This is the third part of the `Kernel booting process` series. In the previous [part](linux-bootstrap-2.md#kernel-booting-process-part-2), we stopped right before the call to the `set_video` routine from [main.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/main.c).
**NOTE** If you don't know anything about protected mode, you can find some information about it in the previous [part](linux-bootstrap-2.md#protected-mode). Also, there are a couple of [links](linux-bootstrap-2.md#links) which can help you.
As I wrote above, we will start from the `set_video` function which is defined in the [arch/x86/boot/video.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/video.c) source code file. We can see that it starts by first getting the video mode from the `boot_params.hdr` structure:
which we filled in the `copy_boot_params` function (you can read about it in the previous post). `vid_mode` is an obligatory field which is filled by the bootloader. You can find information about it in the kernel `boot protocol`:
So we can add the `vga` option to the grub (or another bootloader's) configuration file and it will pass this option to the kernel command line. This option can have different values as mentioned in the description. For example, it can be an integer number `0xFFFD` or `ask`. If you pass `ask` to `vga`, you will see a menu like this:
which will ask to select a video mode. We will look at its implementation, but before diving into the implementation we have to look at some other things.
Earlier we saw definitions of different data types like `u16` etc. in the kernel setup code. Let's look at a couple of data types provided by the kernel:
After we get `vid_mode` from `boot_params.hdr` in the `set_video` function, we can see the call to the `RESET_HEAP` function. `RESET_HEAP` is a macro which is defined in [arch/x86/boot/boot.h](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/boot.h) header file.
If you have read the second part, you will remember that we initialized the heap with the [`init_heap`](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/main.c) function. We have a couple of utility macros and functions for managing the heap which are defined in `arch/x86/boot/boot.h` header file.
Let's try to understand how `__get_heap` works. We can see here that `HEAP` (which is equal to `_end` after `RESET_HEAP()`) is assigned the address of the aligned memory according to the `a` parameter. After this we save the memory address from `HEAP` to the `tmp` variable, move `HEAP` to the end of the allocated block and return `tmp` which is the start address of allocated memory.
which subtracts value of the `HEAP` pointer from the `heap_end` (we calculated it in the previous [part](linux-bootstrap-2.md)) and returns 1 if there is enough memory available for `n`.
Now we can move directly to video mode initialization. We stopped at the `RESET_HEAP()` call in the `set_video` function. Next is the call to `store_mode_params` which stores video mode parameters in the `boot_params.screen_info` structure which is defined in [include/uapi/linux/screen_info.h](https://github.com/torvalds/linux/blob/v4.16/include/uapi/linux/screen_info.h) header file.
If we look at the `store_mode_params` function, we can see that it starts with a call to the `store_cursor_position` function. As you can understand from the function name, it gets information about the cursor and stores it.
First of all, `store_cursor_position` initializes two variables which have type `biosregs` with `AH = 0x3`, and calls the `0x10` BIOS interruption. After the interruption is successfully executed, it returns row and column in the `DL` and `DH` registers. Row and column will be stored in the `orig_x` and `orig_y` fields of the `boot_params.screen_info` structure.
After `store_cursor_position` is executed, the `store_video_mode` function will be called. It just gets the current video mode and stores it in `boot_params.screen_info.orig_video_mode`.
After this, `store_mode_params` checks the current video mode and sets the `video_segment`. After the BIOS transfers control to the boot sector, the following addresses are for video memory:
So we set the `video_segment` variable to `0xb000` if the current video mode is MDA, HGC, or VGA in monochrome mode and to `0xb800` if the current video mode is in color mode. After setting up the address of the video segment, the font size needs to be stored in `boot_params.screen_info.orig_video_points` with:
First of all, we put 0 in the `FS` register with the `set_fs` function. We already saw functions like `set_fs` in the previous part. They are all defined in [arch/x86/boot/boot.h](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/boot.h). Next, we read the value which is located at address `0x485` (this memory location is used to get the font size) and save the font size in `boot_params.screen_info.orig_video_points`.
Next, we get the amount of columns by address `0x44a` and rows by address `0x484` and store them in `boot_params.screen_info.orig_video_cols` and `boot_params.screen_info.orig_video_lines`. After this, execution of `store_mode_params` is finished.
