The Circle

ZX Spectrum Circle Drawing Fun

Table of Contents

1. Program Description
2. Technical Specifications
3. Build Instructions
4. Execution
5. Customization
6. Algorithm Details
7. Optimizations
8. Memory Usage
9. Compatibility
10. Known Issues
11. License

Program Description

This Z80 assembly program implements Bresenham's circle algorithm to draw perfect circles on the ZX Spectrum 48K. The program:

• Clears the screen
• Draws a circle with configurable position and radius
• Halts the CPU when finished

Technical Specifications

• Target Platform: ZX Spectrum 48K (works on all models)
• Memory Requirements:
  • 32768-49151 (0x8000-0xBFFF) - Program location
  • 16384-22527 (0x4000-0x57FF) - Screen memory
• Output Format: .SNA snapshot file
• Assembler: sjasmplus (modern Z80 assembler)
• Dependencies: None

Build Instructions

Prerequisites

1. Install sjasmplus (v1.18.3 or newer recommended)
   • Windows: Download pre-built binary
   • Linux: sudo apt install sjasmplus or build from source
   • macOS: brew install sjasmplus

Compilation

sjasmplus --lst=circle.lst --raw=circle.bin --sym=circle.sym circle.asm 
sjasmplus --lst=circle-optimized.lst --raw=circle-optimized.bin --sym=circle-optimized.sym circle-optimized.asm 

These generates:

• circle.sna - Executable snapshot
• circle.lst - Assembly listing with addresses
• circle.sym - Symbol table
• circle.bin - Raw binary
• circle-optimized.sna - Executable snapshot
• circle-optimized.lst
• circle-optimized.sym
• circle-optimized.bin - Raw binary

Execution

Load circle.sna(or circle-optimized.sna) in your preferred emulator: Fuse (multi-platform) ZX Spin (Windows) ZEsarUX (Linux/macOS)

The program will:

• Immediately clear the screen
• Draw a circle with default parameters
• Halt when complete (press reset to rerun)

Customization

Modify these values in the source code:

Start:
    LD D, 128   ; Center X (0-255)
    LD E, 96    ; Center Y (0-191)
    LD B, 40    ; Radius (1-96 recommended)
For different colors, add after CALL CLEAR_SCREEN:

    LD A,color_code  ; 0-15 (INK+PAPER*8)
    LD (23693),A     ; Set border
    CALL 8859         ; Set colors

Algorithm Details

Bresenham's Circle Algorithm Implementation Uses only integer arithmetic Implements the midpoint circle algorithm Draws all 8 symmetrical octants simultaneously Decision parameter updates use bit shifts for efficiency

Optimizations

Key Optimizations: (circle-optimized.asm)

• IX register usage for storing center coordinates
• Bitmask table for fast bitwise operations
• Register-based calculations minimizing memory access
• Dual addressing modes for faster pixel plotting
• Cycle-optimized code in the Bresenham algorithm

Pixel Plotting

Uses pre-calculated screen address table (SCREEN_Y_LOOKUP) Optimized bit positioning with RRCA rotation Automatic clipping of Y coordinates >191 Screen memory layout compliant with ZX Spectrum standard

Memory Usage

Address Range Usage 0x4000-0x57FF Screen memory 0x8000-0x8020 Main program 0x8020-0x80FF Circle algorithm 0x8100-0x81FF Screen address table 0x8200-0x83FF PlotPixel routine Compatibility

Tested on:

• Original ZX Spectrum 48K hardware
• ZX Spectrum+
• ZX Spectrum 128K (48K mode)
• All major emulators

Known Issues

Maximum reliable radius is 96 pixels (due to 8-bit math) No bounds checking for X coordinate Simple HALT at end (no return to BASIC) Color attributes not set (uses existing screen colors)

License

This work is released into the Public Domain.