Segmentation & O/S Input/Output

Chapter 4 & 5

Tuesday, April 3 , 2007

Today’s Schedule Memory Management - Chapter 4

Segmentation Multics & Intel Pentium – Segmentation & Paging

Input/Output Device Types Goals of I/O Devices for O/S Layers of I/O s/w System

Objectives

You will be able to describe: Advantages of Segmentation for Memory

management Types of I/O Devices Goals of I/O Software from O/S view Layers of I/O s/w and what happens in

each

What is Segmentation? Having two or more separate virtual

address spaces Provide several independent address spaces

called segments Each segment goes from 0 to some max Segments can have varying lengths Length can change during execution Programmer will now have to specify the

segment number

Can now use one segment for shared libraries

Segmentation

One-dimensional address space with growing tables One table may bump into another

Segmentation – Logical View

Allows each table to grow or shrink, independently

Segmentation vs Paging?

Paging is incidental

Page-data is heterogeneous

Statically Sized

Segments are intentional

Segment data is homogenous

Dynamically Sized

Pure Segmentation

Segment size varies and changes External Fragmentation

Also called “Checkerboarding”Swapping large segments for smallResolution: Compaction

Implementation of Pure Segmentation

Removal of the checkerboarding by compaction

MULTICS Implementation

Similar to Multi-Level Paging Address contains Segment # for table Segment table holds Segment Descriptors Descriptor contains page table location Standard Page Table

Has 218 segments 216 Pages With 36-bit entries 234 Total Pages

Segmentation with Paging: MULTICS

Descriptor segment points to page tables Segment descriptor – numbers are field lengths

Segmentation with Paging: MULTICS

A 34-bit MULTICS virtual address

Segmentation with Paging: MULTICS

Conversion of a 2-part MULTICS address into a main memory address

Input & Output Device types

Block DeviceStores fixed size block Independently addressabledisks

Character DevicesStream of charactersNo seek operation, not addressableKeyboard, printer, n/w interface

I/O Hardware

Some typical device, network, and data base rates

Device Controllers I/O devices have components:

mechanical component electronic component

The electronic component is the device controllermay be able to handle multiple devices

Controller's tasksconvert serial bit stream to block of bytesperform error correction as necessarymake available to main memory

Goals of I/O Software Device independence

programs can access any I/O device without specifying device in advance

· (floppy, hard drive, or CD-ROM)

Uniform namingname of a file or device a string or an integernot depending on which machine

Error handlinghandle as close to the hardware as possible

Goals of I/O Software (cont’d) Synchronous vs. asynchronous

transfersblocked transfers vs. interrupt-driven

Bufferingdata coming off a device cannot be stored

in final destination Sharable vs. dedicated devices

disks are sharable tape drives would not be

I/O Software Layers

Layers of the I/O Software System

Interrupt Handlers

Interrupt handlers are best hidden have driver starting an I/O operation block until

interrupt notifies of completion

Interrupt procedure does its task then unblocks driver that started it

Device Drivers

Logical position of device drivers is shown here Communications between drivers and device controllers goes

over the bus

Device-Independent I/O Software

Functions of the device-independent I/O software

Uniform interfacing for device drivers

Buffering

Error reporting

Allocating and releasing dedicated devices

Providing a device-independent block size

Device-Independent I/O Software

(a) Without a standard driver interface

(b) With a standard driver interface

Device-Independent I/O Software

(a) Unbuffered input(b) Buffering in user space(c) Buffering in the kernel followed by copying to user space(d) Double buffering in the kernel

Summary User-Space I/O Software

Layers of the I/O system and the main functions of each layer

Summary

Advantages of Segmentation for Memory management

Types of I/O Devices Goals of I/O Software from O/S view Layers of I/O s/w and what happens in

each

Thursday, April 5

Read Chapter 55.1.1 & 5.1.2 I/O Devices5.2.1 Goals of I/O in O/S5.3.1 to 5.3.35.4 Disks