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

4K ROM provides greater fl exibility than the arcade board, but far less
total storage space.
The TIA also provides registers to set sprite colors: one named
COLUP0 and the other COLUP1. In many early VCS games, including
Combat, sprite colors were set once for the entire game. In later games,
the program stored a different color value in one or both sprite color
registers along with a different bitmap value. Multicolor sprites were
implemented, too. These included Pitfall Harry in Activision’s Pitfall! The
careful observer can note color banding in most of these sprite graphics,
though, which is not seen in the true bitmapped graphics of later platforms
like the NES. This style of “stripe-colored” sprites is a particular
trademark of VCS games. Mercifully for Tod Frye, the iconic Pac-Man of
the arcade game is a single color, so no further ROM space or horizontal
blank logic had to be expended to draw his yellow image convincingly.
Combat uses two sprites, each of which fi res a corresponding missile—
just what the TIA ordered, or what it was originally ordered to do. But
games like Taito’s Space Invaders were not designed with the peculiarities
of the Atari VCS in mind. Sprites were different in many post-1977 arcade
games. Most important, there were often more than two per screen! When
faced with the rows of aliens in Space Invaders or the platoon of ghosts that
chases Pac-Man, VCS programmers needed to discover and use methods
of drawing more than two sprites, even though only two one-byte registers
were available.
As discussed in the previous chapter, the TIA offers a set of horizontal
motion registers for each of the sprites, the missiles, and the ball. The TIA
also exposes another register called HMOVE to execute changes in horizontal
motion. These registers were primarily intended to be set during a
vertical blank—that is, between screen draws. For example, Combat repositions
both player and missile horizontal positions each frame, then
updates variables in RAM to ensure that the objects are drawn on the
appropriate lines, and then updates the horizontal motion registers once
at the start of the frame.
Larry Kaplan, one of the fi rst developers to work on the Stella prototype,
fi gured out that sprite data could be reset more frequently than once
per frame. Because the VCS requires the program to control every line of
the television screen, it is possible to change the sprite graphics’ values
and their horizontal positions more than once per frame. Kaplan fi rst
used this technique in Air-Sea Battle, one of the console’s launch titles. In
the game, multiple rows of enemies, one per row, pass back and forth
across the screen. Each player controls a turret on the ground that can be
aimed and fi red at targets in the air. Multiple targets are presented by
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