Introduction
Last updated
Last updated
A process must have its address space, at least partially, resident in the main memory to be executed.
In a multiprogramming environment, to maximize processor utilization and improve response time (or turnaround time), a computer system must maintain the address spaces of multiple processes resident in the main memory.
But, there may not be room for all.
because, although the main memory has been growing over the years, it is a fact that “data expands to fill the space available for storage”
Ideally, an application programmer would like to have infinitely large, infinitely fast, non-volatile, and inexpensively available memory.
In practice, this is not possible.
Thus, the memory of a computer system is typically organized at different levels, forming a hierarchy.
cache memory – small (tens of KB to some MB), very fast, volatile and expensive.
main memory – medium size (hundreds of MB to hundreds of GB), volatile and medium price and medium access speed.
secondary memory – large (tens, hundreds or thousands of GB), slow, non-volatile and cheap.
The cache memory will contain a copy of the memory positions (instructions and operands) most frequently referenced by the processor in the near past.
The cache memory is located on the processor’s own integrated circuit (level 1).
And on an autonomous integrated circuit glued to the same substrate (levels 2 and 3).
Data transfer to and from main memory is done almost completely transparent to the system programmer.
Secondary memory has two main functions:
File system – storage for more or less permanent information (programs and data).
Swapping area – Extension of the main memory so that its size does not constitute a limiting factor to the number of processes that may currently coexist.
the swapping area can be on a disk partition used only for that purpose or be a file in a file system.
This type of organization is based on the assumption that the further an instruction or operand is away from the processor, the less times it will be referenced.
In these conditions, the mean time for a reference tends to be close to the lowest value.
Based on the principle of locality of reference.
The tendency of a program to access the same set of memory locations repetitively over a short period of time.
The role of memory management in a multiprogramming environment focuses on allocating memory to processes and on controlling the transfer of data between main and secondary memory (swapping area), in order to:
Maintaining a register of the parts of the main memory that are occupied and those that are free.
Reserving portions of main memory for the processes that will need it, or releasing them when they are no longer needed.
Swapping out all or part of the address space of a process when the main memory is too small to contain all the processes that coexist.
Swapping in all or part of the address space of a process when main memory becomes available.
