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#Top 10 List of Week 04

1. Memory Allocation
   Memory Allocation is the process of setting aside sections of memory in a program to be used to store variables,and instances of structures and classes. There are two basic types f memory allocation: Static memory allocation and Dynamic Memory Allocation. When you declare a variable or an instance of a structure or class. The memory for that object is allocated by the operating system. The name you declare for the object can then be used to access that block of memory. When you declare a variable or an instance of a structure or class. The memory for that object is allocated by the operating system. The name you declare for the object can then be used to access that block of memory.

2. Address Binding
   Computer memory uses both logical addresses and physical addresses. Address binding allocates a physical memory location to a logical pointer by associating a physical address to a logical address, which is also known as a virtual address. Address binding is part of computer memory management and it is performed and it is performed by the operating system on behalf of the applications that need access to memory

3. Memory Management Unit (MMU)
   A Memory Management Unit (MMU) is a computer hardware component that handless all memory and caching operations associated with the processor. In other words, the MMU is responsible for all aspects of memory management. It is usually integrated into the processor, although in some systems it occupies a separate IC(Integrated Circuit) chip.

4. Logical Address
   Logical Address is generated by CPU while a program is running. The logical address i virtual address as it does not exist physically,therefore,it is also known as Virtual Address. This address is used as a reference to access the physical memory location by CPU. The term Logical Address Space is used for the set of all logical addresses generated by program’s perspective. The hardware device called Memory-Management Unit is used for mapping logical address to its corresporending physical address.

5. Physical Address
   Physical Address identifies a physical location of required data in a memory. The user never directly deals with the physical address but can access by its corresponding logical address. The user program generates the logical address and thinks that the program is running in this logical address but the program needs physical memory for its execution,therefore, the logical address must be mapped tho physical address by MMU before they are used. The term Physical Address Space is used for all physical addresses corresponding to the logical addresses in a Logical address space.

6. Fragmentation
   As processes are loaded and removed from memory, the free memory space is broken into little pieces. It happens after sometimes that processes cannot be allocated to memory blocks considering their small size and memory blocks remains unused. This problem is known as Fragmentation. Framentation is of two types -
   • External fragmentation : Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous, so it cannot be used.
   • Internal fragmentation : Memory block assigned to process is bigger. Some portion of memory is left unused, as it cannot be used by another process.
7. Contiguous Allocation
   Main memory must support bot OS and user processes. Limited resource, must allocate efficiently. Contiguous allocation is one early method. Main memory usually into two partitions:
   • Resident operating system, usually held in low memory with interrupt vector
   • User processes then held in high memory
   • Each process contained in single contiguous section of memory

8. Translation Look aside buffer
   A Translation look aside buffer can be defined as a memory cache which can be used to reduce the time taken to access the page table again and again. It is a memory cache which is closer to the CPU and the time taken by CPU to access TLB is lesser then that taken to access main memory. In other words, we can say that TLB is faster and smaller than the main memory but cheaper and bigger than the register.

9. Multiple Partition Allocation
   Variable-partition sizes for efficiency(sized to a given process’ needs). Hole - block of availablememory; holes of various size are scattered throughout memory. When a process arrives,allocated memory form a hole large enough to accommodate it. Process exiting frees its partition, adjacent free partitions combined. Operating system maintains information about: a) allocated partitions b) free partitions(hole)

10. Three Level Page Tables
    Linux assumes that ther are three levels of page tables. Each Page Table accessed containts the PFN of the next level of Page Table. each field providing an offset into a particular Page Table. To translate a virtual address into a physical one, the CPU must take the contents of each level fields, convert it into an offset into the physical page containing tha Page Table and read the PFN of the next level of Page Table. This is repeated three times until the PFN of the physical page containing the virtual address is found. Now the final field in the virtual address, the byete offset, is used to find data inside the page.