home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Atari Mega Archive 2
/
Atari Mega Archive CD - Volume 2.iso
/
8bit
/
cislib_b
/
xegs.doc
< prev
next >
Wrap
Text File
|
1995-04-22
|
15KB
|
299 lines
BY MATTHEW RATCLIFF
(synopsis)
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
entertainment center.
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.
KEYBOARD
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
automatically.
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
joystick ports.
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.
MEMORY
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
chips.
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.)
LIGHT GUN
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.
10 LPENH=564
20 LPENV=565
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
Atari.
To modify the Sega gun for the Atari, you'll have to cut
off the incompatible connector.