Next we can see the `save_screen` function which just saves the contents of the screen to the heap. This function collects all the data which we got in the previous functions (like the rows and columns, and stuff) and stores it in the `saved_screen` structure, which is defined as:
The next call is `probe_cards(0)` from [arch/x86/boot/video-mode.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/video-mode.c) source code file. It goes over all video_cards and collects the number of modes provided by the cards. Here is the interesting part, we can see the loop:
but `video_cards` is not declared anywhere. The answer is simple: every video mode presented in the x86 kernel setup code has a definition that looks like this:
is in the `.videocards` segment. Let's look in the [arch/x86/boot/setup.ld](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/setup.ld) linker script, where we can find:
It means that `video_cards` is just a memory address and all `card_info` structures are placed in this segment. It means that all `card_info` structures are placed between `video_cards` and `video_cards_end`, so we can use a loop to go over all of it. After `probe_cards` executes we have a bunch of structures like `static __videocard video_vga` with the `nmodes` (the number of video modes) filled in.
After the `probe_cards` function is done, we move to the main loop in the `set_video` function. There is an infinite loop which tries to set up the video mode with the `set_mode` function or prints a menu if we passed `vid_mode=ask` to the kernel command line or if video mode is undefined.
The `set_mode` function is defined in [video-mode.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/video-mode.c) and gets only one parameter, `mode`, which is the number of video modes (we got this value from the menu or in the start of `setup_video`, from the kernel setup header).
The `set_mode` function checks the `mode` and calls the `raw_set_mode` function. The `raw_set_mode` calls the selected card's `set_mode` function, i.e. `card->set_mode(struct mode_info*)`. We can get access to this function from the `card_info` structure. Every video mode defines this structure with values filled depending upon the video mode (for example for `vga` it is the `video_vga.set_mode` function. See the above example of the `card_info` structure for `vga`). `video_vga.set_mode` is `vga_set_mode`, which checks the vga mode and calls the respective function:
Next, `vesa_store_edid` is called. This function simply stores the [EDID](https://en.wikipedia.org/wiki/Extended_Display_Identification_Data) (**E**xtended **D**isplay **I**dentification **D**ata) information for kernel use. After this `store_mode_params` is called again. Lastly, if `do_restore` is set, the screen is restored to an earlier state.
We can see the last function call - `go_to_protected_mode` - in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/main.c). As the comment says: `Do the last things and invoke protected mode`, so let's see what these last things are and switch into protected mode.
The `go_to_protected_mode` function is defined in [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/pm.c). It contains some functions which make the last preparations before we can jump into protected mode, so let's look at it and try to understand what it does and how it works.
First is the call to the `realmode_switch_hook` function in `go_to_protected_mode`. This function invokes the real mode switch hook if it is present and disables [NMI](http://en.wikipedia.org/wiki/Non-maskable_interrupt). Hooks are used if the bootloader runs in a hostile environment. You can read more about hooks in the [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) (see **ADVANCED BOOT LOADER HOOKS**).
The `realmode_switch` hook presents a pointer to the 16-bit real mode far subroutine which disables non-maskable interrupts. After the `realmode_switch` hook (it isn't present for me) is checked, Non-Maskable Interrupts(NMI) is disabled:
At first, there is an inline assembly statement with a `cli` instruction which clears the interrupt flag (`IF`). After this, external interrupts are disabled. The next line disables NMI (non-maskable interrupt).
An interrupt is a signal to the CPU which is emitted by hardware or software. After getting such a signal, the CPU suspends the current instruction sequence, saves its state and transfers control to the interrupt handler. After the interrupt handler has finished its work, it transfers control back to the interrupted instruction. Non-maskable interrupts (NMI) are interrupts which are always processed, independently of permission. They cannot be ignored and are typically used to signal for non-recoverable hardware errors. We will not dive into the details of interrupts now but we will be discussing them in the coming posts.
Let's get back to the code. We can see in the second line that we are writing the byte `0x80` (disabled bit) to `0x70` (the CMOS Address register). After that, a call to the `io_delay` function occurs. `io_delay` causes a small delay and looks like:
To output any byte to the port `0x80` should delay exactly 1 microsecond. So we can write any value (the value from `AL` in our case) to the `0x80` port. After this delay the `realmode_switch_hook` function has finished execution and we can move to the next function.
The next function is `enable_a20`, which enables the [A20 line](http://en.wikipedia.org/wiki/A20_line). This function is defined in [arch/x86/boot/a20.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/a20.c) and it tries to enable the A20 gate with different methods. The first is the `a20_test_short` function which checks if A20 is already enabled or not with the `a20_test` function:
First of all, we put `0x0000` in the `FS` register and `0xffff` in the `GS` register. Next, we read the value at the address `A20_TEST_ADDR` (it is `0x200`) and put this value into the variables `saved` and `ctr`.
