Daniel Samson
e854f65623
Every doc verified claim-by-claim against the code by parallel audit agents, then fixed and adversarially re-verified. Two waves of staleness corrected: the originally audited findings (higher-half boot handoff, kernel VFS takeover, fault isolation + claim release + driver restart, AML/S5 moving to ring 3, threading's shipped design, USB+FAT landing) and a second pass of adjacent claims the verifiers caught (smp.md 'not built yet' intro, system-requirements' PS/2-only and no-storage claims, halting.md's red-panic and no-IDT text, testing.md's serial mirroring, router-era vfs-protocol wording, capsule-first boot loading). threading.md now documents the shared-fate gap explicitly: the design says a process dies whole, the kernel today kills only the offending thread. Also fixes three stale code comments (isr.s exceptionHandler, acpi.zig sleepValue, build.zig boot-volume) — comments only, no behavior change.
109 lines
4.3 KiB
Markdown
109 lines
4.3 KiB
Markdown
# The Framebuffer
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## What a framebuffer is
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A **framebuffer** is just a big region of memory where each element is one
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pixel's color. The display hardware continuously scans this memory and turns
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each value into light on the screen. There's no drawing API involved — you
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write a 32-bit value to the right address, and a pixel changes color. That's
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exactly what `Console.pixel` does:
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```zig
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self.rowPtr(y)[x] = color; // system/kernel/console.zig
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```
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Our `Framebuffer` struct (`system/boot-handoff.zig`) is the four facts you need to
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address it:
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| Field | Meaning |
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|----------|---------|
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| `base` | the memory address where pixel data starts |
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| `width` | visible pixels per row (e.g. 1920) |
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| `height` | visible rows (e.g. 1080) |
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| `pitch` | **bytes** from the start of one row to the start of the next |
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The bootloader (UEFI GOP, in our case) sets all this up and hands it over. The
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kernel just writes into it: no firmware, no driver — just pixels.
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## The mental model: it's 1D memory pretending to be 2D
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The screen is a grid, but memory is a flat line of bytes. So the pixels are
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stored row after row, laid end to end:
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```
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row 0: [px0][px1][px2]...[width-1] <padding?>
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row 1: [px0][px1][px2]...[width-1] <padding?>
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row 2: ...
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```
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To find pixel `(x, y)` you compute:
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```
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address = base + y * (bytes per row) + x * (bytes per pixel)
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```
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## So what is pitch?
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**Pitch is "bytes per row"** — sometimes called *stride*. The obvious guess
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would be `pitch = width * 4` (4 bytes = 32 bits per pixel). And often it is.
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**But not always** — and that's the whole reason the field exists.
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Hardware frequently wants each row to start at a nicely aligned address (a
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multiple of 32, 64, or a page). If `width` doesn't land on that boundary, the
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firmware pads the end of every row with a few extra unused bytes. That padding
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is invisible — it's never shown — but it's physically there in memory between
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the last pixel of one row and the first pixel of the next.
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Example: a 1366-pixel-wide display at 32bpp:
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- `width * 4` = 1366 × 4 = **5464 bytes** of actual pixels
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- but `pitch` might be **5504 bytes** (padded up to a multiple of 64)
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- those extra 40 bytes per row are dead space
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This is exactly why `rowPtr` uses `pitch`, not `width`, to step between rows:
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```zig
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inline fn rowPtr(self: *Console, y: u32) [*]volatile u32 {
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const base: [*]volatile u8 = @ptrFromInt(self.fb.base);
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return @ptrCast(@alignCast(base + y * self.fb.pitch)); // <- pitch, not width*4
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}
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```
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Note the deliberate detail: `base` is cast to a **byte** pointer
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(`[*]volatile u8`) *before* adding `y * pitch`, because pitch is measured in
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bytes. Then it's cast to a `u32` pointer so that `[x]` indexes whole pixels. If
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you'd done the arithmetic on a `u32` pointer, `+ pitch` would step `pitch`
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*pixels* (4× too far).
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### Why you must use pitch, not `width * 4`
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If you assumed rows were `width * 4` apart on a display where
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`pitch > width * 4`, every row would start a little too early. The error
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accumulates: row 0 is fine, row 1 is off by (pitch − width×4) bytes, row 2 by
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twice that, and so on. The image ends up **skewed diagonally** — a slanted,
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sheared picture — because each row creeps sideways relative to where the
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hardware actually reads it.
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Using `pitch` is what keeps each row landing exactly where the scanout expects
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it.
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## Two subtleties worth noting
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1. **`width` vs `pitch` in the loops.** In `fillRow` we iterate `x` up to
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`self.fb.width` — the *visible* count — but jump between rows with
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`pitch`. That's the correct pairing: touch only real pixels, but skip the
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full stride (including padding) to reach the next row. We never write into
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the padding, which is right. (A `copyRow` used to sit alongside it; it's
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gone — `scroll` was since rewritten as a writes-only screen clear, because
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reading VRAM back is uncached-slow on real hardware.)
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2. **`volatile`.** The pointer is `volatile` because this memory is special —
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it's watched by the display hardware. `volatile` tells the compiler *"don't
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optimize these writes away or reorder/coalesce them"*; every store must
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actually hit memory, because something outside the CPU's knowledge (the
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scanout engine) is reading it.
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Bottom line: **width is how wide the picture is; pitch is how wide the memory
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rows are.** They're usually equal (×4) but not guaranteed to be, so always
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advance rows by pitch.
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