What is Memory Management Unit (MMU)?


Memory management Unit Concepts

The von Neumann rule for the design and activity of computers necessitates that a program must be an essential memory inhabitant to execute. Likewise, a client requires returning to his projects frequently during its advancement. Be that as it may, because of the way that essential memory is unpredictable, a client needs to store his program in some non-volatile store. All computers give a non-volatile secondary memory accessible as online stockpiling. Projects and documents might be plate occupant and downloaded at whatever point their execution is required. Along these lines, some type of memory the board is required at both primary and secondary memory levels.

The secondary memory may store program contents, executable procedure pictures, and information records. It might store applications, just as, system programs. In facts, an acceptable piece of all operating system, the system programs which offer types of assistance (the utilities for example) are stored in the secondary memory. These are ordered varying.

The primary inspiration for the executives of fundamental memory originates from the help of multiprogramming. A few executable procedures live in the primary memory at some random time. At the end of the day, there are a few projects utilizing the principle memory as their location space. Likewise, programs move into, and out of, the principle memory as they end, or get suspended for some input/outputs, or new executable are required to be stacked in primary memory.

In this way, the operating system must have some methodology for main memory on the board. In this section we will talk about the administration issues and systems for both primary memory and secondary memory.

Memory Management
Memory Management

MMU is short for the memory management unit. It is the hardware part that oversees virtual memory system. Regularly, the MMU is a piece of the CPU; however, in certain structures it is a different chip. The MMU incorporates a modest quantity of memory that holds a table coordinating virtual address to physical address. This table is known as the Translation Look-aside Buffer (TLB). All solicitations for information are sent to the MMU, which decides if the information is in RAM or should be gotten from the mass stockpiling gadget. On the off chance that the information isn’t in memory, the MMU issues a page shortcoming intrudes.

MMU is hardware device that maps virtual to physical address

  • In the MMU scheme, the incentive in a migration register is added to each address produced by a client procedure at the time it is sent to memory.
  • The client program manages sensible locations; it never observes the genuine physical address.

Also check: Memory Management In OS

Basic Hardware for MMU

As main memory and registers are incorporated with processor and CPU can get to these as it were, so every instruction ought to be written in direct access stockpiling devices;

  1. If CPU gets to instructions from the register then it tends to be done in one CPU clock cycle as registers are incorporated with CPU.
  2. If an instruction dwells in the primary memory then it will be gotten to by means of a memory transport that will take a great deal of time. So the solution for this includes quick memory in the middle of CPU and principle memory for example including a store for the exchange.
  3. Now we ought to guarantee that procedure dwells in the lawful location.
  4. The legitimate location comprises of the base register (holds littlest physical location) and cutoff register (size of range).

For example:

Base register = 300040

limit register = 120900

then legal address = (300040+120900)= 420940(inclusive).

legal address = base register+ limit register


How processes are mapped from disk to memory

  • Usually process resides in disk in form of binary executable file.
  • So to execute process it should reside in main memory.
  • Process is moved from disk to memory based on memory management in use.
  • The processes wait in disk in form of ready queue to acquire memory.

Procedure of mapping of disk and memory

Normal procedure is that process is selected from input queue and loaded in memory. As process executes it accesses data and instructions from memory and as soon as it completes it will release memory and now memory will be available for other processes.

Also check: Memory Fragmentation In OS

MMU scheme:

CPU——- MMU——Memory

  • CPU will generate logical address for eg: 346
  • MMU will generate relocation register(base register) for eg:14000
  • In Memory physical address is located eg:(346+14000= 14346)


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