Z*Magazine: 25-Jun-86 #6
From: Atari SIG (xx004@cleveland.Freenet.Edu)
Date: 07/03/93-08:27:29 PM Z
From: xx004@cleveland.Freenet.Edu (Atari SIG)
Subject: Z*Magazine: 25-Jun-86 #6
Date: Sat Jul 3 20:27:29 1993
________________________^^^________
Zmagazine June 25 1986 HOT Atari
Vol 1, No.6 News Plus++
Ron Kovacs-Publisher/Editor
Middlesex, New Jersey
___________________________________
Xx In this Issue!!!
This is our biggest issue to date,
due to the many text files sent in
during the past week, I thought I
would include everything I had.
I was also thinking of writing this
weeks edition in 80 column format.
But do to the time required to re-
format all the uploaded information
I received this week, I will try to
do it for next week.
CONTENTS:
Legislation in Congress??
Atari ST news
Beginners Column by Steve Godun
Part 3 of Assembly Programming
Zmag BBS Watch
and more!!!!!!!!
Xx Editors Notes
Next week I will update our Zmag
BBS list. We will also have a BBS
review of two BBS Systems.
ZPRINT2 has been released and I am
looking for someone to modify it
for various printers. When we go
to 80 column format, I would like
readers to be able to print out
the issue (if they choose), I
would like to put the config for
their printers in the program.
Anyone who can help, I will include
their name in Zmag and in the
ZPRINT title screen.
-----------------------------------
Xx Atari News Part One
[Reprinted from InfoWorld]
Atari: $99.95 Modem
by Karen Sorensen InfoWorld
Atari Corp. announaced it is
developing a Hayes-compatible
1,200(bps) modem that will retail
for $99.95 and will run on a
variety of personal computers.
The modem will feature an RS-232
serial port, which will allow it to
run with the Atari line and other
machines, such as the IBM PC, said
Neil Harris, Atari's hardware
products manager. The device will
also have a serial input/output
port, which will allow Atari's 8-
bit computers to work with it,
Harris said.
Users will be able to select from
either 1,200 or 300-bps rates. The
modem will support all standard
Hayes commands, except for one that
allows users to store phone numbers
on nonerasable memory, according to
Harris. Atari plans to offer
separate accessory kits, as well as
software that will allow the
computer to emulate specific
terminals, Harris said. Atari now
hopes to introduce the modem by
late summer.
Atari is also developing a cart-
ridge that will plug into the ST
and allow it to operate like a DEC
VT100 terminal. Many large corp-
orations are interested in using
the St solely as a terminal, Harris
said. Currently, VT100 emulation
is available through a $124 program
called PC/Intercomm from Mark of
the Unicorn, in Cambridge,Mass.
Xx Spelling Aide
by Scott Mace InfoWorld Staff
Batteries Included released Thunder
a 50,000-word real-time spelling
checker for the Atari ST that works
from within most GEM-based applica-
tions.
Thunder checks words as the user
types and allows users to correct
words within 80 characters of mis-
spelling them, said Mark Skapinker,
director of product development for
Batteries Included and the programs
designer.
Skapinker said Thunder's process of
checking spellings and retrieving a
list of possible correct spellings
is a quicker, one-step process than
that used by Turbo Lightning, an
IBM PC program from Borland Inter-
national of scotts Valey,CA.
Like Turbo Lightning, Thunder can
reside in memory at all times,
Skapinker said. But Turbo Lightning
does its initial screening from a
5,000-root-word dictionary, and
users may have to branch to a
50,000 word, disk-based synonym
directory, he said. "But the
Thunder dictionary is always loaded
completely into memory,"
Thunder does not include a synonym
dictionary, said Skapinker, and the
program works only with GEM-based
(Graphics Environment Manager)
applications on the Atari St, such
as Homepak, Paperclip Elite, ST
Talk, BTS The Spreadsheet, and
Timelink from Batteries Included;
Habawriter from Haba/Arrays Inc. of
Van Nuys, California; Regent Base
from Regent Software of Canoga
Park, Caifornia; and 1st Word from
Atari Corp. of Sunnyvale,CA.
