BLOCKBREAKER will run on any
Atari 400/800/XL/XE with a minimum of 32k RAM. A paddle will be required
plugged into jack 1. The Level 1 playfield appears after about one
minute of initialisation. Please note - when RUNning BLOCKBREAKER
you should start out with clear RAM. If you have just switched on
your machine and loaded in the program all will be well (proving
you have typed the program correctly!). Otherwise make sure that
you have SAVEd or CSAVEd a copy of the program and then type POKE
580,1 in direct mode and hit RETURN then RESET before loading. This
process clears RAM in the same manner as switching off and on does
but is kinder to the machine.
THE GAME
Blockbreaker is based on one of
the classic computer games in which you have to demolish a wall
of bricks using a fast moving ball directed by your paddle. Simple
in concept, the game can nevertheless become very addictive (and
not a little frustrating) as you strive to keep the ball in play
to beat the current high score. Two types of brick will be noted
'normal' and 'special', the latter identifiable by pulsing colours.
Both types have a points value dependant on their position in the
wall. Unlike the normal bricks however, hitting one of the 'specials'
will give you additional points as displayed on the bonus counter
top right and also initiate a stacking routine transferring them
out of the playfield area.
STACKING: During this manoeuvre,
attempt to strike any, remaining special bricks because, in this
phase, such hits continue to award the player with bonus points
but the bricks themselves remain intact. Since the bonus counter
resets each time the ball hits a special brick, BIG points can be
amassed using skilful paddle control. As the game progresses a stack
of bricks will be formed on the left. Losing a ball during play
forfeits a part of this stack. Once the whole wall has been demolished
each stacked brick becomes worth 300 additional points. An extra
ball is then awarded (up to a maximum reserve of 9) after which
the next level commences. There are 8 levels in all. Should you
succeed in clearing level 8 (no mean feat), levels 5 to 8 will repeat.
BALL MOTION: The ball has
two possible speeds selected automatically during game play and
is served by pressing the trigger. In the slow phase only two angles
of bounce are possible but during the fast phase, shallow bounce
angles will result if the ball is struck closer to the centre of
the paddle. These angles will be required in order to gain maximum
benefit from the special brick scoring routine and in order to complete
a level.
RESTARTING: When you have
lost all reserve balls the high score is updated, a good target
being in the tens of thousands. Press the START key to begin a new
game.
TECHNICAL NOTES
Those of you who just want to play
the game do not need to read on but if you are interested how the
program works I, have provided some notes on various parts of the
program.
Except for the initialising period
following loading, together with a brief inter-level sequence, the
program runs entirely in machine language. A combination of mainline
code with vertical blank and display list interrupts is used. The
use of interrupt code results in smooth motion (since graphics objects
are moved while the screen is blanked out), the ability to use 4
channel sound independently of other operations and the use of multiple
colours.
The mainline code looks after the
service initialisation for each ball, updates display features and
scoring and takes care of 'normal' brick routines. The VBI is responsible
for moving paddle, ball and 'special' bricks during the stacking
routine. It also checks for collisions between ball and other playfield
objects, monitors sound values and scrolls the high score copyright
message. Twenty two DLI's are incorporated to impart a unique colour
to each row of bricks. In this manner up to 40 colours can be displayed
on the screen simultaneously. Additionally, the colours themselves
are randomised for each new wall.
SOUND: All sound is processed
in a vertical blank interrupt. Because the VBI operates independently
of the mainline program in a precisely timed manner it is ideal
for such use. Blockbreaker's sound is simple but effective - an
example is given in Listing 2. Because our VBI operates 50 times
per second, each sound pulse requires several passes. In the example
given (ball hitting bat), 7 passes are required. Flag1is the master
flag which restarts the routine each time the collision register
is set. It does this regardless of whether the routine is currently
active from a previous strike or is off. Flag2 simply bypasses the
routine if no sound is required. Location 53768 is very important
for sound production although few programmers appear to take advantage
of it. The insistent beat heard during initialisation owes its effect
to this register. As a demonstration try replacing the '7' in line
310 with a default of '0'. The smoothly descending high pitched
note of the falling brick is a result of clocking one sound channel
with a much higher frequency to that normally used - location 53768
again.
