add lab answers

labcodes_answer/lab4_result/kern/trap/trapentry.S

@@ -0,0 +1,49 @@
+#include <memlayout.h>
+
+# vectors.S sends all traps here.
+.text
+.globl __alltraps
+__alltraps:
+    # push registers to build a trap frame
+    # therefore make the stack look like a struct trapframe
+    pushl %ds
+    pushl %es
+    pushl %fs
+    pushl %gs
+    pushal
+
+    # load GD_KDATA into %ds and %es to set up data segments for kernel
+    movl $GD_KDATA, %eax
+    movw %ax, %ds
+    movw %ax, %es
+
+    # push %esp to pass a pointer to the trapframe as an argument to trap()
+    pushl %esp
+
+    # call trap(tf), where tf=%esp
+    call trap
+
+    # pop the pushed stack pointer
+    popl %esp
+
+    # return falls through to trapret...
+.globl __trapret
+__trapret:
+    # restore registers from stack
+    popal
+
+    # restore %ds, %es, %fs and %gs
+    popl %gs
+    popl %fs
+    popl %es
+    popl %ds
+
+    # get rid of the trap number and error code
+    addl $0x8, %esp
+    iret
+
+.globl forkrets
+forkrets:
+    # set stack to this new process's trapframe
+    movl 4(%esp), %esp
+    jmp __trapret