ATARI XEGS INFORMATION
By Matthew Ratcliff
This look inside the Atari XE Game System includes two
programs. XEGS Manager is a utility for convenient control
of all the Game System's built-in features, but many of its
options can also be used on the other Atari XL/XE computers.
The second BASIC program is a short "spray-painting" routine
for testing light guns.
Atari executives asked the heads of several major toy
store chains which product they'd rather sell -- the powerful
65XE home computer for about $80, or a fancy new game system
for about $150. The answer was, "You can keep the computer,
give us that game machine!"
This "game machine" is what we now know as the Atari
XEGS, the XL-compatible Extended memory Game System. It's
simply an enhanced 65XE in a game machine package. It's also
a brilliant idea. The XEGS has been selling out almost as
fast as toy stores can get them in.
The XEGS may not seem like such a hot idea to serious
Atari computer users. But just think about it. If you were
afraid of computers or don't have the foggiest idea what to
do with one, you'd have absolutely no interest in an Atari
65XE -- even if it could play great games. However, you'd
probably have no compunction about buying a great video game
system, the XEGS, as a new addition to the family
Now we'll take a close technical look at the Atari XEGS.
I'll explain how some of its changes in physical design have
affected the operating system software. I'll also present
the XEGS Manager, a utility for controlling all the built-in
features of the XEGS.
The keyboard of the XEGS is detachable. When not
connected, the XEGS console looks (and acts) like just a tame
little game console. In fact, Missile Command is turned on
Plug in the keyboard and turn on the machine without an
external cartridge, and you're running Atari BASIC, Revision
C. The keyboard is virtually identical to the one found on
the 130XE. It's mushy, but you can get used to it.
The cable on the XEGS keyboard is quite short. There
are two brackets at the top of the keyboard case which lock
neatly under the front of the console. The keyboard
connector is a standard DB15 female. My first XEGS project
was to construct an extension cable for the keyboard. I find
it far more comfortable to type for a long time with the
keyboard on my lap than on a desk.
Inside the XEGS keyboard case is a small circuit board.
On it you will find some resistors, capacitors and two CD4051
chips, which decode the keypresses and send an internal
keycode back to the POKEY chip in the XEGS. I already made
an adapter cable to connect the XEGS keyboard to the 800XL
So far my efforts at "scanning" the external keyboard
manually have failed -- the POKEY chip does this
automatically in the XEGS -- but I hope to bring you a laptop
keyboard utility program that lets you use the XEGS keyboard
on any other Atari computer with minimal hardware hacking --
just a cable.
There are only two RAM chips in the XEGS, which deliver
a full 64K of RAM. They're Texas Instruments TMS4464-12
64K-by-4-bit chips. In the XL/XE computers, eight
64K-by-1-bit chips are used. Fewer chips improves
reliability and generally reduces the cost of producing the
machine. In fact, the XEGS contains a total of only 17
ICD has indicated that it is working on a RAM upgrade
kit for the XEGS similar to the RAMBO XL for the Atari 800XL.
I've already received a 128K RAM upgrade kit from Innovative
Concepts for my XEGS, making it fully 130XE-compatible. I'll
review this in a future issue.
Using higher-density ROM chips, the updated XEGS
operating system had 8K of spare ROM. Atari decided to use
that extra ROM for the Missile Command game, which is
bank-switched in and out much like how Atari BASIC is toggled
on and off. Missile Command can be enabled by holding the
[SELECT] key during power-up when a keyboard is connected.
When in BASIC, you can enter BYE to get to the Self Test.
From there, pressing [RESET] while holding the [SELECT] sends
control to Missile Command. Press [RESET] in Missile
Command, while holding [OPTION], to return to the Self Test.
Press [RESET] by itself to return to BASIC. (Reboot if a
disk drive is connected.)
The XEGS comes with a light gun called the XG-1. The
Nintendo light gun is more accurate -- if something is lined
up in its sights, that's exactly what you hit. Not so with
the XG-1. You'll find that it often shoots to the left or
right, depending on the software you're running.
The XG-1 is simply a specialized light pen. Light pen
support was built into the earliest Atari computers, but it
never really caught on. In the shape of a gun, the light pen
has brought a whole new dimension to video games
applications. The light pen horizontal position, LPENH, can
be PEEKed at memory location 564, and the vertical position,
LPENV, is found at location 565.
