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Fix highlighting, grammar and some code additions
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kittyhawk
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@ -4,21 +4,25 @@ Kernel booting process. Part 2.
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First steps in the kernel setup
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--------------------------------------------------------------------------------
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We started to dive into linux kernel internals in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call of the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). Here we will continue to research the kernel setup code and see what `protected mode` is, some preparation for the transition into it, the heap and console initialization, memory detection and much much more. So... Let's go ahead.
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We started to dive into linux kernel internals in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call of the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). Here we will continue to research the kernel setup code and see what `protected mode` is, some preparation for the transition into it, the heap and console initialization, memory detection and much much more. So, Let's go ahead.
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Protected mode
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--------------------------------------------------------------------------------
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Before we can move to the native Intel64 [Long mode](http://en.wikipedia.org/wiki/Long_mode), the kernel must switch the CPU into protected mode. What is protected mode? Protected mode was first added to the x86 architecture in 1982 and was the main mode of Intel processors from the [80286](http://en.wikipedia.org/wiki/Intel_80286) processor until Intel 64 and long mode. The Main reason to move away from real mode is that there is very limited access to the RAM. As you may remember from the previous part, there is only 2^20 bytes or 1 megabyte, sometimes even only 640 kilobytes.
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Before we can move to the native Intel64 [Long mode](http://en.wikipedia.org/wiki/Long_mode), the kernel must switch the CPU into protected mode.
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Protected mode brought many changes, but the main one is different memory management. The 24-bit address bus was replaced with a 32-bit address bus. It allows access to 4 gigabytes of physical address space. Also [paging](http://en.wikipedia.org/wiki/Paging) support was added, which you can read about in the next sections.
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What is [protected mode](https://en.wikipedia.org/wiki/Protected_mode)? Protected mode was first added to the x86 architecture in 1982 and was the main mode of Intel processors from the [80286](http://en.wikipedia.org/wiki/Intel_80286) processor until Intel 64 and long mode came. The Main reason to move away from [real mode](http://wiki.osdev.org/Real_Mode) is that there is very limited access to the RAM. As you may remember from the previous part, there is only 2^20 bytes or 1 megabyte, sometimes even only 640 kilobytes of RAM available in real mode.
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Protected mode brought many changes, but the main one is the difference memory management. The 24-bit address bus was replaced with a 32-bit address bus. It allows access to 4 gigabytes of physical address space vs 1MB of real mode. Also [paging](http://en.wikipedia.org/wiki/Paging) support was added, which you can read about in the next sections.
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Memory management in protected mode is divided into two, almost independent parts:
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* Segmentation
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* Paging
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Here we can only see segmentation. As you can read in the previous part, addresses consist of two parts in real mode:
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Here we will only see segmentation. Paging will be discussed in the next sections.
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As you can read in the previous part, addresses consist of two parts in real mode:
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* Base address of the segment
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* Offset from the segment base
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@ -92,7 +96,7 @@ Don't worry, I know it looks a little scary after real mode, but it's easy. Let'
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| 15 1 1 1 1 | Code | Execute/Read, conforming, accessed
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```
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As we can see the first bit is 0 for a data segment and 1 for a code segment. The next three bits `EWA` are expansion direction (expand-down segment will grow down, you can read more about it [here](http://www.sudleyplace.com/dpmione/expanddown.html)), write enable and accessed for data segments. `CRA` bits are conforming (A transfer of execution into a more-privileged conforming segment allows execution to continue at the current privilege level), read enable and accessed.
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As we can see the first bit is `0` for a _data_ segment and `1` for a _code_ segment. The next three bits `EWA` are expansion direction (expand-down segment will grow down, you can read more about it [here](http://www.sudleyplace.com/dpmione/expanddown.html)), write enable and accessed for data segments. `CRA` bits are conforming (A transfer of execution into a more-privileged conforming segment allows execution to continue at the current privilege level), read enable and accessed.
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4. DPL (descriptor privilege level) defines the privilege level of the segment. It can be 0-3 where 0 is the most privileged.
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@ -138,9 +142,11 @@ Let's look at [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/mast
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Copying boot parameters into the "zeropage"
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--------------------------------------------------------------------------------
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We will start from the `main` routine in "main.c". First function which is called in `main` is [copy_boot_params](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L30). It copies the kernel setup header into the field of the `boot_params` structure which is defined in the [arch/x86/include/uapi/asm/bootparam.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/bootparam.h#L113).
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We will start from the `main` routine in "main.c". First function which is called in `main` is [`copy_boot_params(void)`](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L30). It copies the kernel setup header into the field of the `boot_params` structure which is defined in the [arch/x86/include/uapi/asm/bootparam.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/bootparam.h#L113).
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The `boot_params` structure contains the `struct setup_header hdr` field. This structure contains the same fields as defined in [linux boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) and is filled by the boot loader and also at kernel compile/build time. `copy_boot_params` does two things: copies `hdr` from [header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L281) to the `boot_params` structure in `setup_header` field and updates pointer to the kernel command line if the kernel was loaded with the old command line protocol.
