CSCI 350 Fall 2001 Quiz 4

Statistics:

mean     37.357 (523.000/14)
stddev   9.013
median   39.500
midrange 32.000-44.000

#   avg
1	7.57 / 8
2	7.14 / 9
3	6.57 / 8
4	6    / 9
5	4.43 / 8
6	5.64 / 8

1. [8 pts] Give an example of a situation where the execve() system call might return with an error code.

   I'll give you several situations.

   • There is not enough memory to allocate to the desired executable file.
   • The user does not have permission to access the directory in which the command lies.
   • The user does not have permission to execute the requested executable file.
   • The command's file does not exist.
   • The executable file is not formatted in a manner compatible with the operating system.

2. [9 pts] In serious operating systems, user processes run on the CPU with restricted permissions: Their access to memory and I/O devices is severely limited. And of course they can't willy-nilly gain unrestricted permissions --- that would be a gaping security hole.

   But sometimes a process nonetheless needs to access an I/O device (like the disk), even though it cannot access it directly. What CPU feature allows it to accomplish this?

   A software interrupt puts the system into unrestricted mode. It also jumps the CPU into operating system code, so that the operating system can govern access to the device.

3. [8 pts] Give a reason why modern operating systems use segments in memory allocation.

   Three reasons are good answers.

   • They allow an OS to locate a program's memory anywhere in chooses.

   • They allow the OS to bound the program's memory space, so that segmentation faults will occur when the program tries to go outside its memory.

   • They permit the OS to move a program's memory to another location while the process is in the middle of running.

4. [9 pts] Name three specific pieces of data found in a typical page table entry.
   • page frame number (telling where page is in physical memory)
   • invalid bit (tracking whether page is currently in physical memory)
   • dirty bit (telling whether the page in memory has been modified)
   • referenced bit (telling whether the page in memory has been accessed)

5. [8 pts] In class we saw that a straightforward implementation of paging would require two memory accesses every time the CPU wants to fetch something from memory --- effectively halving memory efficiency. We also talked about a way of making this more time-efficient. What was it? (Either the exact term or its definition would do.)

   The TLB (transition lookaside buffer) caches page table entries on the CPU itself, permitting quick parallel lookup of the entries that it can fit into the TLB.

6. [8 pts] Give a reason why an OS might keep track of a process's working set.
   • It allows the OS to prepage the process's working set before it puts the process into the CPU to run again.

   • The OS might want the working sets to determine how to allocate page frames among processes.

   • The OS might take processes' working sets into consideration when deciding what page to eject on a page fault.