# Initramfs

"An initial RAM filesystem, or **initramfs**, is a compressed **cpio** archive.
**cpio** is an old Unix archive format, similar to TAR and ZIP but easier to
decode and so requiring less code in the kernel. You need to configure your
kernel with **CONFIG_BLK_DEV_INITRD** to support **initramfs**."

## Create an initramfs

To create the archive, compress it and add a U-Boot header, run these commands:
```bash
cd ~/rootfs
find . | cpio -H newc -ov --owner root:root > ../initramfs.cpio
cd ..
gzip initramfs.cpio
mkimage -A arm -O linux -T ramdisk -d initramfs.cpio.gz uRamdisk
```

To embed the initramfs into the kernel, set the *CONFIG_INITRAMFS_SOURCE*
kernel configuration to the full path of the just created cpio archive.
With menuconfig, this step is done in
**General setup | Initramfs source file(s)**.
The file has to be an uncompressed cpio file ending in *.cpio*, not
*.gzip*, *.zip*, etc.

## Booting the initramfs

Here are two options displayed on how to boot the initramfs.
The first one is QEMU on the host system, the other one on the board itself.

### Booting with QEMU

```bash
QEMU_AUDIO_DRV=none \
    qemu-system-arm -m 256M -nographic -M versatilepb \
    -kernel zImage
    -append "console=ttyAMA0 rdinit=/bin/sh" \
    -dtb versatile-pb.dtb
```

### Booting the BeagleBoneBlack

Needed to boot the image on the BeagleBoneBlack is the previously prepared
SD-Card with the Kernel, Bootloader and Rootfs built, using the
arm-cortex_a8-linux-gnueabihf toolchain. The previously created uRamdisk shall
be copied onto the boot partition on the microSD card.
Then the board should be booted, up to the point where the U-Boot prompt comes
in.
Booting it this way will need us to enter these commands:
```bash
fatload mmc 0:1 0x80200000 zImage
fatload mmc 0:1 0x80f00000 am335x-boneblack.dtb fatload mmc 0:1 0x81000000 uRamdisk
setenv bootargs console=ttyO0,115200 rdinit=/bin/sh
bootz 0x80200000 0x81000000 0x80f00000
```

To avoid entering this everytime the board boots, just chain them all into one
String called "bootcmd" and enter "saveenv" after. Bootcmd is the initial
command that U-Boot runs at boot.
More about U-Boot [here](../bootloader/U-boot.md)!
```bash
setenv bootcmd "fatload mmc 0:1 0x80200000 zImage; fatload mmc 0:1 0x80f00000 am335x-boneblack.dtb fatload mmc 0:1 0x81000000 uRamdisk; setenv bootargs console=ttyO0,115200 rdinit=/bin/sh; bootz 0x80200000 0x81000000 0x80f00000"
saveenv
run bootcmd
```

After this is done, the procfs has to be mounted.
```bash
#!/bin/sh
/bin/mount -t proc proc /proc
# Other boot-time commands go here
/bin/sh
```

This process can be automized, as explained in the following.

## Init Programm

The init program starts by reading the config file under */etc/inittab*.
It can look something like this:
```bash
::sysinit:/etc/init.d/rcS
::askfirst:-/bin/ash
```

The first line starts the script *rcS* under */etc/init.d* when init is
started.
The second line prints the message
"**Please press Enter to activate this console**"
to the console and starts a shell when *Enter* is pressed.
The *dash* before the following path means that it will become a login shell,
which sources */etc/profile* and *~/.profile* before giving the shell prompt.

BusyBox init provides a default inittab if none is present in the root
filesystem. It is a little more extensive than the preceding one.

The file */etc/init.d/rcS* can look something like this:
```bash
#!/bin/sh
mount -t proc proc /proc
mount -t sysfs sysfs /sys
```

The rcS script **must** be **executable**!


*source:* Mastering Embedded Linux Programming - Third Edition By Frank
Vasquez, Chris Simmonds