BREAKVIC is a videogame written in Commodore VIC-20 BASIC. BREAKVIC was written for a competition using only 20 lines of code.
The project goal is to insert in only 20 lines full gameplay, sounds, and graphics. For the graphics, only PETSCII characters are used. The gameplay consists of breaking all the blocks on the screen. The game ends with a final score when the player destroys all the blocks. The score is computed as:
- 300 points at the begin of the game
- -100 points when the ball hits the lower border of the screen outside the player paddle
- +100 points when the ball breaks a block or hits the player paddle
The player can control the paddle by using the joystick.
- In order to compress the code, the keywords print and poke are replaced by their basic abbreviations (? and pO)
- The code is written through the "CBM .prg Studio". The code contains some particular control sequences, for example {clear}, {white}, ... When you load the prg file on both the emulator or the real hardware, you can list the program and see the original control characters used by the VIC-20.
- The file breakvic_joy.bas is the source file for the competition. The breakvic.prg and breakvic_joy.bas are generated by this source file.
- The color of the ball changes when the ball is on a screen location previously occupied by a block
- The x,y coordinates of the ball are always incremented/decremented by 1, only when the ball hits the player paddle the x coordinate is incremented/decremented by 2
Line 10 initializes the sound volume (poke 36878,15) and the screen color (poke 36879,12), then it clears the screen and sets the character color to white. Some variables are initialized:
- x, y: the ball coordinates
- dx,dy: the values used to increment/decrement x and y
- bx: the x coordinate of the player paddle
- c: the position in the screen memory where the first block is inserted
- p: as c but it points to the color memory
- v: the ball velocity
10 pO36878,15:pO36879,12:?"{clear}{white}":x=10:y=21:dx=-1:dy=-1:bx=10:c=7727:p=38447:v=1
Line 20 initializes some variables: t (the player score) and m (the number of blocks). The other instructions on the line draw the blocks on the screen: 15 blocks for each line plus an empty line. In the cycle, both the screen memory (poke c+i,102) and the color memory (poke p+i,3) are written.
20 t=300:m=75:fori=1to15:pOc+i,102:pOp+i,3:nexti:c=c+44:p=p+44:ifc<7944goto20
Line 30 draws the player paddle (character 160) on the last row of the screen, the length of the paddle is two characters. Moreover, the space (code 32) is written on the ball coordinates (poke7680+y*22+x,32) for deleting the previous ball on the screen.
30 poke8164+bx,160:poke8164+bx+1,160:poke7680+y*22+x,32
Line 40 and 50 report standard basic code for reading the joystick. If left is detected and bx>0 then the paddle is moved on the left by goto line 110. Line 50 detects the right and if bx<20 it moves the paddle on the right by goto line 120
40 j=peek(37151):if(jand16)=0andbx>0thengoto110
50 poke37154,127:j=peek(37152):poke37154,255:if(jand128)=0andbx<20thengoto120
Line 60 stops the first voice of the VIC and increases the x,y coordinates. The x coordinate is multiplied by the velocity. The instruction if checks if the ball hits the left/right screen border or the ball hits a block (peek(7680+y*22+x)=102).
60 poke36874,0:x=x+dx*v:y=y+dy:ifx<=0orx>=21orpeek(7680+y*22+x)=102thendx=-dx
Line 80 stops the second voice of the VIC and checks the y coordinate for screen border and blocks detection.
poke36875,0:ify=0ory=21orpeek(7680+y*22+x)=102thendy=-dy
Line 90 and 91 check the x coordinate. This control is necessary because when the x coordinate is incremented/decremented with a velocity greater than 1, the coordinate can overcome the left/right screen border.
90 ifx<0thenx=0
91 ifx>21thenx=21
Line 92 checks if the ball hits the player paddle. If the ball hits the paddle gosub to line 140.
92 ify=21andx>bx-2andx<bx+3thengosub140
Line 93 checks if the ball hits a block. If the ball hits a block gosub to line 130.
93 ifpeek(7680+y*22+x)=102thengosub130
If the ball hits the lower border of the screen but not the player paddle then it decreases the score.
94 ify=21andnot(peek(7680+(y+1)*22+x))=160thent=t-100
If the number of blocks is equal to 0 goto to line 150 (the end of the game).
97 ifm=0thengosub150
Line 100 draws the ball and goto to the start of the game cycle (line 30).
100 poke7680+y*22+x,81:goto30
Lines 110 and 120 are executed when the player uses the joystick (see lines 40 and 50) for moving the player paddle.
110 poke8164+bx,32:poke8164+bx+1,32:bx=bx-1:goto60
120 poke8164+bx,32:poke8164+bx+1,32:bx=bx+1:goto60
Line 130 is executed when the ball hits a block (see line 92). The line plays the voice 1 of the VIC, modifies the ball velocity, increments the score and decrements the number of blocks.
130 poke36874,135:v=1:t=t+100:m=m-1:return
Line 140 is executed when the ball hits the player paddle (see line 93). The line plays the voice 2 of the VIC and modifies the ball velocity.
140 poke36875,135:v=2:return
This line is executed when the game ends (see line 97). The line sets the volume to 0 and prints the player final score.
150 poke36878,0:?"{clear}{white}{down*9}{right*5}game over{down*2}{left*9}score: ";t:end