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.
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.
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.
Memory segmentation was completely redone in protected mode. There are no 64 kilobyte fixed-size segments. All memory segments are described by the `Global Descriptor Table` (GDT) instead of segment registers. The GDT is a structure which resides in memory. There is no fixed place for it in memory, but its address is stored in the special `GDTR` register. Later we will see the GDT loading in the linux kernel code. There will be an operation for loading it into memory, something like:
where the `lgdt` instruction loads the base address and limit of global descriptor table to the `GDTR` register. `GDTR` is a 48-bit register and consists of two parts:
3. Type (40-47 bits) defines the type of segment and kinds of access to it. Next `S` flag specifies descriptor type. if `S` is 0 then this segment is a system segment, whereas if `S` is 1 then this is a code or data segment (Stack segments are data segments which must be read/write segments). If the segment is a code or data segment, it can be one of the following access types:
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.
Segment registers don't contain the base address of the segment as in real mode. Instead they contain a special structure - `segment selector`. `Selector` is a 16-bit structure:
Where `Index` shows the index number of the descriptor in the descriptor table. `TI` shows where to search for the descriptor: in the global descriptor table or local. And `RPL` is the privilege level.
Every segment register has a visible and hidden part. When a selector is loaded into one of the segment registers, it will be stored into the visible part. The hidden part contains the base address, limit and access information of the descriptor which pointed to the selector. The following steps are needed to get the physical address in the protected mode:
We will see the transition to protected mode in the linux kernel in the next part, but before we can move to protected mode, we need to do some preparations.
Let's look at [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). We can see some routines there which perform keyboard initialization, heap initialization, etc... Let's take a look.
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).
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:
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
2. Updates pointer to the kernel command line if the kernel was loaded with the old command line protocol.
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:
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.
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:
*`ax` will contain the address of the `boot_params.hdr` in bytes
*`dx` will contain the address of `hdr` in bytes
*`cx` will contain the size of `hdr` 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 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.
After the `hdr` is copied into `boot_params.hdr`, the next step is console initialization by calling the `console_init` function which is defined in [arch/x86/boot/early_serial_console.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/early_serial_console.c).
It tries to find the `earlyprintk` option in the command line and if the search was successful, it parses the port address and baud rate of the serial port and initializes the serial port. Value of `earlyprintk` command line option can be one of the:
The definition of `puts` is in [tty.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/tty.c). As we can see it prints character by character in a loop by calling The `putchar` function. Let's look into the `putchar` implementation:
`__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).
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:
Here `initregs` takes the `biosregs` structure and first fills `biosregs` with zeros using the `memset` function and then fills it with register values.
As you can read above, it uses the `fastcall` calling conventions like the `memcpy` function, which means that the function gets parameters from `ax`, `dx` and `cx` registers.
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.
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`.
After that `biosregs` structure is filled with `memset`, `bios_putchar` calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt which prints a character. Afterwards it checks if the serial port was initialized or not and writes a character there with [serial_putchar](https://github.com/torvalds/linux/blob/master/arch/x86/boot/tty.c#L30) and `inb/outb` instructions if it was set.
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.
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:
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).
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.
```c
check_cpu(&cpu_level, &req_level, &err_flags);
if (cpu_level <req_level){
...
return -1;
}
```
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.
The next step is memory detection by the `detect_memory` function. It uses different programming interfaces for memory detection like `0xe820`, `0xe801` and `0x88`. We will see only the implementation of 0xE820 here. Let's look into the `detect_memory_e820` implementation from the [arch/x86/boot/memory.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/memory.c) source file. First of all, the `detect_memory_e820` function initializes the `biosregs` structure as we saw above and fills registers with special values for the `0xe820` call:
The `ax` register must contain the number of the function (0xe820 in our case), `cx` register contains size of the buffer which will contain data about memory, `edx` must contain the `SMAP` magic number, `es:di` must contain the address of the buffer which will contain memory data and `ebx` has to be zero.
Next is a loop where data about the memory will be collected. It starts from the call of the 0x15 bios interrupt, which writes one line from the address allocation table. For getting the next line we need to call this interrupt again (which we do in the loop). Before the next call `ebx` must contain the value returned previously:
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.
The next couple of steps are queries for different parameters. We will not dive into details about these queries, but will get back to it in later parts. Let's take a short look at these functions:
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:
It fills the `ah` register with `0xc0` and calls the `0x15` BIOS interruption. After the interrupt execution it checks the [carry flag](http://en.wikipedia.org/wiki/Carry_flag) and if it is set to 1, the BIOS doesn't support `MCA`. If carry flag is set to 0, `ES:BX` will contain a pointer to the system information table, which looks like this:
There is inline assembly which gets the value of the `seg` parameter and puts it into the `fs` register. There are many functions in [boot.h](https://github.com/torvalds/linux/blob/master/arch/x86/boot/boot.h) like `set_fs`, for example `set_gs`, `fs`, `gs` for reading a value in it etc...
The next step is getting [Intel SpeedStep](http://en.wikipedia.org/wiki/SpeedStep) information by calling the `query_ist` function. First of all it checks the CPU level and if it is correct, calls `0x15` for getting info and saves the result to `boot_params`.
The following [query_apm_bios](https://github.com/torvalds/linux/blob/master/arch/x86/boot/apm.c#L21) function gets [Advanced Power Management](http://en.wikipedia.org/wiki/Advanced_Power_Management) information from the BIOS. `query_apm_bios` calls the `0x15` BIOS interruption too, but with `ah` - `0x53` to check `APM` installation. After the `0x15` execution, `query_apm_bios` functions checks `PM` signature (it must be `0x504d`), carry flag (it must be 0 if `APM` supported) and value of the `cx` register (if it's 0x02, protected mode interface is supported).
Next it calls the `0x15` again, but with `ax = 0x5304` for disconnecting the `APM` interface and connect the 32bit protected mode interface. In the end it fills `boot_params.apm_bios_info` with values obtained from the BIOS.
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.
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.
where `0x80` is the first hard drive and the `EDD_MBR_SIG_MAX` macro is 16. It collects data into the array of [edd_info](https://github.com/torvalds/linux/blob/master/include/uapi/linux/edd.h#L172) structures. `get_edd_info` checks that EDD is present by invoking the `0x13` interrupt with `ah` as `0x41` and if EDD is present, `get_edd_info` again calls the `0x13` interrupt, but with `ah` as `0x48` and `si` containing the address of the buffer where EDD information will be stored.
This is the end of the second part about linux kernel internals. In the next part we will see video mode setting and the rest of preparations before transition to protected mode and directly transitioning into it.
**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 a PR to [linux-internals](https://github.com/0xAX/linux-internals).**
