Name: Virtual Memory: 2 Three problems with Memory
Uploaded: 2014-07-14T11:33:12.000Z
Duration: 14 min 27 s
Description: Interactive lecture at http://test.scalable-learning.com, enrollment key YRLRX-25436. Virtual Memory Problems: 1) What if we don't have enough memory? 2) Holes in our address space? 3) Keeping programs secure with virtual memory. How does virtual memory solve these problems? Indirection between the program's memory space and the hardware memory space.

Virtual Memory: 2 Three problems with Memory

Memory Problems in MIPS

Insufficient Memory

The first problem discussed is the limitation of memory in MIPS architecture, where each program is promised a 32-bit address space.

A 32-bit address allows access to 2^32 bytes or 4 gigabytes of memory; however, practical limitations often reduce this to about 2 gigabytes due to operating system reservations.

If a computer has less than 4 gigabytes of RAM (e.g., only 1 gigabyte), accessing addresses beyond the physical memory leads to crashes without virtual memory support.

Memory Fragmentation

When multiple programs run simultaneously, they share the available memory, which can lead to fragmentation.

For example, if two programs occupy a total of 3 gigabytes and one quits, there may be free space but not contiguous enough for another program that requires it (e.g., needing 2 gigabytes).

Data Corruption Between Programs

Another significant issue arises when multiple programs attempt to access the same memory address.

An example illustrates two programs writing data to the same address (1024), leading to corruption as they overwrite each other's data.

Summary of Memory Issues

The problems identified include: insufficient physical memory leading to crashes, fragmentation preventing efficient use of available memory, and potential data corruption from shared access between programs.

Virtual Memory as a Solution

Introduction to Virtual Memory

The lecture introduces virtual memory as a solution that provides each program with its own separate virtual address space.

Mapping Virtual Space to Physical RAM

Virtual Memory and Its Mapping

Understanding Virtual Memory

Virtual memory involves a mapping system that connects program memory spaces to physical RAM, allowing for efficient memory management.

When physical RAM is insufficient, data can be moved to disk storage, effectively using the hard disk as an extension of memory. This concept underpins the term "virtual memory."

A discussion on shared address spaces highlights potential issues when multiple programs share a 32-bit address space with less than four gigabytes of available memory.

Address Space Limitations