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Supported Video Modes

This document describes the video modes supported by the emulator, along with their restrictions.

Text Modes

  • TEXTMODE_40x25_COLOR: 40x25 text mode with 16 colors.
  • TEXTMODE_40x25_BW: 40x25 text mode with 2 colors (black and white).
  • TEXTMODE_80x25_COLOR: 80x25 text mode with 16 colors.
  • TEXTMODE_80x25_BW: 80x25 text mode with 2 colors (black and white).

Restrictions:

  • The font is fixed to an 8x16 font for 80-column modes and an 8x8 font for 40-column modes.
  • Blinking text is supported, but the blink rate is fixed.
  • The cursor is supported, but its blink rate is fixed.

Graphics Modes

  • CGA_320x200x4: 320x200 with 4 colors. The palette is one of the standard CGA palettes.
  • CGA_320x200x4_BW: 320x200 with 2 colors (black and white).
  • CGA_640x200x2: 640x200 with 2 colors (black and a configurable foreground color).
  • HERC_640x480x2: 640x480 with 2 colors (black and white). This is a Hercules-compatible mode.
  • HERC_640x480x2_90: 640x348 with 2 colors (black and white). This is a Hercules-compatible mode with a 90-column text mode layout.
  • TGA_160x200x16: 160x200 with 16 colors. This is a Tandy Graphics Adapter (TGA) mode.
  • TGA_320x200x16: 320x200 with 16 colors. This is a TGA mode.
  • TGA_640x200x16: 640x200 with 16 colors. This is a TGA mode.
  • VGA_320x200x256: 320x200 with 256 colors. This is a standard VGA mode (Mode 13h).
  • VGA_320x200x256x4: 320x200 with 256 colors, using a planar memory layout. This is a VGA "Mode X" style mode.
  • VGA_640x480x2: 640x480 with 2 colors. This is a standard VGA mode.
  • EGA_320x200x16x4: 320x200 with 16 colors, using a planar memory layout. This is an EGA-compatible mode.
  • COMPOSITE_160x200x16: 160x200 with 16 colors, emulating the color composite output of a CGA card.
  • COMPOSITE_160x200x16_force: 160x200 with 16 colors, forcing the composite color mode even if the BIOS does not request it.

Implementation Details & Restrictions

The video signal generation is handled by dma_handler_VGA and has the following characteristics and limitations:

  • Hardware Dependency: The implementation is specific to the Raspberry Pi Pico and its PIO/DMA capabilities. It is not thread-safe and assumes a specific system clock frequency.
  • Resolution Limits:
    • Maximum visible resolution is 640x480. Video modes with a higher vertical resolution will be cropped.
    • Maximum horizontal resolution is 640 pixels. Video modes with a higher horizontal resolution will be cropped.
  • Refresh Rate: Fixed at 60Hz.
  • Memory Layout: Each video mode has a fixed, non-configurable video memory layout.
  • Palette: The color palette is fixed for most modes. Only TGA and VGA modes allow for a programmable palette.
  • Special Modes:
    • HERC_640x480x2_90: Uses a non-standard 90-column text mode memory layout.
    • Composite modes: An approximation of real CGA hardware; colors may not be an exact match.
    • VGA "Mode X": This is a custom mode, not a standard VGA mode, that uses a planar memory layout.
  • Debug Console: A debug console may be displayed at the bottom of the screen, which is not part of the emulated video output.

NTSC TV Output

The NTSC TV output mode generates a composite video signal.

  • Resolution: 320x200 pixels.
  • Color: 256-color programmable palette.
  • Signal: Composite video signal generated via PWM on GPIO 22.
  • Text Mode: 40x25 text mode is supported.

Implementation Details & Restrictions

  • The NTSC signal generation is highly dependent on the RP2040's PIO, DMA, and a specific system clock frequency (315MHz).
  • The refresh rate is ~60Hz (NTSC standard).