If you have read my previous [blog posts](http://0xax.blogspot.com/search/label/asm), you can see that some time ago I started to get involved with low-level programming. I wrote some posts about x86_64 assembly programming for Linux. At the same time, I started to dive into the Linux source code. It is very interesting for me to understand how low-level things work, how programs run on my computer, how they are located in memory, how the kernel manages processes and memory, how the network stack works on low-level and many many other things. I decided to write yet another series of posts about the Linux kernel for **x86_64**.
Note that I'm not a professional kernel hacker, and I don't write code for the kernel at work. It's just a hobby. I just like low-level stuff, and it is interesting for me to see how these things work. So if you notice anything confusing, or if you have any questions/remarks, ping me on twitter [0xAX](https://twitter.com/0xAX), drop me an [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-insides/issues/new). I appreciate it. All posts will also be accessible at [linux-insides](https://github.com/0xAX/linux-insides) and if you find something wrong with my English or the post content, feel free to send pull request.
Note that I'm not a professional kernel hacker, and I don't write code for the kernel at work. It's just a hobby. I just like low-level stuff, and it is interesting for me to see how these things work. So if you notice anything confusing, or if you have any questions/remarks, ping me on twitter [0xAX](https://twitter.com/0xAX), drop me an [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-insides/issues/new). I appreciate it. All posts will also be accessible at [linux-insides](https://github.com/0xAX/linux-insides) and if you find something wrong with my English or the post content, feel free to send a pull request.
*Note that this isn't the official documentation, just learning and sharing knowledge.*
@ -23,7 +23,7 @@ All code is actually for kernel - 3.18, if there are changes, I will update the
Despite that this is a series of posts about linux kernel, we will not start from kernel code (at least in this paragraph). Ok, you pressed the magic power button on your laptop or desktop computer and it started to work. After the motherboard sends a signal to the [power supply](https://en.wikipedia.org/wiki/Power_supply), the power supply provides the computer with the proper amount of electricity. Once motherboard receives the [power good signal](https://en.wikipedia.org/wiki/Power_good_signal), it tries to run the CPU. The CPU resets all leftover data in its registers and sets up predefined values for every register.
Despite that this is a series of posts about Linux kernel, we will not start from kernel code (at least in this paragraph). Ok, you pressed the magic power button on your laptop or desktop computer and it started to work. After the motherboard sends a signal to the [power supply](https://en.wikipedia.org/wiki/Power_supply), the power supply provides the computer with the proper amount of electricity. Once motherboard receives the [power good signal](https://en.wikipedia.org/wiki/Power_good_signal), it tries to run the CPU. The CPU resets all leftover data in its registers and sets up predefined values for every register.
[80386](https://en.wikipedia.org/wiki/Intel_80386) and later CPUs define the following predefined data in CPU registers after the computer resets:
@ -206,7 +206,7 @@ hdr:
boot_flag: .word 0xAA55
```
The bootloader must fill this and the rest of the headers (only marked as `write` in the linux boot protocol, for example [this](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L354)) with values which it either got from command line or calculated. We will not see description and explanation of all fields of kernel setup header, we will get back to it when kernel uses it. Anyway, you can find description of any field in the [boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L156).
The bootloader must fill this and the rest of the headers (only marked as `write` in the Linux boot protocol, for example [this](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L354)) with values which it either got from command line or calculated. We will not see description and explanation of all fields of kernel setup header, we will get back to it when kernel uses it. Anyway, you can find description of any field in the [boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L156).
As we can see in kernel boot protocol, the memory map will be the following after kernel loading:
@ -245,7 +245,7 @@ where `X` is the address kernel bootsector loaded. In my case `X` is `0x10000` (
Ok, now the bootloader has loaded linux kernel into the memory, filled header fields and jumped to it. Now we can move directly to the kernel setup code.
Ok, now the bootloader has loaded Linux kernel into the memory, filled header fields and jumped to it. Now we can move directly to the kernel setup code.
Here we can see `jmp` instruction opcode - `0xeb` to the `start_of_setup-1f` point. `Nf` notation means following: `2f` refers to the next local `2:` label. In our case it is label `1` which goes right after jump. It contains rest of setup [header](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L156) and right after setup header we can see `.entrytext` section which starts at `start_of_setup` label.
Actually it's the first code which starts to execute besides previous jump instruction. After kernel setup got the control from bootloader, first `jmp` instruction is located at `0x200` (first 512 bytes) offset from the start of kernel real mode. This we can read in linux kernel boot protocol and also see in grub2 source code:
Actually it's the first code which starts to execute besides previous jump instruction. After kernel setup got the control from bootloader, first `jmp` instruction is located at `0x200` (first 512 bytes) offset from the start of kernel real mode. This we can read in Linux kernel boot protocol and also see in grub2 source code:
This is the end of the first part about Linux kernel internals. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part we will see first C code which executes in linux kernel setup, implementation of memory routines as `memset`, `memcpy`, `earlyprintk` implementation and early console initialization and many more.
This is the end of the first part about Linux kernel internals. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part we will see first C code which executes in Linux kernel setup, implementation of memory routines as `memset`, `memcpy`, `earlyprintk` implementation and early console initialization and many more.
**Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).**
Waqar144
9 years ago