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The `boot_params` structure contains the `struct setup_header hdr` field. This structure contains the same fields as defined in [linux boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) and is filled by the boot loader and also at kernel compile/build time. `copy_boot_params` does two things:
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1. Copies `hdr` from [header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L281) to the `boot_params` structure in `setup_header` field
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2. Updates pointer to the kernel command line if the kernel was loaded with the old command line protocol.
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Note that it copies `hdr` with `memcpy` function which is defined in the [copy.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/copy.S) source file. Let's have a look inside:
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@ -162,7 +168,7 @@ GLOBAL(memcpy)
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ENDPROC(memcpy)
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```
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Yeah, we just moved to C code and now assembly again :) First of all we can see that `memcpy` and other routines which are defined here, start and end with the two macros: `GLOBAL` and `ENDPROC`. GLOBAL is described in [arch/x86/include/asm/linkage.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/linkage.h) which defines `globl` directive and the label for it. ENDPROC is described in [include/linux/linkage.h](https://github.com/torvalds/linux/blob/master/include/linux/linkage.h) which marks `name` symbol as function name and ends with the size of the `name` symbol.
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Yeah, we just moved to C code and now assembly again :) First of all we can see that `memcpy` and other routines which are defined here, start and end with the two macros: `GLOBAL` and `ENDPROC`. GLOBAL is described in [arch/x86/include/asm/linkage.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/linkage.h) which defines `globl` directive and the label for it. `ENDPROC` is described in [include/linux/linkage.h](https://github.com/torvalds/linux/blob/master/include/linux/linkage.h) which marks `name` symbol as function name and ends with the size of the `name` symbol.
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Implementation of `memcpy` is easy. At first, it pushes values from `si` and `di` registers to the stack because their values will change during the `memcpy`, so it pushes them on the stack to preserve their values. `memcpy` (and other functions in copy.S) use `fastcall` calling conventions. So it gets its incoming parameters from the `ax`, `dx` and `cx` registers. Calling `memcpy` looks like this:
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@ -170,7 +176,12 @@ Implementation of `memcpy` is easy. At first, it pushes values from `si` and `di
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memcpy(&boot_params.hdr, &hdr, sizeof hdr);
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```
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So `ax` will contain the address of the `boot_params.hdr`, `dx` will contain the address of `hdr` and `cx` will contain the size of `hdr` (all in bytes). memcpy puts the address of `boot_params.hdr` into `si` and saves the size on the stack. After this it shifts to the right on 2 size (or divide on 4) and copies from `si` to `di` by 4 bytes. After it we restore the size of `hdr` again, align it by 4 bytes and copy the rest of the bytes from `si` to `di` byte by byte (if there is more). Restore `si` and `di` values from the stack in the end and after this copying is finished.
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So,
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* `ax` will contain the address of the `boot_params.hdr` in bytes
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* `dx` will contain the address of `hdr` in bytes
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* `cx` will contain the size of `hdr` in bytes.
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`memcpy` puts the address of `boot_params.hdr` into `si` and saves the size on the stack. After this it shifts to the right on 2 size (or divide on 4) and copies from `si` to `di` by 4 bytes. After this we restore the size of `hdr` again, align it by 4 bytes and copy the rest of the bytes from `si` to `di` byte by byte (if there is more). Restore `si` and `di` values from the stack in the end and after this copying is finished.
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Console initialization
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--------------------------------------------------------------------------------
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@ -205,7 +216,7 @@ void __attribute__((section(".inittext"))) putchar(int ch)
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}
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```
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`__attribute__((section(".inittext")))` means that this code will be in the .inittext section. We can find it in the linker file [setup.ld](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L19).
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`__attribute__((section(".inittext")))` means that this code will be in the `.inittext` section. We can find it in the linker file [setup.ld](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L19).
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First of all, `put_char` checks for the `\n` symbol and if it is found, prints `\r` before. After that it outputs the character on the VGA screen by calling the BIOS with the `0x10` interrupt call:
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@ -257,7 +268,7 @@ As you can read above, it uses the `fastcall` calling conventions like the `memc
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Generally `memset` is like a memcpy implementation. It saves the value of the `di` register on the stack and puts the `ax` value into `di` which is the address of the `biosregs` structure. Next is the `movzbl` instruction, which copies the `dl` value to the low 2 bytes of the `eax` register. The remaining 2 high bytes of `eax` will be filled with zeros.
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The next instruction multiplies `eax` with `0x01010101`. It needs to because `memset` will copy 4 bytes at the same time. For example we need to fill a structure with `0x7` with memset. `eax` will contain `0x00000007` value in this case. So if we multiply `eax` with `0x01010101`, we will get `0x07070707` and now we can copy these 4 bytes into the structure. `memset` uses `rep; stosl` instructions for copying `eax` into `es:di`.
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The next instruction multiplies `eax` with `0x01010101`. It needs to because `memset` will copy 4 bytes at the same time. For example, we need to fill a structure with `0x7` with memset. `eax` will contain `0x00000007` value in this case. So if we multiply `eax` with `0x01010101`, we will get `0x07070707` and now we can copy these 4 bytes into the structure. `memset` uses `rep; stosl` instructions for copying `eax` into `es:di`.
