Thus far we have discussed the three levels of computer languages. It is also important to consider the layers of software that are used inside a computer. First, remember that all computer instructions are software. However, the software inside a computer serves many different purposes. You have already learned that a computer contains several "entities:" the Boot program, DOS and Application programs. Each of these sets of instructions are programs. Boot instructions are stored in a special way inside a computer, using a special form of memory called ROM, for Read Only Memory, a set of computer "chips" that store special instructions to get the computer started when it is turned on.
By contrast, we saw that the RAM [Random Access Memory, Main Memory] used to hold DOS and Applications programs is erased each time the machine is turned off. ROM is necessary for the Boot program, but since we need to change the conditions under which DOS works, and we want to switch applications in a general purpose computer, we never use ROM except for permanent, fixed programs like the Boot program.
DOS is also a program, one that controls the user's interaction with the computer, once Boot has checked it out to make sure it is in working order. Through DOS, the user can access Application Programs. Applications come in several different broad categories, five of which are most common in today's personal computers:
One final application area not mentioned in the list above is the large number of High-Level Languages that allow users like yourself to develop your own, customized software. In this class, you have been working with MUMPS or Scheme - programming languages that are especially powerful for handling text-related problems with a computer.
In summary, remember the layers of software in a computer. The Boot program starts and then gives control to the Operating System. The operating system interfaces directly with the hardware of the computer. Applications programs and other environments form additional layers "on top" of the operating system. Users are able to customize their environment more and more as software becomes more flexible. In this lecture and the next the performance of operating systems and the reasons for having them are described in more detail.
Operating system - A set of programs that lies between the applications software and the computer hardware.
Operating systems are extremely important - if there is a problem with the operating system, it is difficult to get any work done. Much of what the operating system does is accomplished silently. We usually only notice the operating system if something goes wrong.
System software - The operating system plus programming language compilers, interpreters, utilities, etc.
There are 3 categories of files most people use:
Operating systems count when purchasing software. Software/applications programs run on only one operating system (i.e. Mac, DOS, Windows, Unix, ...). If you want to run a program on more than one operating system, you must buy a different version for each one. However, if you have the source code in a high-level language and compilers for that language on each machine, then you can compile the program to create a version for each different machine and operating system.
The 3 Primary Functions of the operating system are to manage resources, establish a user interface, and execute and perform services for applications programs.
Resource allocation - The process of assigning (allocating) computer resources to certain programs for their use. This includes the CPU and RAM, as well as devices like floppy disks, hard disks, printers, and CDROM drives. This is much more complex than it seems at first. Imagine trying to coordinate 2 or more children (programs) playing with a dozen toys (resources). There might be conflicts if more than one child (program) wants to play with the same toy (resource) at the same time.
User Interface - Common user interfaces are text versions and graphical versions (GUIs). GUIs work with on-screen icons (small pictures) and pull-down menus with a mouse as a pointing device. The keyboard is also used for commands and for data entry.
You have experience with both types with DOS (text) and Windows95 (GUI). In order for the user to perform file management (create directories or copy files, for example) tasks, there must be some way for the operating system to accept user commands. This is the function of the user interface to the operating system.
Applications software execution and services - The operating system is responsible for executing (starting) applications programs like spreadsheets and wordprocessors, as well as providing services for those applications. We have seen how to execute programs in both DOS and Windows95.
What do me mean by providing services? Applications programs do not have permission to access the devices attached to the computer (what if more than one program did so at the same time..). That is an operating system responsibility. So when you save a file in a wordprocessor or spreadsheet, the application program asks the operating system to perform that service.
PC operating systems (DOS) were originally restricted to
We will see below and in the following lecture, the additional capabilities demanded of operating systems for large computers to accommodate not only multiple tasks, but multiple programs executing by multiple users.
Concurrently - Two or more tasks happening within the same time frame, but one at a time.
Simultaneously - Two or more tasks happening at the same exact time.
Multitasking - The capability of running several programs on the same CPU in the same time frame (may only be a single user, like Windows). This is one of the major advantages of Windows95 (and could also be accomplished in Win 3.1 by running a shell in DOS).
Multiprogramming - permits the running of several programs, probably from many users, in the same time frame. Also called concurrently. This is an additional capability of operating systems for large computers that we will discuss next.
Operating environments (like Windows 3.1) - Provide a GUI - graphical user interface, and add an additional layer between the user and the hardware.
In order for operating systems to accomplish this without being destructive, they must handle not only the multiprogramming aspects of the task, but also the multiple users. Multiple users means that file protection is necessary. There is really no file protection to speak of on PCs. Whoever is using the PC has access to everything.
Each large operating system has a method for protecting files in a user's directory, and of making those files "available" if the user so desires. We will talk more about file protection below and how it is accomplished in Unix.
