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

contemplated all at once. It offers a variety of actions built from a few
core possibilities, each of which provides a unique experience and
demands a different skill. And fi nally, it gives the player choices—even
if limited ones—about where to go and what route to take to get there.
Attention to Detail
The Pitfall! jungle features multiple areas on each screen, making the
game’s kernel much more complex than earlier games, even though the
overall program could not be larger. The routine that draws the screen
closely matches Crane’s original visualization of a setting for his little
running man. Like many Activision games, Pitfall! creates a detailed onscreen
image by splitting the screen up into horizontal sections. First the
kernel draws the score and timer. Next come the trees and branches. Then
it draws the top of the liana, then the liana and Pitfall Harry, then the
ground, the pits, next the ladder and underground area, and fi nally the
Activision logo and copyright.
Within each of these sections, Crane makes deft use of graphics
drawing techniques that had become second nature by this time; namely,
using the TIA graphics registers for visual elements different from their
intended purposes and reusing those registers multiple times down the
screen (see fi gure 6.2). For example, the leaves are drawn with a mirrored
playfi eld, as are the tree trunks, but the more detailed branch segments
where the trunks meet the leaves are rendered using sprite graphics, carefully
positioned to match the location of the trunks and wide-doubled
using the number-size registers. The liana is rendered with the ball
graphic, which is moved slightly between each scan line to make it
swing.
Those at Activision prided themselves on their richly colored, graphically
sophisticated games, and players certainly noticed the difference.
However, understanding the function of the TIA helps us appreciate the
true attention to detail in a game like Pitfall! One such detail can be found
in all Activision titles. Close inspection of the sides of a Pitfall! screen
reveals a black bar at the left edge, but not at the right. The reason for this
bar is partly aesthetic, partly technical.
To move a sprite, missile, or ball, the program must strobe the horizontal
move (HMOVE) register after setting the horizontal position offset
for the desired objects in their corresponding horizontal motion registers
(HMP0/1, HMM0/1, HMBL). Once HMOVE is strobed (which is accomplished
by writing any value to the register), the TIA executes the motion
changes. However, to complete this process, the TIA requires that HMOVE
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