Lecture 9: SHELL PROGRAMMING (continued) Creating shell scripts!

1. Lecture 9:SHELL PROGRAMMING (continued)Creating shell scripts!

2. NAMEchex-change a file to be executable SYNOPSISchex filename DESCRIPTIONThis is the outline for chexSelect shApply chmod u+x to file named as argument ($1)Inform user that file is executableUse ls-l to show the file modes Making a File Executable

3. NAMEmywc-labeled word count SYNOPSISmywc filename DESCRIPTIONThis is the outline for mywcSelect shRun wc on $1 and capture output with setPrint the filename ($4)Print the number of lines ($1)Print the number of words ($2)Print the number of characters ($3) Labeling the output from wc: mywc

4. NAMEdel- delete a file interactively SYNOPSISdel filename DESCRIPTIONThis is the outline for delSelect shGet the filename from the command line ($1)If there is no file with that nameprint an error message OtherwiseAsk if the user wants to delete the fileRead the user's choice (y/n)If the choice is yes (y)remove the file and print a messageotherwise print a message Removing files safely: del

5. NAMEtickle- a daily reminder service SYNOPSIStickle DESCRIPTIONThis is the outline for tickleSelect shUse set to capture the output from datePrint a messageCheck day ($1) and print an appropriate message A daily reminder system: tickle

6. NAMEmyspell- an improved spelling-checker SYNOPSISmyspell file DESCRIPTIONHere is the outline for myspell:Select shRun shell on the file; for each misspellingdoRun grep to find lines with misspellingsPrint the misspelled wordPrint the line(s) containing misspellingsdone An improved spell program

7. Memory Management

8. Memory Allocation • Information in memory are used in many different ways. Some classifications are: • Programming Language • Instructions: specify the operations to be performed • Variables: information that change during the program execution • Constants: information used as operands, but never change • Changeable • Read-only: program code , constants • Read-Write: variables

9. Memory Allocation (ctd) • Initialised • Yes: for code, constants, and some variables • No: Most variables • Static or Dynamic location • Static: data are stored at a fixed address • Dynamic: the location can be changed if the memory is rearranged (garbage collection, relocation) • Binding time (when the memory is allocated to objects) • Static: location is determined before the program execution (compiler, linker, loader) • Dynamic: location is defined at runtime and it may change

10. 0x000... Code Data Heap Stack 0x7fff... Memory Allocation (ctd) • What does a process in memory look like? • In Unix OS, it is divided into areas called segments • Code (text) segment, Data and Stack segments • Why a process has different segments? • Isolate read-only data from read-write data

11. Memory Management • Memory management is vital in computer systems: memory needs to be allocated efficiently to pack as many processes into memory as possible • The OS process that manages memory is called memory manager • The memory manager process has the following roles: • Keep track of which parts of memory is in use which parts are not • Allocate memory to processes when they need it and de-allocates it when they are done • Manage swapping between main memory and disk when the main memory can not hold all the processes

12. Multiprogramming Systems • Multiprogramming • Providing interactive service to several people simultaneously • Ability to have more than one process in memory at once in order to achieve reasonable performance • Improving performance by attempting to keep the processor(s) busy all the time • suspend processes when they are executing I/O operations • run processes which are not executing I/O operations

13. I/O Performance

14. Memory Systems • During the process execution the memory unit • sees only a stream of memory addresses • does not know how addresses are generated • The memory unit is interested only in the sequence of memory addresses

15. Binding and Relocation • Compile Time • Absolute code can be generated • Need to recompile if the starting location has changed • Example: MS-DOS “.COM” programs • Load Time • Re-locatable code is generated at compile time • The binding is done at load time • No need to recompile if the starting location has changed • Run-Time • The binding is done at the execution time • The process can be moved from one memory segment to another • Special hardware must be provided to implement this scheme (MMU)

16. OS in ROM OS in ROM User Program No swapping & No paging User Program User Program OS in RAM OS in RAM MM Systems • MM systems can be divided into 2 classes: • no swapping and no paging – Mono-programming • swapping and paging – Multiprogramming

17. Memory Organisation • How should memory be organised in order to have more than one process in it ? • First solution: divide the memory into n fixed partitions not necessarily equal • Second solution: divide the memory into variable partitions (dynamic partitioning) • Third solution: virtual memory technique

18. Multiple input queues Single input queue Partition 4 Partition 4 Partition 3 Partition 3 Partition 2 Partition 2 Partition 1 Partition 1 OS OS Fixed Partitions

19. Fixed Partitions (ctd) • Advantages • simple implementation and effective on batch systems • Disadvantages • Create internal fragmentation problem • A multiple input queues strategy can lead to dramatic loss of memory performance (when large partitions are empty and the queue for small jobs is full) • Not suitable for a large number of processes • Not suitable for time-sharing model because of the dynamic behaviour of the system • The size of processes is not taken into account

20. Variable Partitions • Features • Used when the number and size of the processes in memory vary dynamically • The size of processes is taken into account • Number, location and size of the partitions vary dynamically depending on the processes running in the system • Creates external fragmentationproblem • The external fragmentation can be solved by compaction

21. Variable Partitions 128K 128K 128K 128K O/S O/S O/S O/S Process 1 320K Process 1 320K Process 1 320K Process 2 224K Process 2 224K Process space Process space Process 3 896K 576K 288K 352K 64K 128K 128K 128K 128K O/S O/S O/S O/S Process 2 Process 1 320K Process 1 320K 320K 224K 96K Process 4 128K Process 4 128K 224K 96K 96K Process 4 128K Process 3 288K Process 3 Process 3 288K 288K 96K 64K 288K Process 3 64K 64K 64K

22. Paging Technique • Overlays • When a program is too big to fit in the available memory, the programmer has to split it into pieces • The swapping is done by the OS • Paging • The physical memory is partitioned into small equal fixed-size chunks (called frames) • The logical memory is also divided into small and equal fixed-size chunks (called pages)

23. Paging Technique (ctd) • Features • Internal fragmentation: only a fraction of the last page of a process • No external fragmentation • In order to load the pages of a process into non contiguous frames the OS needs page table for each process • Logical address consists of a page number and an offset within the page • Translation logical-to-physical address is done by the processor hardware

24. Logical Address Physical Memory CPU P d F d Physical Address P Page table Paging Hardware

25. Logical Address CPU P d Physical Memory TLB hit F d Physical Address P TLB miss Page table Paging Hardware

26. Analysis • Definition • Hit ratio: percentage of times that a page number is found in the associative registers • Performance • Effective Access Time: • Hit ratio x TLB access time + Miss ratio x Memory access time

27. Segmentation Technique • Segmentation • The logical memory is divided into a number of segments of different length • Logical address consists of two parts: segment number and an offset • Features • With segmentation a process may occupy more than one partition • Internal fragmentation: solved • External fragmentation: Not solved

28. trap: addressing error No Physical Memory Yes < CPU S d + S Segment table limit Base