OSTEP：通过分段管理内存

第十六章：分段

1. 这里记录一个样例作为例子，其他的答案则跳过重复的计算步骤

   运行第一个seed可以得到以下输出

   ARG seed 0
   ARG address space size 128
   ARG phys mem size 512
   
   Segment register information:
   
   Segment 0 base  (grows positive) : 0x00000000 (decimal 0)
   Segment 0 limit                  : 20
   
   Segment 1 base  (grows negative) : 0x00000200 (decimal 512)
   Segment 1 limit                  : 20
   
   Virtual Address Trace
   VA  0: 0x0000006c (decimal:  108) --> PA or segmentation violation?
   VA  1: 0x00000061 (decimal:   97) --> PA or segmentation violation?
   VA  2: 0x00000035 (decimal:   53) --> PA or segmentation violation?
   VA  3: 0x00000021 (decimal:   33) --> PA or segmentation violation?
   VA  4: 0x00000041 (decimal:   65) --> PA or segmentation violation?

   对于VA 0: 0x0000006c (decimal: 108)的情况计算如下

   1. 由Address Size为128(2^7)得到高位为第7位
   2. 将VA的0x6c(108)转为二进制，按7位来算则是1 101100，因此可以知道这是SEG 1的地址
   3. 栈是从最下方的内存反方向增加，VA最底部内存位为0x80(128)，因此0x6c(108)的VA对应偏移量为108 - 128 = -20
   4. VA中0x80(128)的地址对应PA为0x200(512)，因此按偏移量-20算可以得到VA中0x6c(108)对应的PA为0x1ec(492)

   

   答案

   Virtual Address Trace
   VA  0: 0x0000006c (decimal:  108) --> VALID in SEG1: 0x000001ec (decimal:  492)
   VA  1: 0x00000061 (decimal:   97) --> SEGMENTATION VIOLATION (SEG1)
   VA  2: 0x00000035 (decimal:   53) --> SEGMENTATION VIOLATION (SEG0)
   VA  3: 0x00000021 (decimal:   33) --> SEGMENTATION VIOLATION (SEG0)
   VA  4: 0x00000041 (decimal:   65) --> SEGMENTATION VIOLATION (SEG1)

2. 以第一题为参考；在虚拟地址中SEG 0的范围是0-19，SEG 1的范围是108-127，非法地址为20-107。

   以下为通过运行-A标志对分界点进行分别测试有如下结果

   Virtual Address Trace
     VA  0: 0x00000000 (decimal:    0) --> VALID in SEG0: 0x00000000 (decimal:    0)
     VA  1: 0x00000013 (decimal:   19) --> VALID in SEG0: 0x00000013 (decimal:   19)
     VA  2: 0x00000014 (decimal:   20) --> SEGMENTATION VIOLATION (SEG0)
     VA  3: 0x0000006b (decimal:  107) --> SEGMENTATION VIOLATION (SEG1)
     VA  4: 0x0000006c (decimal:  108) --> VALID in SEG1: 0x000001ec (decimal:  492)
     VA  5: 0x0000007f (decimal:  127) --> VALID in SEG1: 0x000001ff (decimal:  511)

3. 设置为以下指令即可

   ./segmentation.py -a 16 -p 128 -A  0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 --b0 0 --l0 2 --b1 16 --l1 1 -c

   输出结果

   Virtual Address Trace
     VA  0: 0x00000000 (decimal:    0) --> VALID in SEG0: 0x00000000 (decimal:    0)
     VA  1: 0x00000001 (decimal:    1) --> VALID in SEG0: 0x00000001 (decimal:    1)
     VA  2: 0x00000002 (decimal:    2) --> SEGMENTATION VIOLATION (SEG0)
     VA  3: 0x00000003 (decimal:    3) --> SEGMENTATION VIOLATION (SEG0)
     VA  4: 0x00000004 (decimal:    4) --> SEGMENTATION VIOLATION (SEG0)
     VA  5: 0x00000005 (decimal:    5) --> SEGMENTATION VIOLATION (SEG0)
     VA  6: 0x00000006 (decimal:    6) --> SEGMENTATION VIOLATION (SEG0)
     VA  7: 0x00000007 (decimal:    7) --> SEGMENTATION VIOLATION (SEG0)
     VA  8: 0x00000008 (decimal:    8) --> SEGMENTATION VIOLATION (SEG1)
     VA  9: 0x00000009 (decimal:    9) --> SEGMENTATION VIOLATION (SEG1)
     VA 10: 0x0000000a (decimal:   10) --> SEGMENTATION VIOLATION (SEG1)
     VA 11: 0x0000000b (decimal:   11) --> SEGMENTATION VIOLATION (SEG1)
     VA 12: 0x0000000c (decimal:   12) --> SEGMENTATION VIOLATION (SEG1)
     VA 13: 0x0000000d (decimal:   13) --> SEGMENTATION VIOLATION (SEG1)
     VA 14: 0x0000000e (decimal:   14) --> SEGMENTATION VIOLATION (SEG1)
     VA 15: 0x0000000f (decimal:   15) --> VALID in SEG1: 0x0000000f (decimal:   15)

4. 要让90%的地址可以被访问，则对于SEG 0的界限寄存器到SEG 1的界限寄存器中间地址差要为总虚拟地址的10%即可。

   $\frac{(b1 - l1 - (b0 + l0))}{Address Size} < 10\%$

5. 所以地址都失效代表没有可以访问的地址，因此l0和l1界限寄存器都设置为0即可