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` 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`.
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`.
```assembly
GLOBAL(protected_mode_jump)
movl %edx, %esi # Pointer to boot_params table
xorl %ebx, %ebx
movw %cs, %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`.
```assembly
shll $4, %ebx
addl %ebx, 2f # Add %ebx to the value stored at label 2
jmp 1f # Short jump to serialize on 386/486
```
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
movw $__BOOT_TSS, %di
movw $__BOOT_DS, %cx
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.
@ -529,9 +547,9 @@ As you can read above `GDT_ENTRY_BOOT_CS` has index 2 and every GDT entry is 8 b
Next, we set the `PE` (Protection Enable) bit in the `CR0` control register:
0xAX
3 years ago
committed by