Viper 2.08 User's Guide download pdf (Page 15)

I

CHIP-

8E

by

Gilles

Detillieux

It seems

that

everyone

has oome up with

extensions to

CHIP-8.

Many

of these

would

be

very

useful

but

obviously

they

could

not

all

be

incorporated

in a

5t2

byte i-nterpreter.

This

gives

rise

to

many

incompatible

systems.

,

The approach

I took

was to

rewrite

the

interpreterr

taking the

following

points

into considerati.on

r

1)

It should be

cornpatible with

all-

prevj-ously

written

CHIP-8

programs"

Execution should. begin at 0200 and all

instructions should

remain

unchanged.

(with

the exception

of BMMM and 004e

which

are

rarely if

ever

used

in

any

CHIP-8

programs*.

)

2)

It should be

readily

adaptable

to

various

ISOZ machines.

3)

It should

incorporate

T/O instructions.

tl)

fhe

ad.ded.

j-nstructions

shoutd

be

useful enough

to

warrant

their

implementation in

the interpretero Less useful

ones

(or

those

used

less

often)

can be handled as

machine

language

subroutingF.

My

interpreter,

which

I

call CHIP-ffi

(for

extended),

features

the

followj-ng

new

instructions

r

OOED

STOP

Replaces

filIer

at location

00ED

with instruction

23

DEC

R3. Since

R3 is

the

program

cowrterr

execution

stops at

this

point.

ot57

WAIT FoR

TIME IIP

Examines

content

of

timer and

exits

once timer

equals

00

OOFz

NO

OPERATION

UsefuL

for deleting

instructions in

a

program.

Executes

a D4 SEP

R4

at

00F2, which makes it

return

to fetch the

next

instruetion.

O18B

SKIP

Unconditionally

skips next

two byte

instruction.

sxY,t

SKIP IF vX>vY

,^+:^* ir r,r, I

Skips

next

two

byte instruction if

VX

is

greater

than

vY.

*So

far, the only CHIP-8

program

I

have seen

which uses

an

instructj.on

not

a.rraiLable

in

CHIP-8E is

"VIP

Bowling"

in

the

VIP

Game

Manual.

This

prograrn

contains

a BJJA

instruction

at 0538,

In

this case, the

new

F01B

instruction

can

directly

replace

it.

2.OB/O9

,

L

5

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