---
title: Documentation for findroot()
---
In x86/x86_autoconf.c.
`findroot()` attempts to find the device (a `device_t`) from which the kernel was loaded and changes `rootdev` (another `device_t`), if allowed to do so.
It does this in 6 steps:
1. If `booted_device` (a `device_t`) is already set, it just returns, not setting `rootdev`. The assumption seems to be, that the kernel was booted from the network and `device_register()` already has set `booted_device`.
2. If `BTINFO_NETIF` is available (but `booted_device` isn't set), it just returns, logging a message that the netboot interface wasn't found.
3. `BTINFO_ROOTDEVICE` is non-NULL.
`biv->devname` is expected to be of the form `"drivername<unitdigit><partitionletter>"`.
It loops over all disk devices looking for a device who's `cfname` is the same as `drivername`.
Iff such a device is found it sets the following variables:

| var name | description |
|----------|-------------|
| booted_device| device_t that the kernel was loaded from|
| booted_partition| the partition number derived from the `partitionletter`|
| booted_nblks| 0, indicating that no geometry information was passed|
|booted_method | `"bootinfo/rootdevice"`|

If it does not find a matching device(`dv` being `NULL` after exiting the loop) and `biv->devname` is not the empty string (`biv->devname[0] != '\0'`) it copies the devname to `bootspec` and returns.

| var name | description |
|----------|-------------|
| bootspec | name of the booted device if not found in device list |

BUGS Note that we can't boot off e.g. sd10 and that the nonsense is stored in `booted_partition` if the last character of `biv->devname` is not a valid partition letter ('a'-'v' c.f. sys/disklabel.h). In particular: "sd0z" and "sd0", in the latter case the "partition letter" is the NUL character terminating the device name.
This is handled better in the xen_machdep.c.
XXX there was other code that converted device unit with strtod(). But where?
4. `BTINFO_BOOTWEDGE` is non-NULL
`biw->biosdev` is expected to have the `0x80` bit set, indicating a hard disk. XXX why? Possibly because we floppies don't have wedges.
`BTINFO_BOOTDISK` should be available.
It loops over all the disk devices looking for disk with `is_valid_disk()` that `match_bootwedge()` the device.
`match_bootwedge()` calculates the MD5 hash of the device blocks `biw->matchblk` to `biw->matchblk+biw->matchnblks` returning true if that hash is equal to `biw->matchhash`.
`biw->matchblk == -1` indicates that no sector(block) was specified, hence no match.
XXX why not compare just devclass as in `BTINFO_ROOTDEVICE` case?
If a matching device was found the following variables are set:

| var name | description |
|----------|-------------|
| booted_device | device_t that the kernel was loaded from |
| booted_partition | `bid->partition` if `BTINFO_BOOTDISK` was available, else `0` |
| booted_nblks | `biw->nblks` (possibly `0`) XXX when? |
| booted_startblk | `biw->startblk`, the starting block of the partition |
| booted_method | `"bootinfo/bootwedge"` |

5. `BTINFO_BOOTDISK` is non-NULL
It loops over all disk and floppy devices, looking for either a floppy device who's `device_unit` is equal to `bid->biosdev` or a disk who's disklabel's type, checksum, and packname match the information passed in `bid->label` (via `match_bootdisk()`).
It sets the following variables:

| var name | description |
|----------|-------------|
| booted_device | the `device_t` of the device that the kernel was loaded from |
| booted_partition | `bid->partition` |
| booted_nblks | `0`, indicating that no geometry was passed |
| booted_method | `"bootinfo/bootdisk"`

6. `BTINFO_BIOSGEOM` is non-NULL and `bid->biosdev` indicates a CDROM (by being bigger than the number of biosgeom entries declared by the BIOS).
It find the first CDROM device and sets the following variables:

| var name | description |
|----------|-------------|
| booted_device | the `device_t` of the first probed CDROM |
| booted_partition | `0` |
| booted_nblks | `0`, indicating that bios geometry is not relevant |
| booted_method | `"bootinfo/biosgeom"` |

7. if nothing matched, `findroot()` return without setting any variables.

Summary: it stores the device from where the system was booted from, in `booted_device`, along with other device associated variables. it does this in one of 4 cases (types of booting method) . those cases are true if bootinfo of bootloader has information corresponding to their type. if the bootinfo has no information regarding those types of bootmethod it returns without setting any varibles.