You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
50 lines
2.2 KiB
Markdown
50 lines
2.2 KiB
Markdown
Tutorial 0E - Initial RamDisk
|
|
=============================
|
|
|
|
Many OS uses initial ramdisk to load files into memory during boot. I felt the need for such
|
|
a tutorial as most hobby OS developer's never learned how to do this properly.
|
|
|
|
First of all, we're not going to reinvent the wheel a come up with a new format and an awful
|
|
image creator tool. We're going to use the POSIX standard `tar` utility to create our initrd. It's format
|
|
is really simple, first comes an 512 bytes long header with file meta information, followed by the
|
|
file contents padded with zeros to round up to multiple of 512 bytes. This repeats for every file in the archive.
|
|
If you want a compressed initrd, you can use for example the [tinf](https://bitbucket.org/jibsen/tinf) library to
|
|
deflate. The uncompressed buffer can be parsed by the method described here.
|
|
|
|
Second, about loading it into memory, we have several options:
|
|
|
|
### Load a file on our own
|
|
You can use the `fat_readfile()` from the previous tutorial. In that case your initrd's address
|
|
will be returned by the function.
|
|
|
|
### Ask the GPU to do so
|
|
You can use `config.txt` to tell start.elf to load the initrd for you. With this you won't need
|
|
any SD card reader or FAT parser at all, resulting in a much smaller kernel. As for the
|
|
[config.txt](https://www.raspberrypi.org/documentation/configuration/config-txt/boot.md),
|
|
you have two options:
|
|
|
|
`initramfs (filename) followkernel` - this will load (filename) after your kernel. You can access it at the label
|
|
*&_end* defined by our linker script.
|
|
|
|
`initramfs (filename) (address)` - load (filename) into a specified location. You can access it at *(address)*.
|
|
|
|
### Statically link
|
|
This is not very practical because you have to build your kernel every time you want to change the initrd. But
|
|
it is the simplest method, and to keep this tutorial simple we'll use this. You can access the initrd by the label
|
|
*_binary_initrd_tar_start*.
|
|
|
|
Makefile
|
|
--------
|
|
I've added a tar.o to the usual Makefile. This rule will dinamically create a tar file and convert it into an
|
|
object file.
|
|
|
|
Initrd.h, initrd.c
|
|
------------------
|
|
|
|
`initrd_list(buf)` list the contents of a tar archive in the buffer.
|
|
|
|
Main
|
|
----
|
|
|
|
We initialize console and the pass the initrd buffer to lister.
|