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Merge pull request #273 from ruthgrace/bootstrap_3_actual_transition_protected

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fixed grammar in linux-bootstrap-3.md, Actual transition into protected mode section
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0xAX 2015-11-02 13:39:38 +06:00
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Booting/linux-bootstrap-3.md
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@ -483,20 +483,20 @@ asm volatile("lgdtl %0" : : "m" (gdt));
Actual transition into protected mode
--------------------------------------------------------------------------------
It is the end of `go_to_protected_mode` function. We loaded IDT, GDT, disable interruptions and now can switch CPU into protected mode. The last step we call `protected_mode_jump` function with two parameters:
This is the end of the `go_to_protected_mode` function. We loaded IDT, GDT, disable interruptions and now can switch the CPU into protected mode. The last step is calling the `protected_mode_jump` function with two parameters:
```C
protected_mode_jump(boot_params.hdr.code32_start, (u32)&boot_params + (ds() << 4));
```
which is defined in the [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S#L26). It takes two parameters:
which is defined in [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S#L26). It takes two parameters:
* address of protected mode entry point
* address of `boot_params`
Let's look inside `protected_mode_jump`. As I wrote above, you can find it in the `arch/x86/boot/pmjump.S`. First parameter will be in `eax` register and second is in `edx`.
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 second is in `edx`.
First of all we put address of `boot_params` in the `esi` register and address of code segment register `cs` (0x1000) in the `bx`. After this we shift `bx` by 4 bits and add address of label `2` to it (we will have physical address of label `2` in the `bx` after it) and jump to label `1`. Next we put data segment and task state segment in the `cs` and `di` registers with:
First of all we put the address of `boot_params` in the `esi` register and the address of code segment register `cs` (0x1000) in `bx`. After this we shift `bx` by 4 bits and add the address of label `2` to it (we will have the physical address of label `2` in the `bx` after this) and jump to label `1`. Next we put data segment and task state segment in the `cs` and `di` registers with:
```assembly
movw $__BOOT_DS, %cx
@ -505,7 +505,7 @@ movw $__BOOT_TSS, %di
As you can read above `GDT_ENTRY_BOOT_CS` has index 2 and every GDT entry is 8 byte, so `CS` will be `2 * 8 = 16`, `__BOOT_DS` is 24 etc.
Next we set `PE` (Protection Enable) bit in the `CR0` control register:
Next we set the `PE` (Protection Enable) bit in the `CR0` control register:
```assembly
movl %cr0, %edx
@ -513,7 +513,7 @@ orb $X86_CR0_PE, %dl
movl %edx, %cr0
```
and make long jump to the protected mode:
and make a long jump to protected mode:
```assembly
.byte 0x66, 0xea
@ -522,7 +522,7 @@ and make long jump to the protected mode:
```
where
* `0x66` is the operand-size prefix which allows to mix 16-bit and 32-bit code,
* `0x66` is the operand-size prefix which allows us to mix 16-bit and 32-bit code,
* `0xea` - is the jump opcode,
* `in_pm32` is the segment offset
* `__BOOT_CS` is the code segment.
@ -534,7 +534,7 @@ After this we are finally in the protected mode:
.section ".text32","ax"
```
Let's look at the first steps in the protected mode. First of all we setup data segment with:
Let's look at the first steps in protected mode. First of all we set up the data segment with:
```assembly
movl %ecx, %ds
@ -544,7 +544,7 @@ movl %ecx, %gs
movl %ecx, %ss
```
If you read with attention, you can remember that we saved `$__BOOT_DS` in the `cx` register. Now we fill with it all segment registers besides `cs` (`cs` is already `__BOOT_CS`). Next we zero out all general purpose registers besides `eax` with:
If you paid attention, you can remember that we saved `$__BOOT_DS` in the `cx` register. Now we fill it with all segment registers besides `cs` (`cs` is already `__BOOT_CS`). Next we zero out all general purpose registers besides `eax` with:
```assembly
xorl %ecx, %ecx
@ -560,9 +560,9 @@ And jump to the 32-bit entry point in the end:
jmpl *%eax
```
Remember that `eax` contains address of the 32-bit entry (we passed it as first parameter into `protected_mode_jump`).
Remember that `eax` contains the address of the 32-bit entry (we passed it as first parameter into `protected_mode_jump`).
That's all we're in the protected mode and stop at it's entry point. What happens next, we will see in the next part.
That's all. We're in the protected mode and stop at it's entry point. We will see what happens next in the next part.
Conclusion
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