Memory Management Questions and Answers

Answer: B

Correct answer: Access point

Coaxial cable, fibre and shielded twisted pair are all cabling used in Ethernet. An access point belongs to a wireless network, which is a different medium.

Answer: C

Correct answer: changes

Because transient code is loaded and released as it is needed, the amount of memory the operating system occupies goes up and down while programs run. It neither only grows nor only shrinks, which is why the answer is that it changes.

Answer: C

Correct answer: either low or high memory (depending on the location of interrupt vector)

The operating system is placed at whichever end of memory the interrupt vector occupies, since the vector must be reachable and is fixed by the hardware. On most machines that is low memory, but the principle rather than the address is what matters.

Answer: D

Correct answer: protection bit with each page

Each page table entry carries protection bits saying whether the page may be read, written or executed, and the hardware checks them on every reference. Protection is therefore per page and enforced by the same lookup that translates the address.

Answer: C

Correct answer: the routine is loaded into memory only when it is first called

Dynamic loading defers bringing a routine into memory until it is first called. All routines are held on disk in a relocatable format; when one is called, the loader checks whether it is resident, loads and relocates it if not, and transfers control. The benefit is that unused routines are never loaded at all. Error-handling code, which may account for a large part of a program and is rarely executed, costs nothing unless an error occurs. It requires no support from the operating system - it is the responsibility of the program - though libraries are usually provided to make it convenient. It should be distinguished from dynamic linking, where the linking of a library is postponed until execution and a stub in the program locates the shared routine on first use. Dynamic linking allows a single copy of a library in memory to serve every process, and lets the library be patched without relinking every program that uses it. Both differ from static loading and linking, where everything is bound before execution begins.

Answer: A

Correct answer: if then statement

Where only one action is to be taken when a condition holds, and nothing when it does not, the if then form is enough; ELSE is needed only when there is a second action.

Answer: C

Correct answer: a process references a page that is not currently in physical memory

A page fault is raised when a process references a page whose valid bit in the page table indicates it is not resident in physical memory. Servicing it follows a fixed sequence: the hardware traps to the operating system, which checks that the reference is legal, finds a free frame or selects a victim to evict, writes the victim back if it is dirty, reads the required page in from the backing store, updates the page table, and restarts the instruction that faulted. It is not an error, despite the name. Demand paging depends on page faults as its ordinary mechanism, and a process starts with none of its pages resident, faulting them in as it runs. The cost is what matters: a memory access takes nanoseconds and a disk access milliseconds, so the effective access time is dominated by the fault rate. A rate of one in a hundred thousand can still double the average access time, which is why locality of reference is essential to the scheme working at all.

Answer: A

Correct answer: page table base register

When the table is too large for registers it is kept in memory and the page table base register holds its address, so a context switch need only reload that one register.

Answer: C

Correct answer: allocates the first free block large enough to satisfy the request

First Fit scans the list of free blocks from the beginning and takes the first one large enough. It is the fastest of the placement strategies because the search stops as soon as a candidate is found. Best Fit searches the whole list for the smallest adequate block, on the reasoning that it wastes least - but it must examine every block, and it leaves behind a scatter of tiny remainders too small to use, so it often produces worse fragmentation in practice. Worst Fit takes the largest block, leaving a remainder big enough to be useful; it performs poorly in most studies. Simulation generally finds First Fit and Best Fit better than Worst Fit in both storage utilisation and time, with First Fit faster. All three apply to contiguous allocation and all three suffer external fragmentation, which is what paging was introduced to remove. A variant, Next Fit, resumes the search where the previous one ended rather than starting over.

Answer: C

Correct answer: hardware that translates logical addresses into physical addresses

The MMU is hardware, part of the processor, that translates every logical address the CPU generates into a physical address before it reaches memory. It must be hardware because the translation happens on every single memory reference, and software could not possibly keep up. In the simplest scheme it adds a relocation register and checks a limit; in a paged system it walks the page table and consults the TLB. It also enforces protection on the same pass, checking the valid bit and the permission bits and trapping to the operating system on a violation - which costs nothing extra, since the entry has to be read for translation anyway. The operating system’s role is to set up the structures the MMU uses - page tables, base and limit registers - and to handle the traps it raises. The division of labour is exact: policy in software, mechanism in hardware. A system without an MMU cannot support virtual memory or genuine process isolation, which is why simple embedded controllers run everything in one address space.