Computer Organization

and Architecture

Operating System Support

Objectives and Functions

Convenience Making the computer easier to use

Efficiency Allowing better use of computer resources

Layers and Views of a Computer System

Operating System Services

Program creation Program execution Access to I/O devices Controlled access to files System access Error detection and response Accounting

Types of Operating System

Interactive Batch Single program (Uni-programming) Multi-programming (Multi-tasking)

Early Systems

Late 1940s to mid 1950s No Operating System Programs interact directly with hardware Two main problems:

Scheduling Setup time

Single Program

Multi-Programming with Two Programs

Multi-Programming with Three Programs

Time Sharing Systems

Allow users to interact directly with the computer i.e. Interactive

Multi-programming allows a number of users to interact with the computer

Scheduling

Key to multi-programming Long term Medium term Short term I/O

Memory Management

Uni-program Memory split into two One for Operating System (monitor) One for currently executing program

Multi-program “User” part is sub-divided and shared among

active processes

Swapping

Problem: I/O is so slow compared with CPU that even in multi-programming system, CPU can be idle most of the time

Solutions: Increase main memory

Expensive Leads to larger programs

Swapping

What is Swapping? Long term queue of processes stored on disk Processes “swapped” in as space becomes available As a process completes it is moved out of main

memory If none of the processes in memory are ready (i.e. all

I/O blocked) Swap out a blocked process to intermediate queue Swap in a ready process or a new process But swapping is an I/O process...

Partitioning

Splitting memory into sections to allocate to processes (including Operating System)

Fixed-sized partitions May not be equal size Process is fitted into smallest hole that will take it

(best fit) Some wasted memory Leads to variable sized partitions

Fixed Partitioning

Variable Sized Partitions (1)

Allocate exactly the required memory to a process This leads to a hole at the end of memory, too small to use

Only one small hole - less waste When all processes are blocked, swap out a process and

bring in another New process may be smaller than swapped out process Another hole

Variable Sized Partitions (2)

Eventually have lots of holes (fragmentation) Solutions:

Coalesce - Join adjacent holes into one large hole Compaction - From time to time go through

memory and move all hole into one free block (c.f. disk de-fragmentation)

Effect of Dynamic Partitioning

Virtual Memory

Virtual Memory

In order to be executed or data to be accessed, a certain segment of the program has to be first loaded into main memory; in this case it has to replace another segment already in memory

Movement of programs and data, between main memory and secondary storage, is performed automatically by the operating system. These techniques are called virtual-memory techniques

Virtual Memory

Virtual Memory Organization

The virtual programme space (instructions + data) is divided into equal, fixed-size chunks called pages.

Physical main memory is organized as a sequence of frames; a page can be assigned to an available frame in order to be stored (page size = frame size).

The page is the basic unit of information which is moved between main memory and disk by the virtual memory system.

Demand Paging

The program consists of a large amount of pages which are stored on disk; at any one time, only a few pages have to be stored in main memory.

The operating system is responsible for loading/ replacing pages so that the number of page faults is minimized.

Demand Paging

We have a page fault when the CPU refers to a location in a page which is not in main memory; this page has then to be loaded and, if there is no available frame, it has to replace a page which previously was in memory.

Address Translation

Accessing a word in memory involves the translation of a virtual address into a physical one: - virtual address: page number + offset - physical address: frame number + offset

Address translation is performed by the MMU using a page table.

Example

Address Translation

The Page Table

The page table has one entry for each page of the virtual memory space.

Each entry of the page table holds the address of the memory frame which stores the respective page, if that page is in main memory.

The Page Table

Each entry of the page table also includes some control bits which describe the status of the page: whether the page is actually loaded into main

memory or not; if since the last loading the page has been

modified; information concerning the frequency of access,

etc.

Memory Reference with Virtual Memory

Memory Reference with Virtual Memory

Memory access is solved by hardware except the page fault sequence which is executed by the OS software.

The hardware unit which is responsible for translation of a virtual address into a physical one is the Memory Management Unit (MMU).

Translation Lookaside Buffer

Every virtual memory reference causes two physical memory access Fetch page table entry Fetch data

Use special cache for page table TLB

TLB Operation

TLB and Cache Operation

Pentium II Address Translation Mechanism

Page Replacement

When a new page is loaded into main memory and there is no free memory frame, an existing page has to be replaced

The decision on which page to replace is based on the same speculations like those for replacement of blocks in cache memory

LRU strategy is often used to decide on which page to replace.

Page Replacement

When the content of a page, which is loaded into main memory, has been modified as result of a write, it has to be written back on the disk after its replacement.

One of the control bits in the page table is used in order to signal that the page has been modified.

Summary

Objective and functions OS Scheduling Memory management

virtual memory page replacement