add some exercises for lab0

related_info/lab0/lab0_ex2.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+int
+main(void)
+{
+    printf("Hello, world!\n");
+    return 0;
+}

related_info/lab0/lab0_ex2.md

@@ -0,0 +1,6 @@
+#README
+Try below command
+```
+$gcc -g -m32 lab0_ex2.c
+```
+Then you will get a.out. Try to use gdb to debug lab0_ex2. 

related_info/lab0/lab0_ex3.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+
+#define STS_IG32        0xE            // 32-bit Interrupt Gate
+#define STS_TG32        0xF            // 32-bit Trap Gate
+
+typedef unsigned uint32_t;
+
+#define SETGATE(gate, istrap, sel, off, dpl) {            \
+    (gate).gd_off_15_0 = (uint32_t)(off) & 0xffff;        \
+    (gate).gd_ss = (sel);                                \
+    (gate).gd_args = 0;                                    \
+    (gate).gd_rsv1 = 0;                                    \
+    (gate).gd_type = (istrap) ? STS_TG32 : STS_IG32;    \
+    (gate).gd_s = 0;                                    \
+    (gate).gd_dpl = (dpl);                                \
+    (gate).gd_p = 1;                                    \
+    (gate).gd_off_31_16 = (uint32_t)(off) >> 16;        \
+}
+
+ /* Gate descriptors for interrupts and traps */
+ struct gatedesc {
+    unsigned gd_off_15_0 : 16;        // low 16 bits of offset in segment
+    unsigned gd_ss : 16;            // segment selector
+    unsigned gd_args : 5;            // # args, 0 for interrupt/trap gates
+    unsigned gd_rsv1 : 3;            // reserved(should be zero I guess)
+    unsigned gd_type : 4;            // type(STS_{TG,IG32,TG32})
+    unsigned gd_s : 1;                // must be 0 (system)
+    unsigned gd_dpl : 2;            // descriptor(meaning new) privilege level
+    unsigned gd_p : 1;                // Present
+    unsigned gd_off_31_16 : 16;        // high bits of offset in segment
+ };
+ 
+int
+main(void)
+{
+    unsigned before;
+    unsigned intr;
+    unsigned after;
+    struct gatedesc gintr;
+    
+    intr=8;
+    before=after=0;
+
+    gintr=*((struct gatedesc *)&intr);
+    SETGATE(gintr, 0,1,2,3);
+    intr=*(unsigned *)&(gintr);
+    printf("intr is 0x%x\n",intr);
+    printf("gintr is 0x%llx\n",gintr);
+    
+    return 0;
+}

related_info/lab0/lab0_ex3.md

@@ -0,0 +1,53 @@
+#README
+Try below command
+```
+gcc -g -m32 lab0_ex3.c 2>&1|tee make.log
+```
+If you get gcc's error, try to read make.log and fix the bugs.
+
+If gcc successed, then you will get a.out. 
+
+Try to answer below question. 
+
+对于如下的代码段，
+```
+...
+#define STS_CG32        0xC            // 32-bit Call Gate
+#define STS_IG32        0xE            // 32-bit Interrupt Gate
+
+#define SETGATE(gate, istrap, sel, off, dpl) {            \
+    (gate).gd_off_15_0 = (uint32_t)(off) & 0xffff;        \
+    (gate).gd_ss = (sel);                                \
+    (gate).gd_args = 0;                                    \
+    (gate).gd_rsv1 = 0;                                    \
+    (gate).gd_type = (istrap) ? STS_TG32 : STS_IG32;    \
+    (gate).gd_s = 0;                                    \
+    (gate).gd_dpl = (dpl);                                \
+    (gate).gd_p = 1;                                    \
+    (gate).gd_off_31_16 = (uint32_t)(off) >> 16;        \
+}
+
+ /* Gate descriptors for interrupts and traps */
+ struct gatedesc {
+    unsigned gd_off_15_0 : 16;        // low 16 bits of offset in segment
+    unsigned gd_ss : 16;            // segment selector
+    unsigned gd_args : 5;            // # args, 0 for interrupt/trap gates
+    unsigned gd_rsv1 : 3;            // reserved(should be zero I guess)
+    unsigned gd_type : 4;            // type(STS_{TG,IG32,TG32})
+    unsigned gd_s : 1;                // must be 0 (system)
+    unsigned gd_dpl : 2;            // descriptor(meaning new) privilege level
+    unsigned gd_p : 1;                // Present
+    unsigned gd_off_31_16 : 16;        // high bits of offset in segment
+ };
+ 
+... 
+
+```
+如果在其他代码段中有如下语句，
+```
+unsigned intr;
+intr=8;
+SETGATE(intr, 0,1,2,3);
+```
+请问执行上述指令后， intr的值是多少？
+