If you have read my previous [blog posts](http://0xax.blogspot.com/search/label/asm), you can see that sometime 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. I have a great interest in understanding 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. So, 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 a pull request.
Anyway, if you just started to learn some tools, I will try to explain some parts during this and the following posts. Ok, little introduction finished and now we can start to dive into the kernel and low-level stuff.
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.
The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode) and we need to back up a little to understand memory segmentation in this mode. Real mode is supported in all x86-compatible processors, from [8086](https://en.wikipedia.org/wiki/Intel_8086) to modern Intel 64-bit CPUs. The 8086 processor had a 20-bit address bus, which means that it could work with 0-2^20 bytes address space (1 megabyte). But it only has 16-bit registers, and with 16-bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). [Memory segmentation](http://en.wikipedia.org/wiki/Memory_segmentation) is used to make use of all of the address space available. All memory is divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we cannot address memory below 64 KB with 16 bit registers, an alternate method to do it was devised. An address consists of two parts: the beginning address of the segment and the offset from the beginning of this segment. To get a physical address in memory, we need to multiply the segment part by 16 and add the offset part:
which is 65519 bytes over first megabyte. Since only one megabyte is accessible in real mode, `0x10ffef` becomes `0x00ffef` with disabled [A20](https://en.wikipedia.org/wiki/A20_line).
`CS` register consists of two parts: the visible segment selector and hidden base address. We know predefined `CS` base and `IP` value, logical address will be:
We get `0xfffffff0` which is 4GB - 16 bytes. This point is the [Reset vector](http://en.wikipedia.org/wiki/Reset_vector). This is the memory location at which CPU expects to find the first instruction to execute after reset. It contains a [jump](http://en.wikipedia.org/wiki/JMP_%28x86_instruction%29) instruction which usually points to the BIOS entry point. For example, if we look in [coreboot](http://www.coreboot.org/) source code, we will see it:
We can see here the jump instruction [opcode](http://ref.x86asm.net/coder32.html#xE9) - 0xe9 to the address `_start - ( . + 2)`. And we can see that `reset` section is 16 bytes and starts at `0xfffffff0`:
Now the BIOS has started to work. After initializing and checking the hardware, it needs to find a bootable device. A boot order is stored in the BIOS configuration. The function of boot order is to control which devices the kernel attempts to boot. In the case of attempting to boot a hard drive, the BIOS tries to find a boot sector. On hard drives partitioned with an MBR partition layout, the boot sector is stored in the first 446 bytes of the first sector (512 bytes). The final two bytes of the first sector are `0x55` and `0xaa` which signals the BIOS that the device is bootable. For example:
This will instruct [QEMU](http://qemu.org) to use the `boot` binary we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (the origin is set to `0x7c00`, and we end with the magic sequence). QEMU will treat the binary as the master boot record(MBR) of a disk image.
In this example we can see that this code will be executed in 16 bit real mode and will start at 0x7c00 in memory. After the start it calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt which just prints `!` symbol. It fills rest of 510 bytes with zeros and finish with two magic bytes `0xaa` and `0x55`.
A real-world boot sector has code for continuing the boot process and the partition table instead of a bunch of 0's and an exclamation point :) Ok so, from this point onwards BIOS hands over the control to the bootloader and we can go ahead.
Same as I mentioned before. But we have only 16 bit general purpose registers. The maximum value of 16 bit register is: `0xffff`; So if we take the biggest values the result will be:
Where `0x10ffef` is equal to `1MB + 64KB - 16b`. But a [8086](https://en.wikipedia.org/wiki/Intel_8086) processor, which was the first processor with real mode. It had 20 bit address line and `2^20 = 1048576.0` is 1MB. So, it means that the actual memory available is 1MB.
But stop, at the beginning of post I wrote that first instruction executed by the CPU is located at the address `0xFFFFFFF0`, which is much bigger than `0xFFFFF` (1MB). How can CPU access it in real mode? As I write about it and you can read in [coreboot](http://www.coreboot.org/Developer_Manual/Memory_map) documentation:
There are a number of bootloaders which can boot Linux, such as [GRUB 2](https://www.gnu.org/software/grub/) and [syslinux](http://www.syslinux.org/wiki/index.php/The_Syslinux_Project). The Linux kernel has a [Boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt) which specifies the requirements for bootloaders to implement Linux support. This example will describe GRUB 2.
Now that the BIOS has chosen a boot device and transferred control to the boot sector code, execution starts from [boot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/boot.S;hb=HEAD). This code is very simple due to the limited amount of space available, and contains a pointer that it uses to jump to the location of GRUB 2's core image. The core image begins with [diskboot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/diskboot.S;hb=HEAD), which is usually stored immediately after the first sector in the unused space before the first partition. The above code loads the rest of the core image into memory, which contains GRUB 2's kernel and drivers for handling filesystems. After loading the rest of the core image, it executes [grub_main](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/kern/main.c).