On the GEM desktop, Thunder appears
as a desk accessory, Skapinker
said. Thunder will not work as a
desk accessory with non-GEM Atari
ST programs, said Skapinker. But
the program can be run alone to
check spelling on files from
virtually any ST application.
The program also allows users to
predefine two-character
abbreviations for longer words.
When the users types the
abbreviaton followed by a space,
Thunder automatically expands it to
the full word.
Thunder also includes a document
analyzer, which reports on each
document's word count, character
count, percentage of words with
more than three syllables, number
of sentences, average words per
sentence, and readability ratings.
Xx Legislation
HOUSE PASSES COMPUTER CRIME BILL
June 1986
The House of Representatives today
made quick work of a new computer
crime bill that would establish
severe penalties for illegally
accessing government computers,
while cracking down on illegal
computer bulletin board systems.
The bill (HR 4718), sponsored by
Rep. William Hughes (D-N.J.),
passed under an expedited procedure
called suspension of the rules
which limits debate and prohibits
amendments. It now must go to the
Senate, where a companion measure
has been languishing in committee,
before going to the president for
his consideration. The fast-track
House procedure requires a two-
thirds vote, but there was no
objection voiced to the bill during
debate. The measure was reported
out last month by the House
Judiciary Committee after hearings
before its crime subcommittee.
Hughes, who chairs the crime
subcommittee, said he expects quick
action on the measure on the
Senate. He predicted that the bill
will become law before the end of
the 99th Congress in December.
The changes in the law, Hughes
said, are needed to eliminate
another glaring example of the
failure of existing law to keep
pace with technological advances.
"With computer crimes," he said,
"the trespassing or theft is done
electronically, not physically.
Although the losses are often just
as great or even greater than
property crime, our laws are not
current enough to keep pace with
the changing technology used by the
criminals."
Hughes was the author of the
nation's first computer crime law
in 1984, a measure that established
a new federal crime for un-
authorized access to classified
information in government computers
and a misdemeanor for accessing any
federal computer or computer
containing financial or credit
information. Hughes said he hopes
the new bill will build on the
existing federal statute.
The new measure would establish a:
-:- New felony for trespassing into
federal interest computers, those
run by or for the federal
government, banks, or states.
Offenders would face five-year
prison terms.
-:- Second felony for "maliciously
trespassing" into a federal
interest computer and causing more
than $1,000 in damage.
-:-Category of federal misdemeanors
involving the use of illegal BBSes
to post private information, such
as credit card data, phone account
information and passwords.
"We need to establish clear
guidelines," Hughes said, "for
protecting the information stored
in computers and for cracking down
on those who knowingly put computers
to criminal of malicious use."
The Judiciary Committee, having
completed its work on computer
crime for the year, next is
expected to turn its attention to
the issue of privacy protection for
databases, electronic mail and
other forms of telecommunications.
The Judiciary subcommittee on the
courts, civil liberties and the
administration of justice has
reported out a bill, sponsored by
its chairman Rep. Robert W.
Kastenmeier (D-Wis.) that would
extend the same protections as
first class mail to tele-
communications. That bill, however,
has yet to be brought up before the
full committee, but staffers for
Judiciary Committee Chairman Rep.
Peter Rodino (D-N.J.) said the
measure will be on the panel's
summer agenda.
A companion bill is moving through
the Senate.
--J. S. Orr
Electronic Privacy
ELECTRONIC PRIVACY ACT HEADED FOR
HOUSE FLOOR
June 1986
The House is preparing to act on a
bill that extends protections under
the 1968 federal wiretap act to all
kinds of data communications,
including electronic mail.
The Electronic Communication
Privacy Act (HR 4952) was reported
out last week by the House
Judiciary Committee and could find
its way to the floor as early as
this week, according to committee
sources. The measure is a "clean
bill," replacing HR 3378, also
called the Electronic Communication
Privacy Act, introduced by Rep.
Robert Kastenmeier (D-Wis.) last
September.
