Overlays in MSDOS By Andrew C Vogan For

Overlays in MS-DOS By Andrew C. Vogan For CS 450 12/03/2002 Overlays in MS-DOS by Andrew C. Vogan This work complies with the JMU Honor Code.

Overview of Presentation 1. Introduction to MS-DOS - History of DOS - Limitations and Features 2. DOS Memory Limitations - 80 x 86 Addressing Modes - 8086 Segmentation - DOS Memory Organization and Loading Overlays in MS-DOS 3. Overlays - Concept and Programmer Perspective - Operating System Perspective - Summary by Andrew C. Vogan 2

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 3

Major Features NOT Included in DOS multitasking, CPU scheduler fault tolerance multiprocessor support deadlock management threads paged memory multiuser graphical user interface hard/soft real-time file system security Major Features Included in DOS FAT file system: Direct, complete hardware control: - file attributes - simple file locks - memory/device access - interrupt vector overrides Overlays in MS-DOS by Andrew C. Vogan 4

80 x 86 Addressing Modes CPU Mode 8086 80286 80386/80486 Real Protected Address Physical Bits Lines 32 20 32 24 32 32 Addressable Memory 1 MB 16 MB 4 GB 80 x 86 “Real Mode” Segmentation 16 4 segment 16 segment 0 x 0 16 offset 20 physical address Overlays in MS-DOS by Andrew C. Vogan 5

DOS Memory Organization and OS Loading Diagrams derived from Ray Duncan, Advanced MS-DOS Programming ROM Bootstrap Program top of RAM Transient part of COMMAND. COM Disk Bootstrap Program TPA Resident part of COMMAND. COM DOS kernel, from MSDOS. SYS (temporary location) Drivers, file control blocks and disk buffer cache SYSINIT, from IO. SYS DOS kernel BIOS, from IO. SYS BIOS 0 x 00400 Interrupt vectors 0 x 00000 Overlays in MS-DOS (moved) by Andrew C. Vogan 0 x 00000 6

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 A D E Overlays in MS-DOS B C by Andrew C. Vogan D E 7

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 Unit 1 (interrupt handler) Overlay Manager Unit 2 (code) … Unit 2 Function … CALL Unit 2 Function … Overlays in MS-DOS (interrupt handler) by Andrew C. Vogan 8

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 9