You are represented in this game by a player on the right-hand side
of the screen and the object is to obtain the best score possible by
carefully aiming and firing each of your ten arrows at the target on
the left. If you miss the board altogether you are given the
opportunity to retrieve your arrow.
A hit in the outer ring records 200 points and the red ring scores
300. A bulls-eye is worth 500 but it is protected by a low-scoring
green area worth only 100 points. Sounds easy, doesn't it? Oh, I
forgot to mention that you are in constant motion and to make things
even harder there are random speed changes at the extremes of your
travel.
Press SELECT to choose one of the ten levels and START to begin to
play. Press OPTION to review the highest score between games.
HOW IT WORKS
The main problem I encountered in writing this program was how to
move Player Missiles up and down the screen at a reasonable speed in
Basic. I found that it just could not be done and I have therefore
used a machine language subroutine from an American magazine which
moves Players during the Vertical Blank Interrupt. The code is poked
into page six and is called only once with X=USR(1696). The subroutine
then executes automatically every 50th of a second whilst the screen
is off prior to being redrawn. Locations 1780-1783 become registers
which hold the Y positions of each Player. If for example a 5 is POKEd
into location 1780 then Player 1 will be moved to position X,5
instantly. Four players are supported but as you will see from the
listing I have used only two (PLY and PLY+l). Horizontal movement is
covered by using the normal X registers at 53248 and 53249 (PLX and
PLX+I).
The program is well REMarked so you should have no trouble in
following the code. Lines 1921 create the custom screen by altering
the display list. The main loop of the program is between lines 160
and 195. Note the use of the POP command which should be used whenever
you jump out of a FOR ... NEXT loop before the loop completes.
When writing programs I tend to use long variable names to make the
listing easier to read. A couple of extra bytes are used when defining
the variable but thereafter only one byte is used for any variable no
matter what the length of the name.
I hope that you enjoy Target and that you learn a little from the
listing. I have sent in my program, how about sending in one of yours?
I like to type them in as well.
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