high

byte

the

address

the

new

IRQ

processor.

2008

STA

$0315

Store

the

vector.

200B

CLI

Reenable

maskable

interrupts.

Commodore Plus 4 - Page 293

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