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boot-1: clarify boot address

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Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
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Alexander Kuleshov 2018-05-04 23:46:09 +06:00
parent efed15d860
commit 5d643f8e9f
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2 changed files with 5 additions and 5 deletions
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4
Booting/linux-bootstrap-1.md
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@ -329,15 +329,13 @@ state.gs = state.fs = state.es = state.ds = state.ss = segment;
state.cs = segment + 0x20;
```
This means that segment registers will have the following values after kernel setup starts:
In my case, the kernel is loaded at `0x10000` address. This means that segment registers will have the following values after kernel setup starts:
```
gs = fs = es = ds = ss = 0x10000
cs = 0x10200
```
In my case, the kernel is loaded at `0x10000` address.
After the jump to `start_of_setup`, the kernel needs to do the following:
* Make sure that all segment register values are equal

6
Booting/linux-bootstrap-3.md
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@ -509,7 +509,9 @@ It takes two parameters:
Let's look inside `protected_mode_jump`. As I wrote above, you can find it in `arch/x86/boot/pmjump.S`. The first parameter will be in the `eax` register and the second one is in `edx`.
First of all, we put the address of `boot_params` in the `esi` register and the address of the code segment register `cs` (0x1000) in `bx`. After this, we shift `bx` by 4 bits and add it to the memory location labeled `2` (which is `bx << 4 + in_pm32`, the physical address to jump after transitioned to 32-bit mode) and jump to label `1`. Next we put the data segment and the task state segment in the `cx` and `di` registers with:
First of all, we put the address of `boot_params` in the `esi` register and the address of the code segment register `cs` in `bx`. After this, we shift `bx` by 4 bits and add it to the memory location labeled `2` (which is `(cs << 4) + in_pm32`, the physical address to jump after transitioned to 32-bit mode) and jump to label `1`. So after this `in_pm32` in label `2` will be overwritten with `(cs << 4) + in_pm32`.
Next we put the data segment and the task state segment in the `cx` and `di` registers with:
```assembly
movw $__BOOT_DS, %cx
@ -538,7 +540,7 @@ where:
* `0x66` is the operand-size prefix which allows us to mix 16-bit and 32-bit code
* `0xea` - is the jump opcode
* `in_pm32` is the segment offset
* `in_pm32` is the segment offset or `(cs << 4) + in_pm`
* `__BOOT_CS` is the code segment we want to jump to.
After this we are finally in protected mode: