Now we need to do the last preparations before we can see transition to the 64-bit mode. At first we need to update Global Descriptor Table for this:
Now we need to do the last preparations before we can see the transition to 64-bit mode. At first we need to update the Global Descriptor Table for this:
```assembly
leal gdt(%ebp), %eax
@ -290,9 +290,9 @@ Now we need to do the last preparations before we can see transition to the 64-b
lgdt gdt(%ebp)
```
Here we put the address from `ebp` with `gdt` offset to `eax` register, next we put this address into `ebp` with offset `gdt+2` and load Global Descriptor Table with the `lgdt` instruction.
Here we put the address from `ebp` with `gdt` offset into the`eax` register, next we put this address into `ebp` with offset `gdt+2` and load the Global Descriptor Table with the `lgdt` instruction.
Let's look on Global Descriptor Table definition:
Let's look at the Global Descriptor Table definition:
```assembly
.data
@ -307,9 +307,9 @@ gdt:
.quad 0x0000000000000000 /* TS continued */
```
It defined in the same file in the `.data` section. It contains 5 descriptors: null descriptor, for kernel code segment, kernel data segment and two task descriptors. We already loaded GDT in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), we're doing almost the same here, but descriptors with `CS.L = 1` and `CS.D = 0` for execution in the 64 bit mode.
It is defined in the same file as the `.data` section. It contains 5 descriptors: null descriptor, for kernel code segment, kernel data segment and two task descriptors. We already loaded the GDT in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), we're doing almost the same here, but descriptors with `CS.L = 1` and `CS.D = 0` for execution in 64 bit mode.
After we have loaded Global Descriptor Table, we must enable [PAE](http://en.wikipedia.org/wiki/Physical_Address_Extension) mode with putting value of `cr4` register into `eax`, setting 5 bit in it and load it again in the`cr4` :
After we have loaded the Global Descriptor Table, we must enable [PAE](http://en.wikipedia.org/wiki/Physical_Address_Extension) mode by putting the value of the `cr4` register into `eax`, setting 5 bit in it and loading it again into`cr4` :
```assembly
movl %cr4, %eax
@ -317,7 +317,7 @@ After we have loaded Global Descriptor Table, we must enable [PAE](http://en.wik
movl %eax, %cr4
```
Now we finished almost with all preparations before we can move into 64-bit mode. The last step is to build page tables, but before some information about long mode.
Now we are almost finished with all preparations before we can move into 64-bit mode. The last step is to build page tables, but before that, here is some information about long mode.
0xAX
9 years ago