Graphics Output Protocol

Overview

The Graphics Output Protocol (GOP) is the UEFI interface that provides framebuffer-based graphics access to applications and boot loaders.

It replaces many legacy graphics interfaces and offers a standardized way to query display modes, switch resolutions and access video memory directly.

Why It Matters

GOP allows software to display graphics before an operating system is running.

Typical use cases include:

Boot menus
Splash screens
Diagnostic tools
Graphical installers
Kernel boot graphics

In this project, GOP is used to render pixels, lines and rectangles directly into the framebuffer.

Accessing GOP

Applications locate the protocol through Boot Services:

LocateProtocol()
        |
        v
EFI_GRAPHICS_OUTPUT_PROTOCOL

The protocol is identified by a GUID and returned as an interface pointer.

Main Operations

The most important GOP functions are:

Function	Purpose
`QueryMode()`	Retrieve information about a graphics mode
`SetMode()`	Switch to a different graphics mode
`Blt()`	Optional block transfer operations

This project primarily uses QueryMode(), SetMode() and direct framebuffer access.

Querying Modes

QueryMode() returns information such as:

Horizontal resolution
Vertical resolution
Pixel format
Pixels per scan line

Example:

Status = gop->QueryMode(
    gop,
    ModeNumber,
    &InfoSize,
    &Info
);

An Important Ownership Detail

A key finding during development was that QueryMode() allocates the Info buffer internally.

This means:

The callee allocates the memory
The caller must free it using FreePool()

Example:

Status = gop->QueryMode(..., &InfoSize, &Info);

if (!EFI_ERROR(Status)) {
    /* use Info */
    FreePool(Info);
}

This differs from APIs such as GetMemoryMap(), where the caller allocates the buffer.

Setting a Graphics Mode

After selecting a mode, the application activates it:

Status = gop->SetMode(gop, ModeNumber);

The framebuffer configuration in gop->Mode is then updated to reflect the active mode.

The Framebuffer

GOP exposes a linear framebuffer through:

FrameBufferBase
FrameBufferSize

A pixel is written by calculating its memory address:

PixelAddress =
    FrameBufferBase +
    (Y * PixelsPerScanLine + X) * BytesPerPixel

The Graphics Demo uses this mechanism to implement PutPixel() and higher-level drawing primitives.

Console vs Framebuffer

UEFI text output and GOP graphics are separate subsystems.

Text printed with:

Print(L"Hello");

uses the console output protocol.

Framebuffer rendering writes directly to video memory.

This is why the Graphics Demo can display graphical content while still printing status messages to the console.

Relationship to the Project

The Graphics Demo application demonstrates practical GOP usage:

Mode enumeration
Mode switching
Framebuffer access
Pixel plotting
Line drawing
Rectangle rendering

Example output:

Graphics Output Demo

Graphics Demo

Memory Ownership