MOTION: All performed in
the VBI in order to achieve smoothness. 'Players' are used to represent
ball, bat and falling brick, the latter effect produced as follows.
Once the VBI has detected a collision between ball and playfield
register 2 (a 'special' brick), the struck brick is replaced by
Player 1, the playfield brick image is blanked out and motion of
the Player commences. Its image is replaced by a playfield image
once the brick has been 'stacked' thereby releasing the sprite for
further use. When combining playfield graphics with PMG in this
manner reference tables should be kept of related positional values
(lines 1530, 1540) - see Listing 3.
THE BALL: Variables (in
the sense of memory locations) are used to keep track of the ball
and bat X/Y coordinates. Once a collision has been registered, the
X coordinate of the left edge of the bat is subtracted from the
ball's X position to give a value between 0 and 11. This value is
used as an index to two Page 6 tables (lines 1500-1520). The first
table determines the ball's reflected angle, a '1' informing the
VBI to move the image one 'pixel' both horizontally and vertically
on each pass - in other words at 45 degrees. The central table locations
are poked with a '2' during play showing we require movement of
two pixels vertically on each pass to everyone horizontally - that
is to say, at a steeper angle. Table 2's value is simply added to
the ball's X coordinate so as to move it left or right (adding 255
to the byte being equivalent to subtracting 1). The ball's speed
is obtained by simply cycling the above routine once or twice during
each VBI pass.
COLOURS: The vertically
structured form of the Blockbreaker display is ideal for multiple
colour generation by Display List Interrupts. Each DLI (there are
22 of them) selects two colours from tables sited in page 6 of memory.
The first colour is POKEd straight into register 53271 and affects
the 'normal' bricks. The second colour is incremented by 16 on each
DLI pass before POKEing into 53272, the register used for the 'special'
bricks, to give the pulsing effect. These two registers correspond
to the more commonly used shadow registers at 709 and 710 but when
using DLI's we must POKE directly to the GTIA chip. Although 22
DLI'S are operating, because they each perform a similar task, only
one master routine is required (see lines 1023 -1025). All we require
is a counter which is incremented in each DLI and which is then
used as an index to the colour tables. Since we have a VBI operating
we use this to reset our counter to 0 in preparation for the next
image. In addition the colour tables are refreshed by the VBI for
each new level.
SCROLLING: The information
line is scrolled conventionally using register 54276 and the VBI.
After a game ends, the score/hiscore values are compared digit by
digit using offsets from the start of the screen memory/scrolling
memory. The latter resides in the unused area set aside for PMG
(lines 1006-1008). Hiscore is updated when required, by the mainline
code which also sets a flag (1680) to allow BASIC to perform its
little routine of message selection (lines 155,25,400-401). Scrolling
continues throughout a game - you don't see the message during play
because the routine's memory pointer is reset to a clear area of
RAM by a single BASIC poke.
MEMORY MANAGEMENT: Line
1000 sets aside wodges of RAM for Player Missiles Graphics, the
redefined character set, the vertical blank code and the mainline
code - all these reside in the upper reaches of RAM just beneath
RAMTOP and safe from the clutches of BASIC. The unused section of
PMG is utilised for the scrolling routine (including the latter's
unique VBI - lines 1026-1029), the new display list (lines 1010-1022)
and the DLI routine (1023-1025). Most of page 6 contains variables
and tables used by the machine code. BASIC has little work to perform
once initialisation is complete but its speed is optimised by compacting
its code and running the loop (lines 100-585) near the start of
the program.
SCREEN PRINTING: Since Blockbreaker
requires regular attention to the screen display, a machine code
PRINTing routine is incorporated into the mainline code. This sets
the cursor to the required location (see listing 3 for example),
organises IOCB #6 and then JSR's to CIOV itself. The 6502's Y register
temporarily holds the character to be printed (we could use the
stack via PHA) and this is transferred to the accumulator in the
PRINT routine as required by CIO. See Listing 4.
Listing 1
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