`grub_main` initializes console, gets base address for modules, sets root device, loads/parses grub configuration file, loads modules etc. At the end of execution, `grub_main` moves grub to normal mode. `grub_normal_execute` (from `grub-core/normal/main.c`) completes last preparation and shows a menu for selecting an operating system. When we select one of grub menu entries, `grub_menu_execute_entry` begins to be executed, which executes grub `boot` command. It starts to boot the selected operating system.
As we can read in the kernel boot protocol, the bootloader must read and fill some fields of kernel setup header which starts at `0x01f1` offset from the kernel setup code. Kernel header [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S) starts from:
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).
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.
Finally we are in the kernel. Technically kernel didn't run yet, first of all we need to setup kernel, memory manager, process manager etc. Kernel setup execution starts from [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S) at the [_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L293). It is a little strange at the first look, there are many instructions before it.
Actually `header.S` starts from [MZ](https://en.wikipedia.org/wiki/DOS_MZ_executable) (see image above), error message printing and following [PE](https://en.wikipedia.org/wiki/Portable_Executable) header:
It needs this for loading the operating system with [UEFI](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface). Here we will not see how it works (we will these later in the next parts).
Bootloader (grub2 and others) knows about this point (`0x200` offset from `MZ`) and makes a jump directly to this point, despite the fact that `header.S` starts from `.bstext` section which prints error message:
```
//
// arch/x86/boot/setup.ld
//
. = 0; // current position
.bstext : { *(.bstext) } // put .bstext section to position 0
.bsdata : { *(.bsdata) }
```
So kernel setup entry point is:
```assembly
.globl _start
_start:
.byte 0xeb
.byte start_of_setup-1f
1:
//
// rest of the header
//
```
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:
As I wrote above, grub2 loads kernel setup code at `0x10000` address and `cs` at `0x1020` because execution doesn't start from the start of file, but from:
`jump`, which is 512 bytes offset from the [4d 5a](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L47). Also need to align `cs` from `0x10200` to `0x10000` as all other segment registers. After that we setup the stack:
push `ds` value to stack, and address of [6](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L494) label and execute `lretw` instruction. When we call `lretw`, it loads address of label `6` to [instruction pointer](https://en.wikipedia.org/wiki/Program_counter) register and `cs` with value of `ds`. After it we will have `ds` and `cs` with the same values.
Actually, almost all of the setup code is preparation for C language environment in the real mode. The next [step](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L467) is checking of `ss` register value and making of correct stack if `ss` is wrong:
Here we can see aligning of `dx` (contains `sp` given by bootloader) to 4 bytes and checking that it is not zero. If it is zero we put `0xfffc` (4 byte aligned address before maximum segment size - 64 KB) to `dx`. If it is not zero we continue to use `sp` given by bootloader (0xf7f4 in my case). After this we put `ax` value to `ss` which stores correct segment address `0x10000` and set up correct `sp`. After it we have correct stack:
2. In the second case (`ss` != `ds`), first of all put [_end](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L52) (address of end of setup code) value in `dx`. And check `loadflags` header field with `testb` instruction too see if we can use heap or not. [loadflags](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L321) is a bitmask header which is defined as:
If `CAN_USE_HEAP` bit is set, put `heap_end_ptr` to `dx` which points to `_end` and add `STACK_SIZE` (minimal stack size - 512 bytes) to it. After this if `dx` is not carry, jump to `2` (it will not be carry, dx = _end + 512) label as in previous case and make correct stack.
The last two steps that need to happen before we can jump to the main C code, are that we need to set up the [BSS](https://en.wikipedia.org/wiki/.bss) area, and check the "magic" signature. Firstly, signature checking:
This simply consists of comparing the [setup_sig](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L39) against the magic number `0x5a5aaa55`. If they are not equal, a fatal error is reported.
But if the magic number matches, knowing we have a set of correct segment registers, and a stack, we need only setup the BSS section before jumping into the C code.
The BSS section is used for storing statically allocated, uninitialized, data. Linux carefully ensures this area of memory is first blanked, using the following code:
First of all the [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address is moved into `di`, and the `_end + 3` address (+3 - aligns to 4 bytes) is moved into `cx`. The `eax` register is cleared (using an `xor` instruction), and the bss section size (`cx`-`di`) is calculated and put into `cx`. Then, `cx` is divided by four (the size of a 'word'), and the `stosl` instruction is repeatedly used, storing the value of `eax` (zero) into the address pointed to by `di`, and automatically increasing `di` by four (this occurs until `cx` reaches zero). The net effect of this code, is that zeros are written through all words in memory from `__bss_start` to `_end`:
The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). What will be there? We will see it in the next part.
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).**
