Why on a system with paging a process Cannot access memory that it does not own?

which clearly depends heavily on p! Even if only one access in 1000 causes a page fault, the effective access time drops from 200 nanoseconds to 8.2 microseconds, a slowdown of a factor of 40 times. In order to keep the slowdown less than 10%, the page fault rate must be less than 0.0000025, or one in 399,990 accesses.

Because every address a process uses is translated by its page table, so it can't "utter" 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.

1. 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.

1. 10.2 Given m frames, p references, n distinct page numbers

1. What is the lower bound on the number of page faults
   n - these are compulsory misses. This could occur if page replacement is perfect, or there is more memory than needed (m > n)
2. What is the upper bound on the number of page faults
   p - you could potentially miss on every page. This could occur if there is very little memory (m = 1) or if page replacement is particularly bad.
2. 10.3 Explain how the system establishes the physical location of a virtual address in the given computer system. Distinguish between hardware and software operations

    The virtual address in binary form is
   0001 0001 0001 0010 0011 0100 0101 0110
   Since the page size is 212, the page table size is 220. Therefore the low-order 12 bits "0100
   0101 0110" are used as the displacement into the page, while the remaining 20 bits "0001
   0001 0001 0010 0011" are used as the displacement in the page table. This displacement is looked up in hardware in the TLB. If there is a miss in the TLB, than either the hardware may walk the page table for the entry, or it may raise an exception, and the operating system can check the page table.

   If the entry in the page table is valid, then either hardware or the OS puts it in the TLB and the process continues executing.

   If the entry is invalid, the OS decides what to do next - either signal the process that it accessed an invalid address, or bring in the page from somewhere else. If it brings in the page, it must find a free frame (which may cause paging out another page), and then read in the missing page from the disk. Once the page is in, the OS marks the page table entry as valid, possibly sticks the entry in the TLB, and lets the process continue.

3. 10.7 List the costs and benefits of virtual memory. What measures can you take to ensure the costs don't exceed the benefits?

   The costs are additional hardware, additional complexity in the OS, additional memory usage for virtual memory data structures, and slower access time. The benefits are good utilization of memory, protection, and larger logical address space than physical address space.

   You can make sure for that programs that fit in memory, there are no unnecessary overheads - e.g. extra accesses to memory, page faults. Also, you can use good page replacement algorithms, to minimize the number of page faults.

   What happens when we try to access a page which is not present in the memory?

   Pages that are not loaded into memory are marked as invalid in the page table, using the invalid bit. ( The rest of the page table entry may either be blank or contain information about where to find the swapped-out page on the hard drive. )

   How can the system distinguish between the pages that are in main memory from the pages that are on the disk?

   Q12: How can the system distinguish between the pages that are in main memory from the pages that are on the disk? The system uses valid-invalid bit is used.

   How many memory access are required for accessing the data or instruction if the page table is stored in the main memory?

   In multilevel paging whatever may be levels of paging, all the page tables will be stored in the main memory. So it requires more than one memory access to get the physical address of the page frame. One access for each level is needed.

   What is paging and explain effects of paging?

   Paging is a function of memory management where a computer will store and retrieve data from a device's secondary storage to the primary storage. Memory management is a crucial aspect of any computing device, and paging specifically is important to the implementation of virtual memory.