C64 Hi-Res Circle Drawing Demo

This project is a Commodore 64 demo that uses the hi-res bitmap mode and implements the Midpoint/Bresenham circle algorithm (integer-only, no floating point) to draw circles on the bitmap screen. The code takes care of VIC-II initialization, screen/bitmap memory setup, proper color handling (hi-res mode: 1 bit/pixel, one foreground + one background color per cell), and includes a fast screen clear routine.

Features

• Hi-res bitmap mode: bitmap at $6000, screen matrix (color nibbles) at $5C00
• Autostart: BASIC stub (10 SYS 2061) at .pc = $0801
• Midpoint circle algorithm: 8-way symmetry, integer arithmetic only
• Color handling: foreground color written to the upper nibble of the - screen matrix; background comes from $D021
• Fast clear: full bitmap clear via self-modifying loop
• Macro API: simple DrawCircle(px, py, pr, pc) macro to draw circles

Memory and VIC Setup

• VIC bank: $4000–$7FFF
• Set by $DD00 low 2 bits → 10 (bank 1)
• Bitmap: $6000 (configured via $D018 bits)
• Screen matrix: $5C00 (40×25 bytes, nibble pairs: upper = foreground, - lower = background)
• Background/border: $D021 / $D020
• Mode: bitmap enabled via $D011 (bit 5 = 1), bank select via $DD00/$DD02

Main Routines

• Initialization
  • VIC bank/mode setup
  • Screen matrix fill: fg = light green ($D), bg = current $D021 lower nibble
  • Clear bitmap

Demo

• Example circles:

  DrawCircle(110,  70, 35, $02)   ; red
  DrawCircle(210, 130, 45, $04)   ; light green

• Circle drawing (circle_midpoint)

  • d = 3 - 2r decision variable

  • Always increment x, conditionally decrement y

  • Plot with 8-way symmetry: plot_octants_8

• Pixel plotting (plot)

  • Converts (X,Y) → bitmap byte + bitmask (bitable)
  • X ≥ 256 fix: add +32 bytes to bitmap row, +32 to color RAM offset
  • Foreground color written to upper nibble of screen matrix

• Clear screen (clear_screen)

  • Self-modifying loop, 32×250 byte chunks

Building with Kick Assembler

Compile with Kick Assembler:

java -jar KickAss.jar circles.asm -o circles.prg

Run in VICE emulator:

x64sc circles.prg

The program will autostart thanks to the BASIC stub.

Known Limitations

• Only 1 foreground color per 8×8 block (hi-res restriction)
• No clipping → drawing outside screen may cause artifacts
• Very large radii may result in extra drawing time