Operating Systems. Engr. Abdul-Rahman Mahmood MS, PMP, MCP, QMR(ISO9001:2000) alphapeeler.sf.net/pubkeys/pkey.htm

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1 Operating Systems Engr. Abdul-Rahman Mahmood MS, PMP, MCP, QMR(ISO9001:2000) alphapeeler.sf.net/pubkeys/pkey.htm pk.linkedin.com/in/armahmood abdulmahmood-sss alphasecure mahmood_cubix VC++, VB, ASP

2 Operating Systems: Internals and Design Principles Operating System Overview

3 Operating Systems: Internals and Design Principles Operating systems are those programs that interface the machine with the applications programs. The main function of these systems is to dynamically allocate the shared system resources to the executing programs. As such, research in this area is clearly concerned with the management and scheduling of memory, processes, and other devices. But the interface with adjacent levels continues to shift with time. Functions that were originally part of the operating system have migrated to the hardware. On the other side, programmed functions extraneous to the problems being solved by the application programs are included in the operating system. WHAT CAN BE AUTOMATED?: THE COMPUTER SCIENCE AND ENGINEERING RESEARCH STUDY, MIT Press, 1980

4 Operating System A program that controls the execution of application programs An interface between applications and hardware Main objectives of an OS: Convenience Efficiency Ability to evolve

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6 Program development: IDE Program execution: process creation Access I/O devices Controlled access to files System access Error detection and response Accounting: response time, billing

7 Key Interfaces Instruction set architecture (ISA): boundary between hardware and software. Defines the repertoire of machine language instructions User ISA: subset of the instruction repertoire that application programs / utilities can access directly. System ISA: OS has access to additional machine language instructions that deal with managing system resources Application binary interface (ABI):Provides binary portability to programs & system call interface in OS. Application programming interface (API): gives a program access to the hardware resources and services available in a system through the user ISA supplemented with high-level language (HLL) library calls.

8 A computer is a set of (shared) resources for the movement, storage, and processing of data The OS is responsible for managing these resources

9 Functions in the same way as ordinary computer software Program, or suite of programs, executed by the processor Frequently relinquishes control and must depend on the processor to allow it to regain control

10 Operating System as Resource Manager

11 Evolution of Operating Systems A major OS will evolve over time for a number of reasons: Hardware upgrades early versions of UNIX did not employ a paging mechanism New types of hardware graphics terminals and page-mode terminals New services performance management tool Fixes patches

12 Evolution of Operating Systems Stages include: Multiprogrammed Batch Systems Time Sharing Systems Simple Batch Systems Serial Processing

13 Serial Processing Earliest Computers: No OS ( ) programmers interacted directly with the computer hardware Computers had a console with display lights, toggle switches, some form of input device, printer Users have access to the computer in series Scheduling: Problems: most installations used a hardcopy sign-up sheet to reserve computer time Setup time time allocations could run short or long, resulting in wasted computer time a considerable amount of time was spent just on setting up the program to run

14 Serial Processing

15 Simple Batch Systems Early computers were very expensive important to maximize processor utilization Monitor (software ) user no longer has direct access to processor job is submitted to computer operator who batches them together and places them on an input device program branches back to the monitor when finished

16 Monitor controls the sequence of events Resident Monitor is software always in memory Monitor reads in job and gives control Job returns control to monitor

17 Processor executes instruction from the memory containing the monitor Executes the instructions in the user program until it encounters an ending or error condition control is passed to a job means processor is fetching and executing instructions in a user program control is returned to the monitor means that the processor is fetching and executing instructions from the monitor program

18 Job Control Language (JCL) Special type of programming language used to provide instructions to the monitor what compiler to use what data to use $JOB $FTN [FORTRAN program] $LOAD $RUN [data] $END $FTN - execute FORTRAN compiler; $ASM - execute Assembler; $RUN - execute user program; $JOB - first card; $END - last card;

19 A job is a unit of work which can be made up of many job steps. Each job step is specified in a Job Control Language (JCL) through a set of Job Control Statements. The Operating System uses Job Entry System (JES) to receive jobs into the Operating System, to schedule them for processing and to control the output.

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21 //SAMPJCL JOB 1,CLASS=6,MSGCLASS=0,NOTIFY=&SYSUID (1) //* (2) //STEP010 EXEC PGM=SORT (3) //SORTIN DD DSN=JCL.SAMPLE.INPUT,DISP=SHR (4) //SORTOUT DD DSN=JCL.SAMPLE.OUTPUT, (5) // DISP=(NEW,CATLG,CATLG),DATACLAS=DSIZE50 //SYSOUT DD SYSOUT=* (6) //SYSUDUMP DD SYSOUT=C (6) //SYSPRINT DD SYSOUT=* (6) //SYSIN DD * (6) SORT FIELDS=COPY INCLUDE COND=(28,3,CH,EQ,C'XXX') /* (7)

22 (1) JOB statement - Specifies the information required for SPOOLing of the job such as job id, priority of execution, user-id to be notified upon completion of the job. (2) //* statement - This is a comment statement. (3) EXEC statement - Specifies the PROC/Program to be executed. In the above example, a SORT program is being executed (i.e., sorting the input data in a particular order) (4) Input DD statement - Specifies the type of input to be passed to the program mentioned in (3). In the above example, a Physical Sequential (PS) file is passed as input in shared mode (DISP = SHR).

23 (5) Output DD statement - Specifies the type of output to be produced by the program upon execution. In the above example, a PS file is created. If a statement extends beyond the 70th position in a line, then it is continued in the next line, which should start with "//" followed by one or more spaces. (6) There can be other types of DD statements to specify additional information to the program (In the above example: The SORT condition is specified in the SYSIN DD statement) and to specify the destination for error/execution log (Example: SYSUDUMP/SYSPRINT). DD statements can be contained in a dataset (mainframe file) or as in stream data (information hard-coded within the JCL) as given in above example. (7) /* marks the end of in stream data. All the JCL statements except in stream data starts with //. There should be at least one space before and after JOB, EXEC and DD keywords and there should not be any spaces in rest of the statement.

24 Memory protection for monitor while the user program is executing, it must not alter the memory area containing the monitor Timer prevents a job from monopolizing the system Privileged instructions can only be executed by the monitor Interrupts gives OS more flexibility in controlling user programs

25 Modes of Operation User Mode user program executes in user mode certain areas of memory are protected from user access certain instructions may not be executed Kernel Mode monitor executes in kernel mode privileged instructions may be executed protected areas of memory may be accessed

26 Simple Batch System Overhead Processor time alternates between execution of user programs and execution of the monitor Sacrifices: some main memory is now given over to the monitor some processor time is consumed by the monitor Despite overhead, the simple batch system improves utilization of the computer