Section 12.5
Review Questions
Address spaces and page tables
  1. What is the virtual address space?
answer...
the set of all memory addresses that a program can generate regardless of whether they correspond to real memory or not
  1. How large is the virtual address space relative to the physical address space?
answer...
usually much larger
  1. How many bits long are VAX addresses?
answer...
32
  1. If there are K bits in a memory address, what is the function that tells how large the virtual address space is?
answer...
2K
  1. How long were the CDC's addresses?
answer...
18
  1. Draw a picture of a typical page table.
answer...
  1. Of the following fields of a page table, which field(s) always has (have) significance, no matter where the page is (in a frame of real memory or only on the disk.) 
               present/absent bit
               dirty bit
               frame #
               disk address
answer...
present/absent bit
disk address
  1. Order the following steps in a sequence that occurs when a program attempts to access a word of memory. Put 1 next to the first step, 2 next to the second, and so on. 
     _____ A page fault event is sent to the operating system.
     
     _____ The hardware translates the address using the page table.
     
     _____ The operating system schedules a disk read to copy the page into a
           frame.
     _____ The operating system suspends the program, which does not run any
           more instructions.
     _____ The operating system marks the program as able to resume.
     
     _____ The operating system uses a victim selection algorithm to find a
           frame in memory to overwrite.
     _____ The program generates an address during a memory read or write.
     
     _____ The hardware detects via the present/absent bit that the page is not
           in memory.
answer...
     __4__ A page fault event is sent to the operating system.
     
     __2__ The hardware translates the address using the page table.
     
     __7__ The operating system schedules a disk read to copy the page into a
           frame.
     __5__ The operating system suspends the program, which does not run any
           more instructions.
     __8__ The operating system marks the program as able to resume.
     
     __6__ The operating system uses a victim selection algorithm to find a
           frame in memory to overwrite.
     __1__ The program generates an address during a memory read or write.
     
     __3__ The hardware detects via the present/absent bit that the page is not
           in memory.
  1. List all the components in a disk address.
answer...
  1. How does a page's dirty bit always start out, 1 or 0?
answer...
0
  1. If dirty=1, what significance does the frame number have?
answer...
the frame number is not bogus; there really is a page in that frame of physical memory (otherwise it could not be dirty!)
  1. Why should the operating system attempt to minimize page faults?
answer...
Each page fault requires a slow and expensive disk read (and possibly also a disk write if the victim's frame is dirty).