Memory Manager The Memory Manager

is the subset of the operating system that manages the memory of the computer. Its most basic task is to allocate memory to processes when they need it. This allocated memory is by default specific to the process that makes the request.

Memory Manager, User Space, and Kernel Space.

On recent kernel security check failure  note 2 , the memory manager hides the physical location of the memory (in RAM or on hard disk , in the memory space paged ) and presents the program a uniform global memory called virtual memory . Thus, any process believes touch a memory “logical” that has the following properties Note 3  :

the memory can be extended to the theoretical capacities of the machine note 4  ;

the memory is private (protected), one process can not access the memory of another process (except specific allocations and permissions).

The advantage of not indicating to the process the physical location of the data is to allow the memory manager to place and move the data in memory at its convenience, without affecting the processes. This data can in particular be fragmented in the RAM when a process requires a block of memory larger than the largest free physical block. The contents of the memory can also be migrated. This migration is done on the various memory media such as in the physical memory (more or less close to the processor), in the paged memory, in the memory accessible by networks (computing cluster).

Memory virtualization also allows for optimistic management of resources: the memory allocated but not yet used can be virtually allocated to several processes kernel security check failure

Programs in the user space have limited memory powers: they must ask the kernel security check failure of memory. The kernel uses its memory manager to allocate (or not) memory to the process that requests it. If a process tries to use memory areas that do not belong to it, it is automatically evicted. The eviction mechanism is based on a mechanism of the processor, namely a memory management unit , or MMU , which signals to the kernel the existence of a faulty access. It is the nucleus itself that makes the decision to suspend or immediately destroy the faulty process.

System Calls

The system calls are functions:

called from a user space program;

  • whose execution (the processing) is performed in the kernel space;
  • whose return is made in the calling program in the user space.

In addition to a change of mode of execution , the system call supposes at least two commutations of contexts  :

  • Context of the calling program;
  • change of context;
  • Kernel context;
  • change of context;

Context of the calling program.

The cost of a system call is much higher than a simple intra-process function call: while a function call requires only a few primitive instructions (loading and executing a memory area), the cost of a system call is counted in thousands or tens of thousands of primitive instructions, generating both a load and additional turnaround times. For these reasons, functions that are used intensively are moved into the kernel security check failure space. User programs then make a small number of high-level system calls. The many low-level interactions generated by these system calls are made in the kernel space. This includes device drivers .

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