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Overlays in MS-DOS. By Andrew C. Vogan For CS 450 12/03/2002. This work complies with the JMU Honor Code. Overview of Presentation. 3. Overlays - Concept and Programmer Perspective - Operating System Perspective - Summary. 1. Introduction to MS-DOS

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overlays in ms dos

Overlays in MS-DOS

By Andrew C. Vogan

For CS 450

12/03/2002

This work complies with the JMU Honor Code.

slide2

Overview of Presentation

3. Overlays

- Concept and Programmer

Perspective

- Operating System

Perspective

- Summary

1. Introduction to MS-DOS

- History of DOS

- Limitations and Features

2. DOS Memory Limitations

- 80x86 Addressing Modes

- 8086 Segmentation

- DOS Memory Organization

and Loading

Overlays in MS-DOS by Andrew C. Vogan

slide3

History of MS-DOS

  • Seattle Computer Products 86-DOS was loosely based on CP/M.
  • In 1981, Microsoft purchased 86-DOS, and renamed it as “MS-DOS.” From the start, MS-DOS was intended for IBM’s personal computers, designed for the Intel 8086/8088 CPUs.
  • MS-DOS shipped with the original IBM PC in 1981, and quickly grew to be the OS of choice for both IBM PC’s and “clones.”
  • “DOS” stands for “Disk Operating System.” MS-DOS came to provide not only the basic disk features provided by CP/M, but also some UNIX-like features (I/O redirection, piping, filters).

Overlays in MS-DOS by Andrew C. Vogan

slide4

Major Features NOT Included in DOS

multitasking, CPU scheduler

multiprocessor support

threads

multiuser

hard/soft real-time

fault tolerance

deadlock management

paged memory

graphical user interface

file system security

Major Features Included in DOS

FAT file system: - file attributes - simple file locks

Direct, complete hardware control:- memory/device access - interrupt vector overrides

Overlays in MS-DOS by Andrew C. Vogan

slide5

80x86 Addressing Modes

Address

Addressable

Physical

CPU

Mode

Bits

Memory

Lines

8086

Real

32

20

1 MB

80286

Protected

32

24

16 MB

80386/80486

Protected

32

32

4 GB

80x86 “Real Mode” Segmentation

16

4

segment

0x0

16

segment

16

offset

16

offset

20

physical address

Overlays in MS-DOS by Andrew C. Vogan

slide6

ROM Bootstrap Program

ROM Bootstrap Program

top of

RAM

top of

RAM

Transient part of COMMAND.COM

Disk Bootstrap Program

TPA

DOS kernel, from MSDOS.SYS

(temporary location)

Resident part of COMMAND.COM

SYSINIT, from IO.SYS

Drivers, file control blocks

and disk buffer cache

BIOS, from IO.SYS

DOS kernel

0x00400

(moved)

Interrupt vectors

BIOS

0x00000

0x00400

Interrupt vectors

0x00000

DOS Memory Organization and OS Loading

Diagrams derived from Ray Duncan, Advanced MS-DOS Programming

Overlays in MS-DOS by Andrew C. Vogan

slide7

A

A

B

C

D

E

B

C

D

E

  • Overlays -- The Concept
  • Overlaying is the technique of loading different portions of a program into the same memory area.
  • Overlay programming techniques were first developed and refined on mainframes in the 1960’s.
  • This allowed MS-DOS developers to split a program up, that was otherwise would not fit in conventional memory.

Overlays in MS-DOS by Andrew C. Vogan

slide8

Overlay Manager

(interrupt handler)

(code)

CALL Unit2Function

Unit1

  • Overlays -- From OS Perspective
  • Overlay files were loaded in a similar manner to normal spawned executables. The main difference was less control by MS-DOS (no PSP or automatic execution point transfer).
  • One popular mechanism of switching between overlaid code units was raising a custom interrupt, intercepted by a custom overlay manager.

Overlay Manager

(interrupt handler)

(code)

Unit2Function

Unit2

Overlays in MS-DOS by Andrew C. Vogan

slide9

Summary

  • Overlays provided an adequate solution to MS-DOS’s memory limitations for many applications.
  • Problems with overlaid programs included that they did not take advantage of extended memory, and that they were difficult to implement (often requiring intricate modular design).
  • Windows 3.x operating systems (bringing full 80386 protected mode support) were a welcome upgrade for many developers.
  • It is still instructive to analyze techniques like overlaying, particularly for the sake of PDA’s and other small-footprint devices.

Overlays in MS-DOS by Andrew C. Vogan