Handling multiple programs on the same computer can be accomplished in two ways. One is multiprogramming, as we saw above, where two or more programs are executing on a single CPU (by taking turns based on some sharing scheme). The other way is Multiprocessing, which means using a large computer with more than one CPU, where each program runs on one of the CPUs.
Multiprogramming is accomplished either by allowing programs to be interrupted or by giving each program a "time slice" to work in and alternating between the programs.
Printing resources are shared with a process known as spooling. With print spooling, print jobs are not sent directly to the printer, but are put in a queue - usually they are printed in the order that they are sent. A special program in the operating system keeps track of what is waiting to print and what has finished printing. It must monitor the printer's status for things like a paper jam or out of paper, and notify the user.
Unix is the most widely used multi-user, multi-tasking operating system available. Most workstations use a version of the Unix operating system. Unix is the operating system on the IT suns, where your email is located. The two most common versions are called System V and BSD (developed at UC Berkeley).
Unix is expected to continue to dominate as the OS for workstations for many years to come. There are also free versions of Unix that run on PCs (Linux, Free BSD..) that are quite popular.
There is a (free) standard graphical user interface (GUI) common to most Unix systems called X windows. This is in use on the computers in the CS department's Computer Science Instructional Facility. Some computer companies have also produced GUIs specifically for their machines. Most of them work in a manner very similar to the X window system, and to other GUIs like Windows 95.
Unix can be a very complicated OS with many special features. However, it is relatively easy to learn the survival commands you actually need, that are comparable to the DOS commands you already know. We will see those below.
In the Unix operating system, file protection is performed as follows. Users can be divided into three groups
Each group of people (User, Group, Others) can be given any combination of permissions (Read, Write, eXecute), thus there are 9 separate "bits" of permission information stored with each file. The User (owner of the file) can change the permissions for any group at any time. (In Unix, the command is "chmod" with the following options and file name(s): the group(s) represented by 1 or more of ugo, the change in permission indicated by + to add and - to remove permission, and the type of permission indicated by 1 or more of rwx. For example, chmod go+r jokes.txt would give read permission to everyone in your group and the outside world to the file jokes.txt in your current directory).
When using your account on a Unix machine, you will have a specific directory available that is yours. That directory is called your home directory. The name of the directory is usually your login name. When you log in, you automatically start in your home directory (this is your default directory). You have control over all the files in that directory, including creating subdirectories, etc.
In order to see where your home directory is, you can use the command pwd, which stands for print working directory. This will print the entire path from the root directory, up to and including your home directory name. On each computer or for each organization, the computer system administrators decide where (the paths) they will put home directories. On the IT suns, home directories for people are in the following directories: /home /home0 /home22 /home96 /home97 /home98 and /home99.
Your Unix account on the IT machines can be used for much more than just reading email. You have a permanent "home directory" where you can store files. There are many software packages available. The emacs editor is available which is very similar to the Edwin editor you have been using in the Scheme labs. You are encouraged to learn and use your account on these larger computers. If you work for a company in the future and they have a large computer, chances are that computer will have a Unix operating system of one "flavor" or another. You can be confident that you know enough Unix to use that computer and can learn a bit more when necessary!
(note: DOS uses \ while unix uses / to separate directory names) DOS commands Unix command (similar) ------------ -------------------------- List files in a directory DIR [ < dir > ] LS [-l] [ < dir > ] Change current Directory CD < dir > CD < dir > Copy a file to another file or directory COPY < file > < file or directory > CP < file > < file or directory > Getting help with commands HELP < command > MAN -k < topic > FASTHELP (list of commands) MAN < command > Move a file from one location to another, or rename the file MOVE < file > < file or directory > MV < file > < file or directory > Create a new directory MD < dir name > MKDIR < dir name > Delete a file DEL < file(s) > RM < file(s) > Delete a directory RMDIR < dir name > RMDIR < dir name > View the contents of a file all at once TYPE < file name > CAT < file name > View the contents of a file one page at a time TYPE < file name > | MORE MORE < file name > Print a file PRINT < file name > LPR < file name > or LP < file name >
Some commands are found in DOS only. Why?
DISKCOPY - Many workstations don't have diskettes drives.
Instead, they have tape drives or other devices that store larger
amounts of data. For those workstations that do have a diskette drive
(like the smaller workstations in the Computer Science department
labs), one must be within reach of that drive to use it (in other
words, at the console and not logged in via modem or telnet from
another machine located elsewhere). On many large computer systems,
only the support staff have access to the console and drives of the
system (for security reasons).
FORMAT (ditto)
UNDELETE - sometimes works in DOS, impossible in Unix with multitasking
C: Disk drives are not addressed by letters as in DOS. All files on all drives are accessible through a single file hierarchy starting with the root directory "/"
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