danos/docs/framebuffer.md

The Framebuffer

What a framebuffer is

A framebuffer is just a big region of memory where each element is one pixel's color. The display hardware continuously scans this memory and turns each value into light on the screen. There's no drawing API involved — you write a 32-bit value to the right address, and a pixel changes color. That's exactly what Console.pixel does:

self.rowPtr(y)[x] = color;   // system/kernel/console.zig

Our Framebuffer struct (system/danos.zig) is the four facts you need to address it:

Field	Meaning
base	the memory address where pixel data starts
width	visible pixels per row (e.g. 1920)
height	visible rows (e.g. 1080)
pitch	bytes from the start of one row to the start of the next

The bootloader (UEFI GOP, in our case) sets all this up and hands it over. The kernel just writes into it: no firmware, no driver — just pixels.

The mental model: it's 1D memory pretending to be 2D

The screen is a grid, but memory is a flat line of bytes. So the pixels are stored row after row, laid end to end:

row 0: [px0][px1][px2]...[width-1]  <padding?>
row 1: [px0][px1][px2]...[width-1]  <padding?>
row 2: ...

To find pixel (x, y) you compute:

address = base + y * (bytes per row) + x * (bytes per pixel)

So what is pitch?

Pitch is "bytes per row" — sometimes called stride. The obvious guess would be pitch = width * 4 (4 bytes = 32 bits per pixel). And often it is. But not always — and that's the whole reason the field exists.

Hardware frequently wants each row to start at a nicely aligned address (a multiple of 32, 64, or a page). If width doesn't land on that boundary, the firmware pads the end of every row with a few extra unused bytes. That padding is invisible — it's never shown — but it's physically there in memory between the last pixel of one row and the first pixel of the next.

Example: a 1366-pixel-wide display at 32bpp:

• width * 4 = 1366 × 4 = 5464 bytes of actual pixels
• but pitch might be 5504 bytes (padded up to a multiple of 64)
• those extra 40 bytes per row are dead space

This is exactly why rowPtr uses pitch, not width, to step between rows:

inline fn rowPtr(self: *Console, y: u32) [*]volatile u32 {
    const base: [*]volatile u8 = @ptrFromInt(self.fb.base);
    return @ptrCast(@alignCast(base + y * self.fb.pitch));   // <- pitch, not width*4
}

Note the deliberate detail: base is cast to a byte pointer ([*]volatile u8) before adding y * pitch, because pitch is measured in bytes. Then it's cast to a u32 pointer so that [x] indexes whole pixels. If you'd done the arithmetic on a u32 pointer, + pitch would step pitch pixels (4× too far).

Why you must use pitch, not width * 4

If you assumed rows were width * 4 apart on a display where pitch > width * 4, every row would start a little too early. The error accumulates: row 0 is fine, row 1 is off by (pitch − width×4) bytes, row 2 by twice that, and so on. The image ends up skewed diagonally — a slanted, sheared picture — because each row creeps sideways relative to where the hardware actually reads it.

Using pitch is what keeps each row landing exactly where the scanout expects it.

Two subtleties worth noting

1. width vs pitch in the loops. In fillRow/copyRow we iterate x up to self.fb.width — the visible count — but jump between rows with pitch. That's the correct pairing: touch only real pixels, but skip the full stride (including padding) to reach the next row. We never write into the padding, which is right.

2. volatile. The pointer is volatile because this memory is special — it's watched by the display hardware. volatile tells the compiler "don't optimize these writes away or reorder/coalesce them"; every store must actually hit memory, because something outside the CPU's knowledge (the scanout engine) is reading it.

Bottom line: width is how wide the picture is; pitch is how wide the memory rows are. They're usually equal (×4) but not guaranteed to be, so always advance rows by pitch.