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Merge pull request #262 from ruthgrace/bootstrap_4_transition_lon

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fixed grammar in linux-bootstrap-4.md, Transition to long mode section
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0xAX 2015-10-20 13:16:56 +06:00
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Booting/linux-bootstrap-4.md
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@ -430,10 +430,10 @@ Our early page table structure are done, it maps 4 gigabytes of memory and now w
That's all. Now we can see transition to the long mode.
Transition to the long mode
Transition to long mode
--------------------------------------------------------------------------------
First of all we need to set `EFER.LME` flag in the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) to `0xC0000080`:
First of all we need to set the `EFER.LME` flag in the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) to `0xC0000080`:
```assembly
movl $MSR_EFER, %ecx
@ -442,16 +442,16 @@ First of all we need to set `EFER.LME` flag in the [MSR](http://en.wikipedia.org
wrmsr
```
Here we put `MSR_EFER` flag (which defined in the [arch/x86/include/uapi/asm/msr-index.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/msr-index.h#L7)) to the `ecx` register and call `rdmsr` instruction which reads [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executed, we will have result data in the `edx:eax` which depends on `ecx` value. We check `EFER_LME` bit with `btsl` instruction and write data from `eax` to the `MSR` register with `wrmsr` instruction.
Here we put the `MSR_EFER` flag (which is defined in [arch/x86/include/uapi/asm/msr-index.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/msr-index.h#L7)) in the `ecx` register and call `rdmsr` instruction which reads the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executes, we will have the resulting data in `edx:eax` which depends on the `ecx` value. We check the `EFER_LME` bit with the `btsl` instruction and write data from `eax` to the `MSR` register with the `wrmsr` instruction.
In next step we push address of the kernel segment code to the stack (we defined it in the GDT) and put address of the `startup_64` routine to the `eax`.
In the next step we push the address of the kernel segment code to the stack (we defined it in the GDT) and put the address of the `startup_64` routine in `eax`.
```assembly
pushl $__KERNEL_CS
leal startup_64(%ebp), %eax
```
After this we push this address to the stack and enable paging with setting `PG` and `PE` bits in the `cr0` register:
After this we push this address to the stack and enable paging by setting `PG` and `PE` bits in the `cr0` register:
```assembly
movl $(X86_CR0_PG | X86_CR0_PE), %eax
@ -464,9 +464,9 @@ and call:
lret
```
Remember that we pushed address of the `startup_64` function to the stack in the previous step, and after `lret` instruction, CPU extracts address of it and jumps there.
Remember that we pushed the address of the `startup_64` function to the stack in the previous step, and after the `lret` instruction, the CPU extracts the address of it and jumps there.
After all of these steps we're finally in the 64-bit mode:
After all of these steps we're finally in 64-bit mode:
```assembly
.code64