Light gun values range from 0 to 227. You will notice
that your horizontal readings are quite odd. Try the sample
program below, and notice how the GUN-X readings vary as you
sweep the gun across the screen, left to right.
30 GRAPHICS 0:POKE 752,1:POKE 712,15
40 POSITION 0,0:FOR I=1 TO 4:? "0123456789";:NEXT I
50 ? "GUN-X=";PEEK(LPENH);" "
60 ? "GUN-Y=";PEEK(LPENV);" "
70 ? "TRIGR=";STICK(0)
80 GOTO 50
Point the gun to the far left of the display and GUN-X
will read about 88. Moving from left to right, the reading
will reach 227 at about column 34. Then suddenly it drops to
0 and increases again to about 30 at column 39. This offset
is due to the delay between when a pixel is actually lit on
the display and when the information is relayed from the
light gun sensor to the POKEY chip, which latches an internal
scan counter for the pen reading.
The old Atari 400/800 Hardware Technical Reference
recommends a "calibration procedure" each time the light pen
is used, so that the software can compensate for this offset.
A calibration procedure would improve the accuracy of the
light gun. But Atari's Bug Hunt and Barnyard Blaster games
both have "hard-coded" values -- different ones in fact.
While Bug Hunt appears to shoot slightly to the left,
Barnyard Blaster seems to shoot a tad to the right. The Y
readings for the gun are more predictable, equal to half the
number of the currently displayed scan line. You'll notice
with your test program that GUN-Y only varies from about 17
to 115. Note that you get much better performance out of the
light gun near the screen edges, when you use a light colored
border achieved with the POKE 712,15 above.
You'll need to perform some computations to adjust for
these unusual readings, to convert gun coordinates to screen
coordinates. Different conversion factors are required for
each graphics mode (and P/M graphics).
The gun won't return reliable readings at all if the
intensity of the display is too low. That's why the screens
for Atari light gun games may be brighter than usual. The
game screen will momentarily flash white whenever you press
the trigger in either Bug Hunt or Barnyard Blaster. While
the screen is all white, the software reads the gun position
and provides the most accurate values.
Listing 1 presents a simple Graphics mode 8
"spray-painting" program for testing the XG-1. Dots are
drawn whenever the fire button is pressed. Try holding the
gun very steady to see how much jitter you get in the
readings. These inaccuracies are reflected in your games as
well. I feel that, at the higher levels of play, Bug Hunt
and Barnyard Blaster both require more accuracy for continued
play than the XG-1 can deliver.
Atari has informed me, however, that the XG-1 and a
revised Bug Hunt will be released as a separate package. If
you're tired of waiting, you may wish to pick up the Sega
light gun, for about $25 (when on sale) and modify it for the
To modify the Sega gun for the Atari, you'll have to cut
off the incompatible connector. The wires must be stripped
back and soldered into an Atari joystick connector as
SEGA GUN ATARI JOYSTICK PORT
Blue wire Pin 1 stick FWD
Gray wire Pin 6 trigger
Green wire Pin 7 +5 volts
Black wire Pin 8 Ground
Because of the close-fitting connections for the XEGS
ports, don't wire in a DB9 female connector that has "ears."
Most joysticks don't have wires for unused signals, so
cutting up an old joystick cable may not work. Specifically,
an Atari joystick does not need the +5 volts, so there isn't
likely to be a wire connected to Pin 7. However, you can
find joystick extension cables at Radio Shack, which have all
nine pins wired from male to female. ANTIC disclaims
responsibility for any damages that might occur during
improper implementaton of this, or any, hardware modification
project we publish.
Once it's all hooked up, you'll notice that gun fires
when you release the trigger, which is annoying. The Sega
trigger wiring is the opposite of what the Atari light gun
uses. To rewire the trigger switch, remove the five screws
(one is under the Sega logo on the side). Find the trigger
micro switch with three connections. Wire to the normally
closed contacts instead of normally open.
