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fix segmenta address in boot 1

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Thank you to @changuoqiang and @PinoTsao

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
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Alexander Kuleshov 2019-03-09 00:00:31 +06:00
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12
Booting/linux-bootstrap-1.md
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@ -28,7 +28,7 @@ The Magical Power Button, What happens next?
Although this is a series of posts about the Linux kernel, we will not be starting directly from the kernel code - at least not, in this paragraph. As soon as you press the magical power button on your laptop or desktop computer, it starts working. The motherboard sends a signal to the [power supply](https://en.wikipedia.org/wiki/Power_supply) device. After receiving the signal, the power supply provides the proper amount of electricity to the computer. Once the motherboard receives the [power good signal](https://en.wikipedia.org/wiki/Power_good_signal), it tries to start the CPU. The CPU resets all leftover data in its registers and sets up predefined values for each of them.
The [80386](https://en.wikipedia.org/wiki/Intel_80386) CPU and later define the following predefined data in CPU registers after the computer resets:
The [80386](https://en.wikipedia.org/wiki/Intel_80386) CPU and later CPUs define the following predefined data in CPU registers after the computer resets:
```
IP 0xfff0
@ -329,11 +329,11 @@ state.gs = state.fs = state.es = state.ds = state.ss = segment;
state.cs = segment + 0x20;
```
In my case, the kernel is loaded at `0x10000` address. This means that segment registers will have the following values after kernel setup starts:
In my case, the kernel is loaded at `0x10000` physical address. This means that segment registers will have the following values after kernel setup starts:
```
gs = fs = es = ds = ss = 0x10000
cs = 0x10200
gs = fs = es = ds = ss = 0x1000
cs = 0x1020
```
After the jump to `start_of_setup`, the kernel needs to do the following:
@ -356,7 +356,7 @@ First of all, the kernel ensures that the `ds` and `es` segment registers point
cld
```
As I wrote earlier, `grub2` loads kernel setup code at address `0x10000` by default and `cs` at `0x10200` because execution doesn't start from the start of file, but from the jump here:
As I wrote earlier, `grub2` loads kernel setup code at address `0x10000` by default and `cs` at `0x1020` because execution doesn't start from the start of file, but from the jump here:
```assembly
_start:
@ -364,7 +364,7 @@ _start:
.byte start_of_setup-1f
```
which is at a `512` byte offset from [4d 5a](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L46). We also need to align `cs` from `0x10200` to `0x10000`, as well as all other segment registers. After that, we set up the stack:
which is at a `512` byte offset from [4d 5a](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L46). We also need to align `cs` from `0x1020` to `0x1000`, as well as all other segment registers. After that, we set up the stack:
```assembly
pushw %ds