Merge pull request #753 from Mutated1994/patch-2

feat: Add some comments

Initialization/linux-initialization-2.md

@@ -115,7 +115,11 @@ Where:
 * `Offset` - is offset to entry point of an interrupt handler;
 * `DPL` -    Descriptor Privilege Level;
 * `P` -      Segment Present flag;
-* `Segment selector` - a code segment selector in GDT or LDT
+* `Segment selector` - a code segment selector in GDT or LDT (actually in linux, it must point to a valid descriptor in your GDT.)
+```C
+#define __KERNEL_CS	(GDT_ENTRY_KERNEL_CS*8) // 0000 0000 0001 0000
+#define GDT_ENTRY_KERNEL_CS 2
+```
 * `IST` -    provides ability to switch to a new stack for interrupts handling.
 
 And the last `Type` field describes type of the `IDT` entry. There are three different kinds of gates for interrupts:
@@ -164,7 +168,7 @@ where `NUM_EXCEPTION_VECTORS` expands to `32`. As we can see, We're filling only
 
 and inserts an interrupt gate to the `IDT` table which is represented by the `&idt_descr` array. 
 
-The `early_idt_handler_array` array is declaredd in the [arch/x86/include/asm/segment.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/segment.h) header file and contains addresses of the first `32` exception handlers:
+The `early_idt_handler_array` array is declared in the [arch/x86/include/asm/segment.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/segment.h) header file and contains addresses of the first `32` exception handlers:
 
 ```C
 #define EARLY_IDT_HANDLER_SIZE   9
@@ -332,15 +336,39 @@ Next we save general registers on the stack:
 	UNWIND_HINT_REGS
 ```
 
+Okay, now the stack contains following data:
+```
+High |-------------------------|
+     | %rflags                 |
+     | %cs                     |
+     | %rip                    |
+     | error code              |
+     | %rdi                    |
+     | %rsi                    |
+     | %rdx                    |
+     | %rax                    |
+     | %r8                     |
+     | %r9                     |
+     | %r10                    |
+     | %r11                    |
+     | %rbx                    |
+     | %rbp                    |
+     | %r12                    |
+     | %r13                    |
+     | %r14                    |
+     | %r15                    |<-- %rsp
+Low  |-------------------------|
+```
+
 We need to do it to prevent wrong values of registers when we return from the interrupt handler. After this we check the vector number, and if it is `#PF` or [Page Fault](https://en.wikipedia.org/wiki/Page_fault), we put value from the `cr2` to the `rdi` register and call `early_make_pgtable` (we'll see it soon):
 
 ```assembly
-	cmpq $14,%rsi
+	cmpq $14,%rsi            /* Page fault? */
 	jnz 10f
 	GET_CR2_INTO(%rdi)
 	call early_make_pgtable
-	andl %eax,%eax
-	jz 20f
+	andl %eax,%eax           /* It is more efficient, the opcode is shorter than movl 1, %eax, only 2 bytes. */
+	jz 20f                   /* All good */
 ```
 
 otherwise we call `early_fixup_exception` function by passing kernel stack pointer:

contributors.md

@@ -131,3 +131,4 @@ Thank you to all contributors:
 * [Sebastian Fricke](https://github.com/initBasti)
 * [Zhouyi Zhou](https://github.com/zhouzhouyi-hub)
 * [Mingzhe Yang](https://github.com/Mutated1994)
+* [Yuxin Wu](https://github.com/chaffz)