Listing 2, XEGSMGR.BAS, is a BASIC loader that lets you
create a machine language file on disk. Type it in, check it
with TYPO II and SAVE a copy before you RUN it. The file
XEGSMGR.EXE can then be loaded from DOS or renamed
AUTORUN.SYS. Many of the program's features also can be
applied to Atari XL/XE computers.
Normally you must press the [OPTION] key at boot time to
disable BASIC and go directly to DOS. And once BASIC is off,
the only way to get it back on is rebooting. Option 1 of
XEGS Manager is to turn internal BASIC on, and option 2 turns
Disabling BASIC while in DOS provides an additional 8K
buffer for file copying. This is an important feature for
owners of a single drive. Quite often, BASIC must be off
before certain machine language files can be loaded and run.
The XEGS Manager eliminates the need of rebooting every time
that BASIC must be re-enabled.
The XEGS Self Test lets you test the computer's sound
registers, keyboard, and memory. However, BASIC is not
turned off automatically when Self Test is run from the BYE
command. This means that the 8K of RAM under BASIC isn't
tested. Option 3 from the XEGS Manager lets you run the Self
Test with BASIC off, so that the maximum ammount of RAM is
All of the operating system of the XEGS (and 64K or more
XL/XE machines) is "shadowed" by RAM. Some disk operating
systems, such as DOS XL and SpartaDOS, use hidden RAM for
many of their own functions. However, if you're using Atari
DOS 2.0 or 2.5, then there is a lot of RAM going to waste in
Option 4 of the XEGS Manager lets you enable a RAm-based
operating system so you can do some real "hacking" --
disassembling and adjusting parts of the XEGS operating
system to suit your needs. Even if you're not a hacker,
there are other practical features of a RAM OS.
Once the RAM operating system is enabled, you are
prompted for a disk drive number, 1-8 or 0 to exit. A custom
font can be loaded in place of the standard one in the OS
ROMs: enter the drive number to display a directory of all
.FNT files. Then enter the name of the font file to load, or
simply press [RETURN] to change drives or disks.
You needn't enter the drive specifier or extender -- the
XEGS Manager will take care of that for you. The font file
is loaded into memory at $E000 (57344). Then you're prompted
to (1) repeat the process and try a different font, or (2)
exit. (You will find some font files on this month's disk as
Your RAM OS and font are reset-proof, too. Pressing the
[RESET] key causes the XEGS to re-enable the ROM-based
operating system, but a special handler in Page 6 of memory
takes control after that. The handler converts the ROM back
to RAM, recopying all the essential parts of the ROM OS, in
case part of this RAM area got clobbered while you were
hacking about. The handler does not recopy the ROM OS font,
however, leaving yours intact. Should your RAM font get
garbled somehow, press [RESET] while holding the [START]
console key to return to the ROM font. Each time that
[RESET] is pressed, a RAM OS prompt is displayed at the top
of the screen as a reminder.
If you enable a RAM OS while running SpartaDOS, the
XEGS Manager detects it and prevents the installation and
subsequent system crash. The Manager does not automatically
detect any other DOS, such as DOS XL, which may crash when a
RAM OS is enabled.
Again, if you turn on your XEGS without the keyboard
connected (or hold down the [SELECT] key at power-up),
Missile Command fires up automatically. Option 6 of the XEGS
Manager will get you into Missile Command without having to
mess with any console keys or power switches.
The XEGS is a superb little computer. It's still a
hacker's system too. I've found that the PBI ROM routines
are intact, which means that you should be able to hack in
your own custom PBI connector and use the XEGS with ICD's MIO
board, if you're a real solder jockey.
The XEGS has brought along a lot of new software too,
something Atari was counting on. Much of it includes
repackaged classics or cartridge conversions from disk-based
software, but there are a few new titles such as Battle Zone.
Atari's new 256K bank-switch cartridges are not likely to be
pirated. This means that the piracy threat for 8-bit Atari
software should be minimal, thus attracting more new software
vendors from the traditional Apple and Commodore markets.
If Atari can provide a responsive cartridge production
service for third party software vendors (something the old
Atari never would have done), then we're likely to see the
software base for the 8-bit Ataris grow with the popularity
of the game industry, which is definitely on the rise again.
Matt Ratcliff is a St. Louis aerospace engineer and a
longtime ANTIC contributing writer.