Overlays in MS-DOS

By Andrew C. Vogan

For CS 450

12/03/2002

This work complies with the JMU Honor Code.

Overlays in MS-DOS by Andrew C. Vogan 2

Overview of Presentation

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

3. Overlays - Concept and Programmer Perspective - Operating System Perspective - Summary

Overlays in MS-DOS by Andrew C. Vogan 3

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 4

multitasking, CPU scheduler

multiprocessor support

threads

multiuser

hard/soft real-time

Major Features NOT Included in DOS

fault tolerance

deadlock management

paged memory

graphical user interface

file system security

FAT file system:- file attributes- simple file locks

Major Features Included in DOS

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

Overlays in MS-DOS by Andrew C. Vogan 5

80x86 Addressing Modes

CPU Mode AddressBits

PhysicalLines

AddressableMemory

8086 Real 32 20 1 MB80286 Protected 32 24 16 MB80386/80486 Protected 32 32 4 GB

80x86 “Real Mode” Segmentation

segment

offset

16

16

segment16

0x04

offset16

physical address20

Overlays in MS-DOS by Andrew C. Vogan 6

DOS Memory Organization and OS LoadingDiagrams derived from Ray Duncan, Advanced MS-DOS Programming

ROM Bootstrap Program

Disk Bootstrap Program

SYSINIT, from IO.SYS

DOS kernel, from MSDOS.SYS

BIOS, from IO.SYS

Interrupt vectors 0x00000

0x00400

(temporary location)

top of RAM

ROM Bootstrap Program

Transient part of COMMAND.COM

TPA

Resident part of COMMAND.COM

Drivers, file control blocksand disk buffer cache

DOS kernel

BIOS

Interrupt vectors 0x00000

0x00400

(moved)

top of RAM

Overlays in MS-DOS by Andrew C. Vogan 7

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.

A

B C ED

A

B C ED

Overlays in MS-DOS by Andrew C. Vogan 8

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.

(interrupt handler)

(code)

…

CALL Unit2Function…

Overlay Manager

Unit1

(interrupt handler)

(code)

…Unit2Function

…

Overlay Manager

Unit2

Overlays in MS-DOS by Andrew C. Vogan 9

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.