VIC Graphics Test

Source file

VIC Graphics Test

Comprehensive test program for the VIC bitmap mode (320x200 pixels).

Quick Start

# Program is already in data/disk/basic.prg
./sbc6502

In the emulator:

BASIC
LOAD
RUN

What the Program Tests

The program runs 6 different graphics tests:

1. Horizontal Lines — Draws horizontal lines spaced 20 pixels apart
2. Vertical Lines — Draws vertical lines spaced 32 pixels apart
3. Checkerboard Pattern — Draws a checkerboard pattern
4. Rectangle — Draws a rectangle from (50,40) to (270,160)
5. Diagonal Line — Draws a diagonal from top-left to bottom-right
6. Full Screen — Fills the entire screen with white pixels

Each test is displayed for about 2 seconds, then the screen is cleared and the next test starts.

Technical Details

Memory Map:

• VIC Control Register: $9000 (36864)
  • Value 0: Text mode (40x25)
  • Value 1: Bitmap mode (320x200)
• Bitmap RAM: $9010-$AF4F (36880-44879, 8000 bytes)

Pixel Formula:

X = 0-319  (horizontal)
Y = 0-199  (vertical)
BYTE_OFFSET = Y * 40 + INT(X / 8)
BIT_POSITION = 7 - (X AND 7)
ADDRESS = 36880 + BYTE_OFFSET

Set pixel:

POKE ADDRESS, PEEK(ADDRESS) OR (2^BIT_POSITION)

Clear pixel:

POKE ADDRESS, PEEK(ADDRESS) AND (255 - 2^BIT_POSITION)

Creating Your Own Programs

From a text file:

# Create program in examples/mygfx.txt
nano examples/mygfx.txt

# Convert
python3 tools/basic_convert.py examples/mygfx.txt data/disk/basic.prg

# Load in emulator
./sbc6502
BASIC
LOAD
RUN

Minimal example:

10 REM Switch to graphics
20 POKE 36864,1
30 REM Draw pixel at (100,50)
40 A=36880+(50*40+INT(100/8))
50 POKE A,PEEK(A) OR 2^(7-(100 AND 7))
60 REM Wait for key
70 GET K$: IF K$="" THEN 70
80 REM Back to text mode
90 POKE 36864,0
100 END

More Examples

• examples/hello.txt — Simple Hello World
• examples/graphics.txt — Interactive graphics demo
• examples/gfxtest.txt — This automated test (154 lines)

Performance Notes

• Drawing many pixels is slow (BASIC is interpreted)
• Delay loops (FOR W=1 TO 2000: NEXT W) are approximate, not exact
• For faster graphics: use assembly routines via SYS