The new bill, also sponsored by
Kastenmeier, who chairs the panel's
subcommittee on civil liberties,
received a unanimous endorsement
from the full Judiciary Committee
as well as praise from its chairman
Rep. Peter Rodino (D-N.J.).
"Almost 20 years ago, Congress
passed legislation to protect the
privacy rights of telephone users,"
Rodino said. "Since then, the
communication industry has been
revolutionized by new technology,
including cellular and cordless
phones, electronic mail sent by
computers, paging devices and other
electronic information systems,"
The problem of privacy protections
for computer data and forms of
electronic communications has long
worried civil libertarians as well
as private and commercial computer
users.
The 1968 wiretap law makes it a
crime to intercept telephone
conversations and forces law
enforcement authorities to seek
permission from the courts to
install wiretaps. However, the law
refers only to oral communication
meaning data transmissions are
without protection against wiretaps
by government authorities or
private individuals.
The new bill "gives paramount
attention to protecting the basic
privacy interests of all Americans,
It also meets the needs of law
enforcement officials who must
resort to court-approved
surveillance in order to apprehend
suspected criminals, and it ensures
fair competition among
industry-users of new communication
technologies.
Specifically, the bill would:
-:-extend protection against
intercepts to cover all electronic
communications
-:-eliminate the distinction
between common carriers and private
carriers
-:-create penalties for persons who
obtain unauthorized access to
electronic communications
-:-require government authorities
to obtain court orders before
obtaining access to third-party
computer records.
-:-expand the list of crimes for
which a wiretap court order may be
obtained.
-:-require government authorities
to obtain a court order based upon
"reasonable cause" before it can
use a device to record phone
numbers dialed from a certain
phone.
-:-require government authorities
to show "probable cause" to obtain
a court order authorizing a
tracking device.
--J. Scott Orr
Xx Atari News from Current Notes
Excerpts from Current Notes June
1986. By: Joe Waters/Frank Sommers
Hard Disk Drives??
Atari has thousands of the long
awaited hard disk drives piling up
in warehouses. The targeted shipping
date was end of May, so these drives
should be in your local store soon.
We'll see...
1200 Baud Modem??
Expect to see the Atari 1200 baud
modem late this month or early July.
Sources indicate that the MS-DOS
emulator has received FCC approval.
The hardware seems to be in fine
shape, but the software still needs
tuning. Probably wont see this
until fall.
New Monitor??
The SPI3000 will be the same as the
current color monitor with one
significant difference--a built in
disk drive. Final price has not been
set, but should be in the neighborhood
of $500.00. The EST, the next generation
ST, will feature a very high
resolution monitor (1280 x 960??).
However, monitors of this kind of
resolution are very expensive.
Currently priced at $1000.00. Atari
is searching the world to see if it
can find a supplier who can make a
more economical HR monitor. Dont
look for the EST before, at best,
early in 1987.
Personal Prolog??
If you are anxiously awaiting for
Personal Prolog from OSS, you'll
have to wait awhile longer. The
first release of PP will be OSS's
first product for the MacIntosh.
The ST version won't be ready till
the end of the summer. OSS also
noted that they have dropped plans
to produce Personal Diskit -- there
are already too many programs
around performing similar functions.
Xx BEGINNERS' COLUMN: POKEing AROUND
By Steve Godun
Written Exclusively for Z-MAG
Greetings, all!!
This column is for all of the
people who wanted to learn BASIC,
but always thought it was too hard,
or it wasn't worth it, or you
didn't have the time, or you were
just too lazy to learn it. But
before I go on, let me say that
learning BASIC is like learning to
do anything else: It takes practice
but once mastered, you can make it
do anything you want.
If you're reading this, chances
are you've had some experience with
BASIC programs, even if just typing
in a program from ANTIC. But many
beginners, when writing a program
mown, at times will overlook (some-
times on purpose) the ever-useful
POKE command. This "fear of POKEs"
(POKEphobia?) tends to arise from
statements in BASIC manuals that
may read "...The first expression,
aexp1, must be an integer or
arithmatic expression that
evaluates to an integer that
represents the memory address of
the machine language routine to be
performed..." (Taken from ATARI
BASIC REFERENCE MANUAL, Page 36).
