OSTEP Chapter 16

1. Segmentation

Segment    VA    Base  Size
-------  ------  ----  ----
   Code   0-2K    32K   2K
   Heap   4-7K    34K   3K (grows positive)
  Stack  16-14K   28K   2K (grows negative)

2. Address Translation Examples

Segment    VA     Base  Size
-------  ------  -----  ----
   Code   0-2K   32768   2K
   Heap   4-7K   34816   3K (grows positive) [4K = 4096] [34816 + 3K = 37888]
  Stack  16-14K  28672   2K (grows negative) [16K = 16384]

Virtual Address 100 (Code) --> Physical Address 32768 + 100 = 32868

Virtual Address 4200 (Heap) --> Physical Address 34816 + (4200 - 4096) = 34920

Virtual Address 15360 (Stack) --> Physical Address 28672 - (16384 - 15360) = 27648

Segmentation Violation = Segmentation Fault = Illegal Virtual Address:

  Virtual Address 8000 (Heap) --> Physical Address 34816 + (8000 - 4096) = 38720 ≥ 37888

3. Segment Mapping Examples

16K virtual address space --> 14-bit virtual address

max segment size 4K --> 12-bit offset

2-bit segment number

4200 = 01 0000 0110 1000

specify SEG_MASK, SEG_SHIFT, and OFFSET_MASK: ________________

4. Segment Options and Protection

6. Exercises

Exercises from the book using segmentation.py:

1. First let's use a tiny address space to translate some addresses. Here's a simple set of parameters with a few different random seeds; can you translate the addresses?

 segmentation.py -a 128 -p 512 -b 0 -l 20 -B 512 -L 20 -s 0
 segmentation.py -a 128 -p 512 -b 0 -l 20 -B 512 -L 20 -s 1
 segmentation.py -a 128 -p 512 -b 0 -l 20 -B 512 -L 20 -s 2

2. Now, let's see if we understand this tiny address space we've constructed (using the parameters from the question above). What is the highest legal virtual address in segment 0? What about the lowest legal virtual address in segment 1? What are the lowest and highest illegal addresses in this entire address space? Finally, how would you run segmentation.py with the -A flag to test if you are right?