This is the third part of the `Kernel booting process` series. In the previous [part](https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-2.md#kernel-booting-process-part-2), we stopped right before the call of the `set_video` routine from the [main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L181). We will see video mode initialization in the kernel setup code, preparation before switching into the protected mode and transition into it in this part.
This is the third part of the `Kernel booting process` series. In the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md#kernel-booting-process-part-2), we stopped right before the call of the `set_video` routine from the [main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L181). We will see video mode initialization in the kernel setup code, preparation before switching into the protected mode and transition into it in this part.
**NOTE** If you don't know anything about protected mode, you can find some information about it in the previous [part](https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-2.md#protected-mode). Also there are a couple of [links](https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-2.md#links) which can help you.
**NOTE** If you don't know anything about protected mode, you can find some information about it in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md#protected-mode). Also there are a couple of [links](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md#links) which can help you.
As i wrote above, we will start from the `set_video` function which defined in the [arch/x86/boot/video.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/video.c#L315) source code file. We can see that it starts with getting of video mode from the `boot_params.hdr` structure:
which subtracts value of the `HEAP` from the `heap_end` (we calculated it in the previous [part](https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-2.md)) and returns 1 if there is enough memory for `n`.
which subtracts value of the `HEAP` from the `heap_end` (we calculated it in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md)) and returns 1 if there is enough memory for `n`.
That's all. Now we have simple API for heap and can setup video mode.
@ -265,7 +265,7 @@ We can see the last function call - `go_to_protected_mode` in the [main.c](https
`go_to_protected_mode` defined in the [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pm.c#L104). It contains some functions which make last preparations before we can jump into protected mode, so let's look on it and try to understand what they do and how it works.
At first we see call of `realmode_switch_hook` function in the `go_to_protected_mode`. This function invokes real mode switch hook if it is present and disables [NMI](http://en.wikipedia.org/wiki/Non-maskable_interrupt). Hooks are used if bootloader runs in a hostile environment More about hooks you can read in the [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) (see **ADVANCED BOOT LOADER HOOKS**). `readlmode_swtich` hook presents pointer to the 16-bit real mode far subroutine which disables non-maskable interruptions. After we checked `realmode_switch` hook (it doesn't present for me), there is disabling of non-maskable interruptions:
At first we see call of `realmode_switch_hook` function in the `go_to_protected_mode`. This function invokes real mode switch hook if it is present and disables [NMI](http://en.wikipedia.org/wiki/Non-maskable_interrupt). Hooks are used if bootloader runs in a hostile environment. More about hooks you can read in the [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) (see **ADVANCED BOOT LOADER HOOKS**). `readlmode_swtich` hook presents pointer to the 16-bit real mode far subroutine which disables non-maskable interruptions. After we checked `realmode_switch` hook (it doesn't present for me), there is disabling of non-maskable interruptions:
```assembly
asm volatile("cli");
@ -351,7 +351,7 @@ where we can see - 16-bit length of IDT and 32-bit pointer to it (More details a
The next point is setup of the Global Descriptor Table (GDT). We can see `setup_gdt` function which setups GDT (you can read about it in the [Kernel booting process. Part 2.](https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-2.md#protected-mode)). There is definition of the `boot_gdt` array in this function, which contains definition of the three segments:
The next point is setup of the Global Descriptor Table (GDT). We can see `setup_gdt` function which setups GDT (you can read about it in the [Kernel booting process. Part 2.](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md#protected-mode)). There is definition of the `boot_gdt` array in this function, which contains definition of the three segments:
@ -421,7 +421,7 @@ in binary. Let's try to understand what every bit means. We will go through all
* 101 - segment type execute/read/
* 1 - accessed bit
You can know more about every bit in the previous [post](https://github.com/0xAX/linux-insides/blob/master/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).
You can know more about every bit in the previous [post](https://github.com/0xAX/linux-insides/blob/master/Booting/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).
0xAX
9 years ago