Now, if you were (or are) a
beginner BASIC programmer and you
saw this, wouldn't you say "Huh?"
I know I did!
So, in this article, I'll try to
explain POKEs with a little more
clarity. Note that this article is
intended for beginning programmers
only, but feel free to read on if
you're not a beginner.
Basically, a POKE is just
changing, or redefining, the value
of a memory location in the Atari's
Operating System (OS). That's it.
Nothing else. Simple, eh?
So, now you're about ready to
fool around with those POKEs. Boot
up DOS, turn on yor monitor, and
let's get on with it.
In direct mode (NO LINE NUMBER),
it works like this:
POKE x,y [RETURN]
Here, x is the memory location,
and y is the value for that
location. To put a POKE in a
program, just type it like you
would any other program.
Here's an example:
10 POKE x,y
Note that nothing except the line
number was changed, added, or taken
away. When run, this will act the
same way it would if it were
entered in direct mode, but the
program would continue to execute
the rest of the program after
performing the POKE.
If you were to look through some
BASIC programs from, let's say,
ANTIC, you might be able to find a
few common POKEs there. The most
commonly used POKEs are these:
POKE 752,1 (CURSOR - Turns cursor
off: Replace the 1 with a
0 to turn it back on.)
POKE 710,x (BACKGROUND - Changes the
background color. The x is any
number from 0-255. Each number will
change the color of the back-
ground. Experiment a little and
find a setting that you like!
More about colors later on.)
POKE 580,1 (COLD START - Type this
in direct mode and press RESET on
your Atari. The computer will act
as if it were turned off and on
again. A very useful POKE commonly
used to prevent LISTing of a
program. Put a 0 where the 1 is to
return to normal setting.)
POKE 16,64 & POKE 53774,64 (BREAK
KEY - Type both in to disable the
BREAK key. Usually used in
conjunction with the above to keep
programs away from "prying eyes".)
POKE 65,0 (I/O FLAG - Type this in
and quiet the "beep beep" from disk
and cassette reading and writing.
Substitute a 1 where the 0 is to
return to normal
POKE 82,x (MARGINS) in Atari
BASIC, enter this POKE and put
a number from 0-39 to set the
margin. 0 is at the far left,
while moving to the right increases
the left margin space. The default
setting is 2.)
POKE 83,x (RIGHT MARGIN - Same as
above, but for the right margin.
The default is 39.)
Of course, there are many, many
more POKE locations. These are just
a few to start you off. For a near
complete list of POKEs, I suggest
you buy a copy of "MASTER MEMORY
MAP" by EDUCATIONAL SOFTWARE, Inc.
Almost all of the Atari POKEs are
listed there, along with an
explanation of each.
So, experiment!! Just be sure
that, when you start your experi-
ments, your disk is out of the
drive, no program is in memory that
hasn't been saved (In case you
screw up the computer and have to
reboot), and that you have a paper
and pencil handy for writing down
all those important POKEs.
- - - - - - - - - - - - - - - - - -
Want to see the internal workings
of your Atari? Type in this program
and RUN it:
10 GRAPHICS 0
15 POKE 88,0
20 PRINT "I LOVE TO POKE!"
25 GOTO 20
The POKE 88,0 opens a "window" to
the internals of the Atari. To get
out of this, hit RESET. Try other
words or phrases in line 20 and see
what happens.
- - - - - - - - - - - - - - - - - -
Well, that's all for now. Have
fun POKEing around your Atari, and
remember that POKEing is not
illegal. See ya soon...
-Steve the Kid
Sys0p of MJAC BBS
(201)469-4474
-----------------------------------
[Ed. In addition to the book that
Steve recommended, Look for "Mapping
The Atari" By: Ian Chadwick,
Published by Compute Publications,
Inc, ABC Publishing Company.
$16.95]
Xx Zmag BBS Watch
This is a new column that will be
dedicated to messages of interest
from Zmag Message bases on systems
carrying Zmag and/or a Zmag Base.
