# EFI / The Boot Process

## What EFI is

**UEFI** (Unified Extensible Firmware Interface) is the software baked into your
machine's flash chip that runs the instant it powers on — the modern successor
to the legacy BIOS. Its job is to bring the hardware up to a sane state and then
find and launch an operating system. From our point of view it's a small runtime
that hands us a working CPU, a memory map, and a screen, and then gets out of the
way.

The key thing to understand: **UEFI is not our OS, it's a stepping stone.** It
exists to load *us*. Our `boot/efi.zig` is a UEFI *application* — a normal program
that the firmware runs — and its entire purpose is to gather what the kernel needs
and then jump into the kernel.

## How the firmware finds us

UEFI boots by looking for a FAT-formatted partition called the **EFI System
Partition (ESP)** and running a file at a well-known fallback path:

```
EFI/BOOT/BOOTX64.efi         <- the "removable media" default for x86-64
```

The boot volume is the **FHS-shaped `zig-out`** itself (see the repository-layout note
in [README.md](README.md)): `build.zig` installs `boot/efi.zig` (built for the `uefi`
target) to `zig-out/EFI/BOOT/BOOTX64.efi` — the one path UEFI firmware fixes — and lays
the rest out by FHS path: the kernel at `zig-out/system/kernel`, init at
`zig-out/system/services/init`, the initial-ramdisk at `zig-out/boot/`. The
`run-x86-64` step points QEMU at OVMF (UEFI firmware for virtual machines) and presents
`zig-out` to the guest as a FAT drive. The firmware finds `BOOTX64.efi` and runs it —
that's our `main()`, which then loads the kernel and init from their FHS paths.

## Boot services: the firmware's API

While a UEFI app runs, it has access to **boot services** — a table of function
pointers the firmware provides for allocating memory, reading files, locating
hardware protocols, and so on. In `boot()` this is the very first thing we grab:

```zig
const bs = uefi.system_table.boot_services orelse return error.NoBootServices;
```

Everything the firmware offers hangs off tables reachable from
`uefi.system_table`: `boot_services`, `con_out` (the text console we `log()` to),
and the various *protocols* (GOP for graphics, SimpleFileSystem for disk access).

**The critical rule:** boot services are *temporary*. They stop existing the
moment we call `ExitBootServices`. So the loader's structure is dictated by one
constraint — **gather everything the kernel could ever need first, then exit.**
The comment in `boot()` says exactly this:

> Everything the kernel needs must be gathered *before* we exit boot services,
> since afterwards none of these calls are usable.

## What our loader actually does

`boot()` runs four steps in order:

### 1. Query the framebuffer (`queryFramebuffer`)

We ask the firmware for the **Graphics Output Protocol (GOP)**, which describes
the linear framebuffer — its address, resolution, pitch, and pixel format —
and, when we can, switch the display to its native resolution first:

- Locate GOP via its *handle* (not `locateProtocol`), because the same handle
  also carries the display's **EDID**.
- Read the EDID (trying the `EDID_ACTIVE` then `EDID_DISCOVERED` protocol on each
  GOP handle) and parse the first Detailed Timing Descriptor — by convention the
  panel's preferred (native) resolution. This is best-effort: firmware installs
  these protocols inconsistently, and OVMF with QEMU's stdvga doesn't expose them
  at all.
- If we got a native resolution, enumerate the GOP modes with `queryMode` and
  `setMode` to the one that matches it (must be a linear 32bpp layout we can
  paint into). If EDID gave us nothing, we **keep the firmware's current default
  mode** rather than guess — with a valid EDID the firmware normally defaults to
  the native mode itself, so its choice beats second-guessing it with, say, the
  largest advertised mode.
- Copy the resulting address/resolution/pitch/format into our own `Framebuffer`.

All of this *must* happen now, because after exit there's no GOP to ask. (See
[framebuffer.md](framebuffer.md) for what those fields mean.)