Next, we write an updated `ctr` value into `fs:A20_TEST_ADDR` or `fs:0x200` with the `wrfs32` function, then delay for 1ms, and then read the value from the `GS` register into the address `A20_TEST_ADDR+0x10`. In a case when `a20` line is disabled, the address will be overlapped, in other case if it's not zero `a20` line is already enabled the A20 line.
If A20 is disabled, we try to enable it with a different method which you can find in `a20.c`. For example, it can be done with a call to the `0x15` BIOS interrupt with `AH=0x2041`.
If the `enable_a20` function finished with a failure, print an error message and call the function `die`. You can remember it from the first source code file where we started - [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S):
which sets up the Interrupt Descriptor Table (describes interrupt handlers and etc.). For now, the IDT is not installed (we will see it later), but now we just load the IDT with the `lidtl` instruction. `null_idt` contains the address and size of the IDT, but for now they are just zero. `null_idt` is a `gdt_ptr` structure, it is defined as:
where we can see the 16-bit length(`len`) of the IDT and the 32-bit pointer to it (More details about the IDT and interruptions will be seen in the next posts). ` __attribute__((packed))` means that the size of `gdt_ptr` is the minimum required size. So the size of the `gdt_ptr` will be 6 bytes here or 48 bits. (Next we will load the pointer to the `gdt_ptr` to the `GDTR` register and you might remember from the previous post that it is 48-bits in size).
Next is the setup of the Global Descriptor Table (GDT). We can see the `setup_gdt` function which sets up the GDT (you can read about it in the post [Kernel booting process. Part 2.](linux-bootstrap-2.md#protected-mode)). There is a definition of the `boot_gdt` array in this function, which contains the definition of the three segments:
for code, data and TSS (Task State Segment). We will not use the task state segment for now, it was added there to make Intel VT happy as we can see in the comment line (if you're interested you can find the commit which describes it - [here](https://github.com/torvalds/linux/commit/88089519f302f1296b4739be45699f06f728ec31)). Let's look at `boot_gdt`. First of all note that it has the `__attribute__((aligned(16)))` attribute. It means that this structure will be aligned by 16 bytes.
The `GDT_ENTRY_BOOT_CS` has index - 2 here, `GDT_ENTRY_BOOT_DS` is `GDT_ENTRY_BOOT_CS + 1` and etc. It starts from 2, because the first is a mandatory null descriptor (index - 0) and the second is not used (index - 1).
`GDT_ENTRY` is a macro which takes flags, base, limit and builds a GDT entry. For example, let's look at the code segment entry. `GDT_ENTRY` takes the following values:
What does this mean? The segment's base address is 0, and the limit (size of segment) is - `0xfffff` (1 MB). Let's look at the flags. It is `0xc09b` and it will be:
You can read more about every bit in the previous [post](linux-bootstrap-2.md) or in the [Intel® 64 and IA-32 Architectures Software Developer's Manuals 3A](http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html).
This is the end of the `go_to_protected_mode` function. We loaded the IDT and GDT, disabled interrupts and now can switch the CPU into protected mode. The last step is calling the `protected_mode_jump` function with two parameters:
Let's look inside `protected_mode_jump`. As I wrote above, you can find it in `arch/x86/boot/pmjump.S`. The first parameter will be in the `eax` register and the second one is in `edx`.
First of all, we put the address of `boot_params` in the `esi` register and the address of the code segment register `cs` in `bx`.
```assembly
GLOBAL(protected_mode_jump)
movl %edx, %esi # Pointer to boot_params table
xorl %ebx, %ebx
movw %cs, %bx
```
After this, we shift `bx` by 4 bits and add it to the memory location labeled `2` (which is `(cs << 4) + in_pm32`, the physical address to jump after transitioned to 32-bit mode) and jump to label `1`.
```assembly
shll $4, %ebx
addl %ebx, 2f # Add %ebx to the value stored at label 2
jmp 1f # Short jump to serialize on 386/486
```
So after this `in_pm32` in label `2` will be overwritten with `(cs << 4) + in_pm32`.
If you paid attention, you can remember that we saved `$__BOOT_DS` in the `cx` register. Now we fill it with all segment registers besides `cs` (`cs` is already `__BOOT_CS`).
This is the end of the third part about Linux kernel insides. In the next part, we will look at the first steps we take in protected mode and transition into [long mode](http://en.wikipedia.org/wiki/Long_mode).
**Please note that English is not my first language, And I am really sorry for any inconvenience. If you find any mistakes, please send me a PR with corrections at [linux-insides](https://github.com/0xAX/linux-internals).**
