VESA BIOS extensions

VBE is a compatibility interface for discovering and setting graphics modes. lcom-ng exposes a VBE-shaped API so you can practice the same flow without BIOS calls from a VM.

A typical setup asks for mode information, chooses a supported mode, requests and maps a linear framebuffer, draws, and presents. The order matters. Drawing before mode selection or mapping is not meaningful.

Mode information

Mode information gives horizontal resolution, vertical resolution, bits per pixel, pitch, and framebuffer address. Treat it as the source of truth for drawing helpers. A rectangle function should not assume a hardcoded width because the first test uses a familiar mode.

The linear framebuffer flag exposes the frame as a flat memory region rather than an older banked model, keeping the focus on address calculation and clipping.

IMPORTANT

Do not hardcode mode dimensions in drawing helpers. Use the mode information returned by the runtime.

Indexed colors

Mode 0x105 uses 8-bit color indices. The runtime starts with the standard VGA BIOS 256-color palette: 16 EGA colors at indices 0 through 15, 16 grays at 16 through 31, nine 24-color hue rings at 32 through 247, and black at 248 through 255.

lcom_vbe_dac_format selects 6-bit or 8-bit DAC components, and each mode set resets the width to 6 bits. The DAC index and data ports at 0x3C8 and 0x3C9 are available through lcom_port_write8, so a game can program its own palette. For full RGB without palette work, choose a direct-color mode.