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The rest of the `memset` function does almost the same as `memcpy`.
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@ -268,7 +279,7 @@ Heap initialization
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After the stack and bss section were prepared in [header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S) (see previous [part](linux-bootstrap-1.md)), the kernel needs to initialize the [heap](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L116) with the [init_heap](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L116) function.
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First of all `init_heap` checks the `CAN_USE_HEAP` flag from the `loadflags` kernel setup header and calculates the end of the stack if this flag was set:
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First of all `init_heap` checks the [`CAN_USE_HEAP`](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/bootparam.h#L21) flag from the [`loadflags`](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L321) kernel setup header and calculates the end of the stack if this flag was set:
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```C
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char *stack_end;
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@ -289,7 +300,15 @@ CPU validation
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The next step as we can see is cpu validation by `validate_cpu` from [arch/x86/boot/cpu.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/cpu.c).
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It calls the `check_cpu` function and passes cpu level and required cpu level to it and checks that the kernel launched on the right cpu. It checks the cpu's flags, presence of [long mode](http://en.wikipedia.org/wiki/Long_mode) (which we will see more details on in the next parts) for x86_64, checks the processor's vendor and makes preparation for certain vendors like turning off SSE+SSE2 for AMD if they are missing, etc...
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It calls the [`check_cpu`](https://github.com/torvalds/linux/blob/master/arch/x86/boot/cpucheck.c#L102) function and passes cpu level and required cpu level to it and checks that the kernel launches on the right cpu level.
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```c
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check_cpu(&cpu_level, &req_level, &err_flags);
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if (cpu_level < req_level) {
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...
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return -1;
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}
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```
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It checks the cpu's flags, presence of [long mode](http://en.wikipedia.org/wiki/Long_mode) (which we will see in more detail in the next sections) in case of x86_64(64-bit) CPU, checks the processor's vendor and makes preparation for certain vendors like turning off SSE+SSE2 for AMD if they are missing, etc.
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Memory detection
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--------------------------------------------------------------------------------
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@ -334,7 +353,7 @@ You can see the result of this in the `dmesg` output, something like:
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Keyboard initialization
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--------------------------------------------------------------------------------
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The next step is the initialization of the keyboard with the call of the `keyboard_init` function. At first `keyboard_init` initializes registers using the `initregs` function and calling the [0x16](http://www.ctyme.com/intr/rb-1756.htm) interrupt for getting the keyboard status. After this it calls [0x16](http://www.ctyme.com/intr/rb-1757.htm) again to set repeat rate and delay.
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The next step is the initialization of the keyboard with the call of the [`keyboard_init()`](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L65) function. At first `keyboard_init` initializes registers using the `initregs` function and calling the [0x16](http://www.ctyme.com/intr/rb-1756.htm) interrupt for getting the keyboard status. After this it calls [0x16](http://www.ctyme.com/intr/rb-1757.htm) again to set repeat rate and delay.
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Querying
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--------------------------------------------------------------------------------
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@ -343,7 +362,7 @@ The next couple of steps are queries for different parameters. We will not dive
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The [query_mca](https://github.com/torvalds/linux/blob/master/arch/x86/boot/mca.c#L18) routine calls the [0x15](http://www.ctyme.com/intr/rb-1594.htm) BIOS interrupt to get the machine model number, sub-model number, BIOS revision level, and other hardware-specific attributes:
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```C
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```c
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int query_mca(void)
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{
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struct biosregs ireg, oreg;
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@ -398,7 +417,7 @@ Offset Size Description )
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Next we call the `set_fs` routine and pass the value of the `es` register to it. Implementation of `set_fs` is pretty simple:
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```C
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```c
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static inline void set_fs(u16 seg)
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{
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asm volatile("movw %0,%%fs" : : "rm" (seg));
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@ -423,7 +442,7 @@ Note that `query_apm_bios` will be executed only if `CONFIG_APM` or `CONFIG_APM_
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#endif
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```
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The last is the [query_edd](https://github.com/torvalds/linux/blob/master/arch/x86/boot/edd.c#L122) function, which asks `Enhanced Disk Drive` information from the BIOS. Let's look into the `query_edd` implementation.
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The last is the [`query_edd`](https://github.com/torvalds/linux/blob/master/arch/x86/boot/edd.c#L122) function, which asks `Enhanced Disk Drive` information from the BIOS. Let's look into the `query_edd` implementation.
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First of all it reads the [edd](https://github.com/torvalds/linux/blob/master/Documentation/kernel-parameters.txt#L1023) option from kernel's command line and if it was set to `off` then `query_edd` just returns.
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@ -465,3 +484,4 @@ Links
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* [APM](https://en.wikipedia.org/wiki/Advanced_Power_Management)
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* [EDD specification](http://www.t13.org/documents/UploadedDocuments/docs2004/d1572r3-EDD3.pdf)
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* [Previous part](linux-bootstrap-1.md)
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