1. 9.7 Why are page sizes powers of two?

So you can compute the page number and offset using just shift and mask operations

2. 9.9 Why can’t a process access memory it does not own? How could the operating system allow access to other memory? Why should it or should it not?
   
   

Because every address a process uses is translated by its page table, so it can’t “utter” an address in another process. The operating system could allow it access by:

1. Setting up entries for the physical pages use by other processes in its page table

2. Providing a system call for reading/writing from other processes

This is probably a good idea – it allows cooperating processes to communicate very quickly and cheaply.

3. 9.11 What is the effect of allowing two entries in a page table to point to the same page frame in memory? Explain how you could use this to copy memory. What is the effect of updating a byte in one page?
   The effect is that two pages in a single process refer to the same physical page. Hence, if you change a byte at one address, a byte at the same offset in the other page will change. This can be used to perform copies quickly by mapping a page read-only at two addresses. If it is modified, then the copy can be done later (if at all).

4. 9.14 Explain why it is easier to share a reentrant module using segmentation than it is to do so when pure paging is used
With segmentation, you only need a small amount of information – the segment descriptor – to share, instead of changing protection on a large number of pages. In addition, you don’t have to worry about mapping the code to the same address – the addresses within the code are relative to the segment, so it can be relocated more easily.

5. 9.17 Consider the Intel address-translation scheme shown in figure 9.21

· Describe all the steps taken to translate a virtual address to a physical address
The address is split into a segment descriptor and an offset. The descriptor indexes into the segment table (either global or local), which produces an address that is added to the offset to produce the linear address. This linear address is translated in a two level page table – the top 10 bits in the page global directory, and the middle 10 bits in the page table, which produces a physical page number that is added to the lower 12 bits of the address to produce the physical address.

· What are the advantages to the OS of hardware that provides such complex memory-translation hardware?

The advantage is that the OS can provide protection both through segmentation and paging – it doesn’t have to choose just one.

· Are there any disadvantages? If so, what? If not, why doesn’t everybody do it?

The complexity makes it slow and inflexible – OS’s can’t choose a page table format or handle TLB misses. The OS had to be more complex to manage both segments and page tables.