add lab answers

labcodes_answer/lab4_result/kern/init/entry.S

@@ -0,0 +1,49 @@
+#include <mmu.h>
+#include <memlayout.h>
+
+#define REALLOC(x) (x - KERNBASE)
+
+.text
+.globl kern_entry
+kern_entry:
+    # reload temperate gdt (second time) to remap all physical memory
+    # virtual_addr 0~4G=linear_addr&physical_addr -KERNBASE~4G-KERNBASE 
+    lgdt REALLOC(__gdtdesc)
+    movl $KERNEL_DS, %eax
+    movw %ax, %ds
+    movw %ax, %es
+    movw %ax, %ss
+
+    ljmp $KERNEL_CS, $relocated
+
+relocated:
+
+    # set ebp, esp
+    movl $0x0, %ebp
+    # the kernel stack region is from bootstack -- bootstacktop,
+    # the kernel stack size is KSTACKSIZE (8KB)defined in memlayout.h
+    movl $bootstacktop, %esp
+    # now kernel stack is ready , call the first C function
+    call kern_init
+
+# should never get here
+spin:
+    jmp spin
+
+.data
+.align PGSIZE
+    .globl bootstack
+bootstack:
+    .space KSTACKSIZE
+    .globl bootstacktop
+bootstacktop:
+
+.align 4
+__gdt:
+    SEG_NULL
+    SEG_ASM(STA_X | STA_R, - KERNBASE, 0xFFFFFFFF)      # code segment
+    SEG_ASM(STA_W, - KERNBASE, 0xFFFFFFFF)              # data segment
+__gdtdesc:
+    .word 0x17                                          # sizeof(__gdt) - 1
+    .long REALLOC(__gdt)
+