---
name: schunix-pci-drivers
description: PCI enumeration lives in kernel/dev/pci.c; drivers that find their own hardware go in kernel/drivers/ and are listed in registry.c
metadata: 
  node_type: memory
  type: project
  originSessionId: a665e14b-86bd-4b41-a228-51377f3e9225
  modified: 2026-08-04T17:28:31.246Z
---

**`kernel/dev/pci.c` enumerates the PCI bus** (added 2026-08-04), through the
old 0xCF8/0xCFC config mechanism — the one that works without first finding an
ACPI table. The scan recurses through bridges rather than sweeping all 256
buses flat, so it also knows what sits behind what.

BAR sizes are found by writing all ones and reading back the bits the device
does not decode; **the original value must be restored**, since that briefly
points a live device's decoder at the top of the address space. 64-bit BARs
consume the following slot. Only header type 0 has six BARs — a bridge is
type 1 with a completely different layout.

**`kernel/drivers/` is the folder for drivers that have to *find* their
hardware.** Fixed-address legacy devices (console, keyboard, the IDE ports)
stay in `kernel/dev/`. To add one: write `kernel/drivers/<name>.c` exporting a
`struct pci_driver`, declare it in `kernel/drivers/registry.c`, add it to the
array there, and add the object to `KERN_DRIVERS` in the Makefile.

The registry is a plain array rather than the linker-section trick, on
purpose: it keeps the list of what is in the kernel somewhere you can read.
A driver silently dropped by the linker looks identical to one never written.

`kernel/drivers/vga_pci.c` is the first driver and the template. It draws
nothing — vga.c still owns the framebuffer — but it cross-checks the aperture
it finds on the bus against the address VBE reported. That disagreement is
exactly the symptom that would have surfaced first on hardware where the old
hardcoded `0xfd000000` was wrong. See [[schunix-framebuffer-device]].

**`cat /proc/pci`** lists everything found, lspci-shaped, with the binding
driver in brackets (`[-]` when unclaimed). Under QEMU's default i440fx it
should show six devices; `tools/bootcheck.ps1` asserts on the host bridge and
the IDE controller by ID, and on `[vga]` having bound.

`inl`/`outl` were added to include/arch/x86.h for this — config space is only
reachable in dwords, so `pci_read16`/`pci_read8` shift and mask the containing
dword rather than doing a narrower bus cycle.