Racing the Beam : the Atari Video Computer System - Index of

power, graphics, and controller setup—were always signifi cantly different
from those of the Atari VCS, so that having a well-tested and implemented
game design on the arcade platform didn’t mean very much
when it came to the home console’s hardware. A “port” from the arcade
to Atari’s home system was not like a port from one computer system
to another, in which the program being converted would function in the
same way on both platforms with only minor differences, after a few
small changes were made. For one thing, the VCS code for a game always
had to be written from scratch—there was no way to modify and reuse
what had been done for the arcade game. As Warshaw explained, “The
hardware in coin-ops was way beyond the capability of the VCS and the
code frequently wouldn’t even have been compatible. The closest to using
coin-op technology would be when occasionally we would consult with
original coin-op programmers to get questions about AI algorithms
answered. The code never even came close to passing from arcade
machines to VCS.” 3
Given the differences in platform and the need to rewrite each game
from scratch, a port to the Atari VCS had plenty of room to become a fairly
involved adaptation rather than a simple re-creation. This could leave
some fans of the arcade game disappointed, as happened with Pac-Man.
It could also lead to the porting project taking a different and interesting
turn, as happened when the Star Castle project veered off to become Yars’
Revenge.
The most obvious difference between the Star Castle computing system
and the VCS was the arcade machine’s vector graphics, which Atari called
XY graphics. Atari’s successful arcade games Tempest, Battlezone, Asteroids,
and Lunar Lander all use this sort of graphics system, which employs a
fundamentally different type of monitor. All early arcade games used a
CRT, but the ones in vector graphics games are wired differently than are
the ones in standard televisions. The electron beam does not sweep across
the screen from left to right, being turned on and off as it makes its way
down and then returns back to the top sixty times each second. Instead,
the electron gun is pointed at a certain location, turned on, and moved
from that (x, y) coordinate to another point in a straight line, where it is
turned off again. (An oscilloscope functions in a similar way; it just uses
a different method of defl ecting the electron beam.) Because the beam
can be made to move arbitrarily instead of progressively scanning along
the screen, this way of illuminating a phosphor-coated screen was also
called “random scan.” Vector graphics systems draw lines in an (x, y)
coordinate system; raster graphics systems draw a grid of bitmap data
representing a pattern of pixels.
5 Yars’ Revenge [83]