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Merge pull request #21 from jkramarz/master

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Few small fixes
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0xAX 2015-01-05 12:21:31 +06:00
commit bf773eaf4e
1 changed files with 4 additions and 4 deletions
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linux-bootstrap-1.md
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@ -64,7 +64,7 @@ Ok, now we know about real mode and memory addressing, let's get back to registe
0xffff0000:0xfff0
```
which we can translate to the physical address::
which we can translate to the physical address:
```python
>>> hex((0xffff000 << 4) + 0xfff0)
@ -132,7 +132,7 @@ We will see:
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 [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.
Real world boot loader starts at the same point, ends with `0xaa55` bytes, but reads kernel code from device, loads it to memory, parses and passes boot parameters to kernel and etc... instead of printing one symbol :) Ok, so, from this moment bios handed control to the operating system bootloader and we can go ahead.
Real world boot loader starts at the same point, ends with `0xaa55` bytes, but reads kernel code from device, loads it to memory, parses and passes boot parameters to kernel and etc... instead of printing one symbol :) Ok, so, from this moment BIOS handed control to the operating system bootloader and we can go ahead.
**NOTE**: as you can read above CPU is in real mode. In real mode for calculating physical address in memory uses following form:
@ -176,7 +176,7 @@ At the start of execution BIOS is not in RAM, it is located in ROM.
Bootloader
--------------------------------------------------------------------------------
Now bios transfered control to the operating system bootloader and it needs to load operating system into the memory. There are a couple of bootloaders which can boot linux, like: [Grub2](http://www.gnu.org/software/grub/), [syslinux](http://www.syslinux.org/wiki/index.php/The_Syslinux_Project) and etc... Linux kernel has [Boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt) which describes how to load linux kernel.
Now BIOS transfered control to the operating system bootloader and it needs to load operating system into the memory. There are a couple of bootloaders which can boot linux, like: [Grub2](http://www.gnu.org/software/grub/), [syslinux](http://www.syslinux.org/wiki/index.php/The_Syslinux_Project) and etc... Linux kernel has [Boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt) which describes how to load linux kernel.
Let us briefly consider how grub loads linux. GRUB2 execution starts from `grub-core/boot/i386/pc/boot.S`. It starts to load from device its own kernel (not to be confused with linux kernel) and executes `grub_main` after successfully loading.
@ -446,7 +446,7 @@ Ok now we have correct segment registers, stack, need only setup bss and jump to
rep; stosl
```
First of all we put [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address in `di` and `_end + 3` (+3 - align to 4 bytes) in `cx`. Clear `eax` register with `xor` instruction and calculate size of BSS section (put in `cx`). Devide `cx` by 4 and repeat `cx` times `stosl` instruction which stores value of `eax` (it is zero) and increase `di`by the size of `eax`. In this way, we write zeros from `__bss_start` to `_end`:
First of all we put [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address in `di` and `_end + 3` (+3 - align to 4 bytes) in `cx`. Clear `eax` register with `xor` instruction and calculate size of BSS section (put in `cx`). Divide `cx` by 4 and repeat `cx` times `stosl` instruction which stores value of `eax` (it is zero) and increase `di`by the size of `eax`. In this way, we write zeros from `__bss_start` to `_end`:
![bss](http://oi59.tinypic.com/29m2eyr.jpg)