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

Overlays in MS-DOS by Andrew C. Vogan2 Overview of Presentation 1. Introduction to MS-DOS - History of DOS - Limitations and Features 2. DOS Memory Limitations - 80x86 Addressing Modes Segmentation - DOS Memory Organization and Loading 3. Overlays - Concept and Programmer Perspective - Operating System Perspective - Summary

Overlays in MS-DOS by Andrew C. Vogan3 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. Vogan4 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. Vogan5 80x86 Addressing Modes CPUMode Address Bits Physical Lines Addressable Memory 8086Real32201 MB 80286Protected MB 80386/80486Protected32 4 GB 80x86 “Real Mode” Segmentation segment offset 16 segment 16 0x0 4 offset 16 physical address 20

Overlays in MS-DOS by Andrew C. Vogan6 DOS Memory Organization and OS Loading Diagrams 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 0x x00400 (temporary location) top of RAM ROM Bootstrap Program Transient part of COMMAND.COM TPA Resident part of COMMAND.COM Drivers, file control blocks and disk buffer cache DOS kernel BIOS Interrupt vectors 0x x00400 (moved) top of RAM

Overlays in MS-DOS by Andrew C. Vogan7 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 BCED A BCED

Overlays in MS-DOS by Andrew C. Vogan8 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. Vogan9 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 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.