Correct description of btsl $_EFER_LME, %eax

The btsl not only tests the bit, it tests the bit
by transfering it into the carry flag and then sets
it as well. Otherwise the wrmsr would not make sense.

Signed-off-by: Andreas J. Reichel <webmaster@6th-dimension.com>

Booting/linux-bootstrap-4.md

@@ -568,7 +568,7 @@ First of all we need to set the `EFER.LME` flag in the [MSR](http://en.wikipedia
 	wrmsr
 ```
 
-Here we put the `MSR_EFER` flag (which is defined in [arch/x86/include/asm/msr-index.h](https://github.com/torvalds/linux/blob/v4.16/arch/x86/include/asm/msr-index.h)) in the `ecx` register and execute the `rdmsr` instruction which reads the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executes, the resulting data is stored in `edx:eax` according to the `MSR` register specified in `ecx`. We check the `EFER_LME` bit with the `btsl` instruction and write data from `edx:eax` back to the `MSR` register with the `wrmsr` instruction.
+Here we put the `MSR_EFER` flag (which is defined in [arch/x86/include/asm/msr-index.h](https://github.com/torvalds/linux/blob/v4.16/arch/x86/include/asm/msr-index.h)) in the `ecx` register and execute the `rdmsr` instruction which reads the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executes, the resulting data is stored in `edx:eax` according to the `MSR` register specified in `ecx`. We check the current `EFER_LME` bit, transfer it into the carry flag and update the bit, all with the `btsl` instruction. Then we write data from `edx:eax` back to the `MSR` register with the `wrmsr` instruction.
 
 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`.