109 lines
4.3 KiB
Markdown
109 lines
4.3 KiB
Markdown
# 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:
|
||
|
||
```zig
|
||
self.rowPtr(y)[x] = color; // system/kernel/console.zig
|
||
```
|
||
|
||
Our `Framebuffer` struct (`system/boot-handoff.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:
|
||
|
||
```zig
|
||
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` 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. (A `copyRow` used to sit alongside it; it's
|
||
gone — `scroll` was since rewritten as a writes-only screen clear, because
|
||
reading VRAM back is uncached-slow on real hardware.)
|
||
|
||
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.
|