Following text taken from the NYC
BBS.
TO->COMPUTER WIZZZ
FROM->RATBANE
SUBJ->Z-MAG
Everyone thanks you for your kind
thoughts. How exactly do you read
ZMAG in computer class? Does your
teacher acquire a file and then let
everybody read it? Is it read as
part of class time or study time,
or after-school club time? Ask
your teacher if there is anything
we could do to be of help to him in
teaching computers and programming.
There is a Basic class being taught
on Atomic boards by Woden, Sysop of
the Valhalla BBS. We have a lot of
Public Domain utilites, and
firsthand access to many goodies
here in the great city of New York.
Do let us know what your teacher
says. Try to get him/her to log on
here, too.
RATBANE
TO->ALL
FROM->COMPUTER WIZZZ
SUBJ->Z-MAG
WELL HELLO FROM THE GREAT STATE OF
VERMONT, COMPUTER WIZZZ HERE
RESPONDING TO Z-MAG ARTICLES ABOUT
THE ATOMIC NETWORK,N.Y.CITY BBS AND
B.A.T. ALSO Z-MAG ITSELF. Z-MAG IS
THE BEST THING TO HAPPEN ON BBS'S
SINCE MODEMS. I HAVE BEEN LOOKING
AROUND AND HAVE COME TO THE
CONCLUSION THAT N.Y.CITY BBS AND
THE ATOMIC NETWORK ARE IN THE FORE-
FRONT OF PUBLIC SERVICE BBS'S. AS
FOR ME I READ Z-MAG IN MY COMPUTER
CLASS IN SCHOOL, INTERESTING.
B-C-IN-YA
*COMPU-WIZZZ*
TO->SYSUSR:0
FROM->[[The*Mayor]]
SUBJ->NYC*AMUSMENTS
ANOTHER WELCOME CHANGE TO THE N.Y.
CITY BBS WILL BE HERE SHORTLY,SINCE
THE FORMER CO-SYSOP OF NYC
*AMUSMENTS BASE #3 HAS LEFT US,I AM
CHANGEING THE NAME OF THE BASE TO
REFLECT THE GROWTH OF THE ATOMIC
NETWORK AND N.Y.CITY BBS. THE MAIN
REASON FOR OUR SUCCESS BESIDES ALL
YOU GREAT USERS IS THE POPULARITY
OF ZMAG ALL ACROSS THE COUNTRY,
WHICH CARRIES INFORMATION OF ATOMIC
AND N.Y.CITY AND B.A.T. SO
THEREFORE BASE #3 WILL BECOME
|NYC|Z-MAG|BASE| OR SOMETHING CLOSE
TO THAT,IF ANY OF YOU HAVE ANY
SUGGESTIONS ON THE NAME,LET ME KNOW
DURING THE WEEK,ALSO IF ANY OF YOU
ARE BIG FANS OF Z-MAG AND CANT SEEM
TO KEEP QUIET ABOUT HOW GREAT IT
REALLY IS,LET ME KNOW THAT TOO,THAT
USER MAYBE OUR NEXT NYC|Z-MAG|
CORRESPONDENT.
KEEP SMILING
THE MAYOR
TO->ALL
FROM->WODEN
SUBJ->ZMAG
THE LATEST ISSUE OF ZMAG IS NOW
AVAILABLE FOR DOWNLOADING. THE FILE
IS ZMAG611. THIS ISSUE HAS A
SECTION ON BEGINING ASSEMBLY
LANGUAGE PROGRAMMING. LOOK FOR IT
IN THE DOWLOAD SECTION. IF ANYONE
HAS ANYTHING THAT THEY WOULD LIKE
DONATE TO ZMAG. YOU CAN UPLOAD IT
HERE, LEAVE ME A MESSAGE AND I'LL
SEE THAT THE NEWS DEPARTMENT
RECIEVES IT. HERE IS YOUR CHANCE TO
HAVE YOUR WORK SHOWN WITHIN THE
FASTEST GROWING BULLETIN BOARD
MAGAZINE AROUND.
