Understanding Operating Systems. 2. MS-DOS, PC-DOS or DOS. Developed to run single-user, stand-alone desktop computers. Exemplifies early OS because it manages jobs sequentially from single user. Advantages: fundamental operation
Ch. 12 MS-DOS Operating System
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Version 2.0, MS-DOS support dynamic allocation, modification, & release of main memory blocks by applications.
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.
Except for COM files, there can be any number of files in TPA at one time.
Memory Block Allocation
Allocates memory by using first-fit algorithm & linked list of memory blocks.
With Version 3.3, use best-fit or last-fit strategy.
With last-fit, allocates highest addressable memory block big enough to satisfy program’s request.
Size of a block can vary from 16 bytes (paragraph) to maximum available memory.
First 5 Bytes of Memory Block Define Block’s Structural Characteristics
Busy Free Busy Free Busy
Block 1 Block 2 Block 3 Block 4 Block 5
Free/Busy Block List
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.
Well-designed application program releases memory block it no longer needs.
MS-DOS wasn’t written in reentrant code because it was designed for a single-user, single-task environment.
Reentrant code is basis for multitasking.
Programs can’t break out of middle of DOS internal routine & restart routine from somewhere else.
There's no interleaving & no need for sophisticated algorithms or policies to determine which job will run next or for how long.
Some software programs give illusion that two jobs can run together.
E.g., Microsoft Windows versions 1.0 to 3.0.
Possible because MS-DOS interrupt handlers give programmers capability to save all info about parent program that allows its proper restart after child program finishes.
Interrupt Handlers Are Responsible for Synchronizing Processes
PC has 256 interrupts & interrupt handlers, & they are accessed via interrupt vector table residing in lowest bytes of memory.
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.
Terminate and Stay Resident (TSR) -- a process without releasing its memory, thus providing memory-resident programming facilities.
Ability to reorder requests to optimize seek & search time is not feature because it’s designed for a single-user environment.
All requests are handled on a first-come first-served basis.
Version 3.0 BIOS can support spooling so users can schedule several files to be printed one after the other.
MS-DOS written for simple systems using keyboard, monitor, printer, mouse, 1-2 serial ports, & maybe second printer.
Use one of each type of I/O device for each port, so no device channels.
Devices do not require special management from OS.
Device drivers are only items needed by Device Manager to make system work.
File Name Conventions
File name consists of :
Drive name (usually A, B, C, or D) is followed by a colon (:).
Directories/subdirectories are 1-8 characters long & preceded by back slash (\).
Primary file name is 1-8 characters long.
Extension is 1-3 characters long with possible special meaning.
Relative name --primary name & extension, if used.
Absolute name -- drive designation & directory location (path) followed by its relative name.
Earliest versions of MS-DOS kept every file in a single directory.
Version 2.0 implemented hierarchical directory structure.
Inverted tree directory structure.
When disk is formatted its tracks divided into sectors of 512 bytes.
2-8 sectors are grouped into clusters & that's how File Manager allocates space to files.
Supports hidden files—files that are executable but not displayed in response to DIR commands.
FORMAT Creates Three Special Areas on Disk
Boot record -- first sector of every logical disk & contains disk boot program & table of disk's characteristics.
Root directory -- where system begins its interaction with user. 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 formatting errors.
Managing Files in DOS
Looks at data in a disk file as a continuous string of bytes.
I/O operations request data by relative byte (relative to beginning of file) rather than by relative sector.
Transformation from physical sector to relative byte address is done by File Manager.
Supports noncontiguous file storage & dynamically allocates disk space to a file, provided there’s enough disk room.
Compaction (MS-DOS Version 6.0) available via utility used to defragment disk (DEFRAG.EXE).
CHKDSK command used to determine need for compaction.
Restricting user access to computer system & resources isn’t built into MS-DOS.
Command-driven operating system.
When user presses Enter key, shell (COMMAND.COM) interprets command & calls on next lower level routine to satisfy request.
User commands include some or all of these elements in this order:
command source-file destination-file switches
Batch Files & Redirection
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.
MS-DOS can redirect output from one standard input or output device to another.
command > destination
Filter Commands (SORT, MORE)
Filter commands accept input from default device, manipulate data in some fashion, & send results to default output device.
SORT accepts input from keyboard, sorts that data, & displays it on screen.
Sort the file by column.
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.
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
FIND -- filter command that searches for specific string in given file or files & displays all lines that contain string from those files.
PRINT -- allows user to set up series of files for printing while freeing up COMMAND.COM to accept other commands.