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Thursday, February 6, 2020

Main Memory (Memory Management)


Main Memory Basics

Memory is central to the operation of a modern computer system. The CPU can only access its registers and main memory. It cannot, for example, make direct access to the hard drive, so any data stored there must first be transferred into the main memory before the CPU can work with it. User processes must be restricted so that they only access memory locations that "belong" to that particular process. This is usually implemented using a base register and a limit register for each process.

Every memory access made by a user process is checked against these two registers, and if a memory access is attempted outside the valid range, then a fatal error is generated. The OS obviously has access to all existing memory locations, as this is necessary to swap users' code and data in and out of memory.
Address Binding

User programs typically refer to memory addresses with symbolic names such as "i", "count", and "average". These symbolic names must be mapped or bound to physical memory addresses, which typically occurs in several stages:
  • Compile Time - If it is known at compile time where a program will reside in physical memory, then absolute code can be generated by the compiler, containing actual physical addresses. However if the load address changes at some later time, then the program will have to be recompiled. DOS .COM programs use compile time binding.
  • Load Time - If the location at which a program will be loaded is not known at compile time, then the compiler must generate relocatable code, which references addresses relative to the start of the program. If that starting address changes, then the program must be reloaded but not recompiled.
  • Execution Time - If a program can be moved around in memory during the course of its execution, then binding must be delayed until execution time. This requires special hardware, and is the method implemented by most modern operating systems.

Logical Versus Physical Address Space

The address generated by the CPU is a logical address, whereas the address actually seen by the memory hardware is a physical address.
Addresses bound at compile time or load time have identical logical and physical addresses.
Addresses created at execution time, have different logical and physical addresses.
o    In this case the logical address is also known as a virtual address, and the two terms are used interchangeably by our text.
o    The set of all logical addresses used by a program composes the logical address space, and the set of all corresponding physical addresses composes the physical address space.

The run time mapping of logical to physical addresses is handled by the memory-management unit, MMU.
o    The MMU can take on many forms. One of the simplest is a modification of the base-register scheme described earlier.
o    The base register is now termed a relocation register, whose value is added to every memory request at the hardware level.

User programs work entirely in logical address space, and any memory references or manipulations are done using purely logical addresses. Only when the address gets sent to the physical memory, the physical memory address is generated.


Dynamic Loading

Rather than loading an entire program into memory at once, dynamic loading loads up each routine as it is called. The advantage is that unused routines will never be loaded; reducing total memory usage and generating faster program startup times. The downside is the added complexity and overhead of checking to see if a routine is loaded every time it is called and then loading it up if it is not already loaded.



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