MS-DOS, PC-DOS or DOS: Microsoft Disk Operating Systemo Developed to run single-user, stand-alone desktop computers. Exemplifies early Operating Systems because it manages jobs sequentially from single user.o Advantages: Simple operation & straight-forward user commands.o Disadvantages: 1. Lack of flexibility & limited ability to meet needs of programmers & experienced users. 2. Written for a single family of microprocessors (Intel family of chips: 8086, 8088, 80186, and 80286).
Design Goalso Accommodate single novice user in single- process environment.o Standard I/O support includes keyboard, monitor, printer, & secondary storage unit.o User commands are based on English words/phrases indicative of action to be performed.o Commands are interpreted by command processor.o Layering approach “protects” user from hardware.
MS-DOS Architecture User COMMAND.COM DOS Kernel MS-DOS BIOS Hardware
First 2 DOS Layers : BIOS & Kernel• BIOS (Basic Input/Output System) -- interfaces directly with various I/ O devices. Device drivers (control flow of data to/from each device). Receives status info about success/failure of each I/O operation & passes it on to the processor.• DOS kernel -- routines needed to interface with the disk drives. Read into memory at initialization time from MSDOS.SYS file on boot disk. Accessed by application programs. Provides a collection of hardware-independent services, such as memory management, & file & record management (system functions).
Third DOS Layer : Command Processor3. Command processor (shell) -- sends prompts to user, accepts commands, executes commands, & issues appropriate responses. Resides in a file called COMMAND.COM, which consists of 2 parts stored in 2 different sections of main memory. Not interpretive. MS-DOS Version 4.0 -- menu-driven DOS shell. OS/2 -- designed to replace MS-DOS.o MS-DOS ran enormous collection of software packages making it difficult to discontinue.
DOS Memory Managemento Memory Manager – Relatively simple job because it’s managing single job for single user. – Uses a first-fit memory allocation scheme since it is most efficient strategy in a single-user environment.
DOS Main Memory Allocationo First versions had simple contiguous memory allocation scheme that gave all of the available memory to resident application program. Applications couldn’t dynamically allocate memory blocks. MS-DOS Version 2.0 supported dynamic allocation, modification, & release of main memory blocks by applications.o Amount of memory each application owns depends on type of file from which program is loaded & size of TPA. Programs.COM -- given all of TPA, whether or not they need it. Programs.EXE -- given amount of memory they need.
Memory Block Allocationo Allocates memory by using first-fit algorithm & linked list of memory blocks.o With Version 3.3, MS-DOS started using best-fit or last-fit strategy.o Size of a block can vary from 16 bytes (paragraph) to maximum available memory.
Free/Busy Block Listo Whenever request for memory comes in, DOS looks through free/busy block list to find free block that fits. If list becomes disconnected, system stops & must be rebooted.o Well-designed application program releases memory block it no longer needed.
Process Managemento MS-DOS doesn’t support multitasking. Programs can’t break out of middle of DOS internal routine & restart routine from somewhere else. Theres no interleaving & no need for sophisticated algorithms or policies to determine which job will run next or for how long.
Interrupt Handlerso Interrupt Handlers - are Responsible for Synchronizing Processeso PC has 256 interrupts & interrupt handlers, & they are accessed via interrupt vector table.o Three types of interrupts:• Internal hardware interrupts -- generated by certain events occurring during program’s execution (e.g., division by zero).• External hardware interrupts -- caused by peripheral device controllers or by coprocessors & assigned by manufacturers.• Software interrupts -- generated by system & application programs to access DOS & BIOS functions, which, in turn, access system resources.
Interrupt Handlers (cont)• Software interrupts (continued) – Some activate specialized application programs • Take control of computer • Example: Borland’s SideKick (type of TSR) – Terminate and Stay Resident (TSR) interrupt handler • Terminates process without releasing memory • Used by subroutine libraries • Sets up memory tables • Execution preparation via DOS interrupt connection • Determines memory required • Sends return code back to parent
Interrupt Handlers (cont)• Interrupt synchronization – CPU senses interrupt • Puts on stack: contents of PSW (program status word), code segment register, and instruction pointer register • Disables interrupt system • Uses eight-bit number to obtain interrupt handler address • Interrupt handler reenables interrupt system: allows higher-priority interrupts to occur • Saves registers and processes interrupt
Device Managemento Ability to reorder requests to optimize seek & search time is not a feature of DOS because it’s designed for a single-user environment. All requests are handled on a first-come first-served basis. Since version 3.0, BIOS can support spooling so users can schedule several files to be printed one after the other.o MS-DOS written for simple systems using keyboard, monitor, printer, mouse, 1-2 serial ports, & maybe second printer. Devices do not require special management from OS. Device drivers are the only items needed by Device Manager to make system work. A device driver is a software module that controls an I/O device and handles its interrupts.
Managing Fileso Earliest versions of MS-DOS kept every file in a single directory.o Version 2.0 implemented hierarchical directory structure.o When disk is formatted, its tracks are divided into sectors of 512 bytes.o 2-8 sectors are grouped into clusters & thats how File Manager allocates space to files.
FORMAT Creates Three Special Areas on Disk• Boot record -- first sector of every logical disk & contains disk boot program & table of disks characteristics.• Root directory -- Lists system’s primary subdirectories & files (name, extension, size, date & time of modification, starting cluster #, file attribute codes).• FAT (File Allocation Table) -- contains status info about disk’s sectors: which are allocated, which are free, and which can’t be allocated because of errors.
Managing Files in DOSo Supports noncontiguous file storage & dynamically allocates disk space to a file, provided there’s enough disk room.o Compaction (MS-DOS Version 6.0) available via utility used to defragment disk (DEFRAG.EXE).o CHKDSK command used to determine need for compaction.o Restricting user access to computer system & resources isn’t built into MS-DOS.
User Interfaceo Command-driven operating system.o When user presses Enter key, shell (COMMAND.COM) interprets command & calls on next lower level routine to satisfy request.o User commands include some or all of these elements in this order:command source-file destination-file switches
Batch Files & Redirectiono By creating customized batch files, users can quickly execute combinations of DOS commands to configure their system, perform routine tasks, or make it easier for non-technical users to run software.o MS-DOS can redirect output from one standard input or output device to another. command > destination E.g., dir > PRN
Filter Commands (SORT, MORE)o Filter commands accept input from default device, manipulate data in some fashion, & send results to default output device.o SORT accepts input from keyboard, sorts that data, & displays it on screen. Sort the file by column.o MORE causes output to be displayed on screen in groups of 24 lines, one screen at a time, & waits until user presses Enter key before displaying next 24 lines.
Pipe• Cause standard output from one command to be used as standard input to another command. – Symbol is a vertical bar, |. – Alphabetically sort directory & display sorted list on screen: DIR | SORT• Combine pipes and other filters. TYPE INVENTRY.DAT | MORE MORE < INVENTRY.DAT DIR | SORT | MORE DIR | SORT > SORTFILE MORE < SORTFILE
Summary• MS-DOS – Written to serve 1980s personal computer users• Limitation – Limited flexibility – Operating system unusable as hardware evolved• First standard operating system – Adopted by personal computing machine manufacturers – Supported by legions of software design groups
Summary (continued)• Advantages – Fundamental operation – Straightforward user commands• Weakness – Design • Single-user/single-task systems – No multitasking, networking, sophisticated applications support