-----------------------------------
Xx Assembly Language
ANTIC PUBLISHING INC., COPYRIGHT
1985. REPRINTED BY PERMISSION.
CHRIS CRAWFORD ASSEMBLY LANGUAGE
LESSON THREE: LOGIC
BOOLEAN LOGIC
A great deal of programming
involves the use of Boolean logic.
This is a standardized system for
handling logical manipulations.
It's sort of like algebra for logic
You must understand Boolean logic
if you are to write assembly
language programs, so let's get
started.
Where algebra deals with numbers
Boolean logic deals with
propositions. A proposition is just
a statement such as "Fred eats
worms." It can take only two
possible values -- True or False.
In our programs we seldom bother
with broad and glorious
propositions such as "Love is the
universal language of truth" or
"War is the extension of policy by
other means". Instead, we normally
deal with propositions such as "The
joystick trigger has been pressed,"
or "There is a diskette in the disk
drive."
When we use Boolean logic with a
computer, we may think in terms of
true and false, but the computer is
actually working with 1's and 0's.
We use the following convention: a
1 corresponds to a Boolean value of
"true", while a 0 corresponds to a
Boolean "false".
Using this system we can
represent propositions inside the
computer. However, programming
requires more than the mere
representation of data; we must
also be able to manipulate that
data. This brings us to the Boolean
operators. There are four common
Boolean operations necessary for
most programming practices:
Not
This is the simplest of Boolean
operators. It takes a single
Boolean value as an input and
produces as its output the logical
converse of the input. Thus, a true
input yields output, while a false
input generates a true input.
Or
This Boolean operator takes two
Boolean values as its input and
generates a single Boolean value as
its output. The value of the
output depends on the values of the
inputs according to the following
rule: If one input is true OR the
other value is true, then the
output is true. Otherwise, the
output is false.
And
This Boolean operator is just like
the or-operator, except that it
uses a different rule. Its rule
is: If one input is true AND the
other input is true, then the
output is true; otherwise the
output is false.
Exclusive-Or
This Boolean operator is just like
the or-operator, except that its
rule is: If one input is true, OR
the other input is true, BUT not
both are true, then the output is
true; otherwise, the output is
false.
When we use the 6502 for Boolean
operations, you must remember that
the operations are eight bits wide.
Instead of working with one bit at
a time, we use all eight bits of a
word in parallel. The bits in a
byte are independent and do not
affect each other in any way -- at
least as far as Boolean operations
are concerned.
The 6502 has three instructions
for performing Boolean operations.
These are AND, EOR, and ORA. The
first performs an and-operation.
For example, consider the following
code:
LDA FISH
AND GOAT
This will first Load the
accumulator with the value of FISH.
It will then And the contents of
the accumulator with the contents
of GOAT. The result of the and-
operation will be left in the
accumulator.
The AND-instruction can use an
immediate operand if you desire,
just as the ADC-instruction can.
The EOR-instruction provides the
exclusive-or operator. It works
just like the AND-instruction. The
ORA instruction provides the or-
operator in just the same way.
If you wish to obtain the NOT-
operation, just use EOR #$FF; this
will invert each bit in the
accumulator. Because NOT is so
easily reproduced with EOR, there
is no special NOT instruction in
the 6502.
APPLICATIONS OF BOOLEAN LOGIC
If you have any sense at all,
you are probably asking, "What good
is all this Boolean nonsense? What
would I use it for?" Four
applications are available:
Program Logic
Many times our programs
encounter rather complex logical
situations. The program must be
able to load a file; if the FMS is
in place and there is a diskette in
the disk drive, and the diskette
has the file we are looking for, or
the file specification calls for a
cassette load, then we will load
the program. Many programming
problems involve such Boolean
operations, Keeping them straight
is certainly a headache.