### 2. Load the kernel (`loadKernel` + `loadElf`)

- Use the **LoadedImage** protocol to discover which device we booted from, then
  **SimpleFileSystem** to open that volume.
- Open the kernel ELF at its FHS path (`system\kernel`), seek to the end to learn its
  size, rewind, and read the whole ELF into a firmware-allocated pool buffer. (`read`
  may return short, so we loop.)
- Parse the ELF: validate the `\x7fELF` magic and the `x86_64` machine type, then
  walk the program headers. For every `PT_LOAD` segment we:
  - reserve the exact physical pages it's linked at (`p_paddr`) via
    `allocatePages`,
  - `@memcpy` the file-backed bytes to that address,
  - `@memset` the `.bss` tail (the part where `p_memsz > p_filesz`) to zero.

The kernel is linked to load at physical `0x100000` (1 MiB) — set by
`exe.image_base` in `build.zig` and the linker script. UEFI identity-maps memory,
so the physical address the ELF asks for is the address it actually runs at. If a
segment's `p_paddr` collided with firmware-reserved memory, `allocatePages` would
fail and we'd need to move `image_base`.

`loadElf` returns `e_entry`, the kernel's entry-point address.

### 3. Exit boot services (`exitBootServices`)

This is the handoff's trickiest step. To exit, the firmware demands the current
**memory map** and its *key* — proof that we've seen the latest state of memory.
But allocating the buffer to hold the memory map can itself *change* the map,
invalidating the key. So it's a retry loop:

```
get map info -> allocate buffer (+ spare descriptors) -> get map
             -> try exit with map.key
             -> if it failed, the map moved: free, retry
```

Once `exitBootServices` succeeds, **the firmware's services are gone for good** —
we must never touch `bs`, `con_out`, or any protocol again. The machine is now
entirely ours.

### 4. Jump to the kernel

```zig
const kernel: *const fn (*const BootInfo) callconv(boot_handoff.kernel_abi) noreturn =
    @ptrFromInt(entry);
kernel(&boot_info);
```

We cast the entry address to a function pointer and call it, passing a pointer to
the `BootInfo` we filled in. This never returns.

## The ABI subtlety: RCX vs RDI

There's a deliberate detail worth calling out. A UEFI binary is compiled with the
**Microsoft x64** calling convention (first argument in register **RCX**). Our
kernel is freestanding and uses the **SysV AMD64** convention (first argument in
**RDI**). If we let each side use its target's default, the loader would place
`boot_info` in RCX while the kernel looked for it in RDI — and the kernel would
read garbage.

So both sides pin the convention explicitly to SysV via the shared
`boot_handoff.kernel_abi` (defined in `system/boot-handoff.zig`). The loader's
function-pointer type and the kernel's `_start` both reference it, so the pointer lands
in the register the kernel expects. This is the whole reason `kernel_abi` lives in the
shared `boot-handoff` module: it's a contract both binaries must agree on. See
[sysv.md](sysv.md) for what "SysV" means and where else it shows up.

## The handoff contract

The loader and kernel are two *separate* binaries built for two different targets,
so everything they exchange must have an identically-defined memory layout. That's
what `system/boot-handoff.zig` provides — imported by both as the `boot-handoff` module.
It is *only* the handoff: the kernel↔user ABI (`system/abi.zig`) and the device types
(`system/devices/device-abi.zig`) are separate contracts the bootloader never sees.

- `BootInfo` — the top-level struct passed to the kernel (currently just the
  framebuffer; this is where future handoff data like the memory map will go).
- `Framebuffer`, `PixelFormat`, `kernel_abi` — the shared field layouts and the
  calling convention.

Both are `extern struct`, giving them a stable, C-compatible layout so the bytes
the loader writes are the bytes the kernel reads.

## The whole flow at a glance

```
power on
  -> UEFI firmware initialises hardware
  -> finds EFI/BOOT/BOOTX64.efi on the FHS volume, runs it   (our efi.zig main)
       -> grab boot services
       -> queryFramebuffer   (via GOP: EDID native res, setMode, describe fb)
       -> loadKernel         (read system/kernel ELF, load PT_LOAD segments to 0x100000)
       -> exitBootServices   (retry until the memory-map key holds)
       -> jump to e_entry, boot_info pointer in RDI
  -> kernel _start           (system/kernel/kernel.zig: framebuffer console, then halt)
```

Bottom line: **UEFI's job is to give us a CPU, memory, and a framebuffer, then
disappear.** `boot/efi.zig` is the thin bridge that collects those gifts into a
`BootInfo`, tears down the firmware, and jumps into the kernel — after which we're
on our own.
