22.1 RetroU pääsykoe Commodore 64 ympyrä

In the Commodore 64's version of BASIC (2.0), there's no built-in command to directly draw a circle. But we can create a circle using a simple mathematical concept.

Here's a small program that will draw a circle on the screen using PRINT statements:

BASIC

10 PRINT CHR$(147)

20 FOR T = 0 TO 6.28 STEP 0.1

30 X = 20 + 10 * COS(T)

40 Y = 11 + 10 * SIN(T)

50 POKE (1024 + INT(Y) * 40 + INT(X)), 81

60 NEXT T

70 GET A$: IF A$="" THEN 70

80 PRINT CHR$(147)

Here's how it works:

    • Line 10 clears the screen.

    • Lines 20 to 60 draw the circle:

        ◦ We're looping over the angles of the circle (from 0 to 2π or roughly 6.28).

        ◦ We're using the trigonometric functions COS and SIN to get the X and Y coordinates of a point on the circle.

        ◦ We're adding offsets to the X and Y coordinates to position the circle in the middle of the screen.

        ◦ The POKE statement puts a character at the calculated screen position.

    • Line 70 waits for a key press.

    • Line 80 clears the screen again.

Note that the circle might not look perfect because of the aspect ratio of the C64's text mode, and we're using a text character (81) to draw the circle. In high-resolution graphics mode, you would be able to draw a more perfect circle, but that requires a much more complex program.