Masking Bits
Sometimes we need to isolate
particular bits in a byte. For
example, in Eastern Front (1941) I
used the character value to store
the unit type. The color of the
unit was encoded in the upper two
bits of the byte, the type in the
lower six bits. If I wanted to get
only the unit type, I had to mask
out the upper two bits. This I did
with the following code fragment:
LDA UNITCODE
AND #$3F
The AND-instruction eliminated
the upper two bits, leaving me with
just the unit type. Bit-masking
like this is useful in many
situations. We use it frequently
when we pack bits into a byte to
save memory. It is also handy with
input handling. If you want to
read the joystick port, you
frequently mask out the bits in
turn to see which is active.
By the way, you mask out bits
set to 1 with the AND-instruction.
You mask out bits set to 0 with the
ORA instruction. The logic is
reversed.
Setting and Clearing Individual
Bits
We also use the AND and ORA
instructions to set or clear
individual bits within a byte.
This is most often useful for
handling arrays of flag bits.
Folding Bytes Together
This little fragment of code
will fold bytes together:
LDA FISH
EOR GOAT
AND MASK
EOR GOAT
STA ANSWER
This is a magical piece of code.
See if you can figure out what it
does. Experiment with two values
of MASK: $OF and $FO.
SHIFT AND ROTATE INSTRUCTIONS
The 6502 also has instructions
that allow you to shift the bits
around inside a byte. The first of
these are the shift instructions.
One, ASL, shifts a byte to the
left; the other, LSR, shifts a byte
to the right. Thus, the byte
%01101011, when shifted left,
becomes %11010110. Each bit is
shifted one position to the left.
The leftmost bit is rudely pushed
right out of the byte and falls
away ("Aaaaaaaaarrrrrggggg!"). A
zero is shifted into the rightmost
bit. The LSR instruction does the
same thing in the opposite
direction.
Note that ASL also doubles the
value of the byte, while LSR halves
it. Two ASL's multiply by four;
three multiply by eight. This
makes it easy to do simple
multiplication, but be careful with
round-off error here. What happens
if you try to multiply by 256?
What do you get if you halve 3?
A variation on the shift
instructions are the rotate
instructions. There are two:
rotate left (ROL) and rotate right
(ROR). These function just like
the shift instructions, except that
the bit that gets shoved into the
bottom is not necessarily a zero;
it is the contents of the Carry
bit. The bit that gets pushed off
the edge of the byte goes into the
Carry bit, so it is not lost.
Thus, if you rotate either way nine
times, you'll be right back where
you started.
Rotate instructions are a handy
way to get a particular bit into
the carry bit where you can work on
it. Conversely, once you get your
desired bit into the carry bit the
way you want it, you can put it
back into a byte with some rotate
instructions.
INCREMENT AND DECREMENT INSTRUCTIONS
The last instructions I will
cover are the increment and
decrement instructions. These allow
you to add one (increment) or
subtract one (decrement) from a
memory location. These are not
considered to be arithmetic
operations so they do not affect
the Carry flag, nor are they
affected by it.
You cannot increment or
decrement the accumulator, only RAM
locations.
Next week Part 4
Xx Fine Tuning DOS
By: Divemaster
A) How to delete "Twin" files
Have you ever ended up with 2
(or more) files on your disk with
the same filename? Do you end up
cursing and screaming when you try
to delete one of them? There is a
way around that, folks:
1) Boot up DOS with BASIC in.
2) In the immediate mode, type
POKE 3118,0
3) Type DOS, and press RETURN.
Now, you'll be able to delete
without losing both files, because
by POKEing 3118,0, DOS will erase
ONLY the first "TWIN" file.
[Reprinted from the March Issue of
the Alamo Area Atari Users Group
Newsletter]
Xx Zmag Notes
Well there you have it. Our longest
issue and hopefully one of our best
to date.
Please pass the word about Zmag,
if you know of a BBS that is
interested, please have them leave
me a message on The Syndicate BBS
201-968-8148 or any of the other
Systems carrying Zmag.
Thanks for reading... See you next
week.
-----------------------------------
Zmagazine June 25, 1986
Please contribute!!
-----------------------------------
(c)1986 Ron Kovacs
(c)1988 SPC/Ron Kovacs
-----------------------------------------
Return to message index
