Chapter 6. An Introduction to System Software and Virtual Machines. INVITATION TO Computer Science

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1 Chapter 6 An Introduction to System Software and Virtual Machines INVITATION TO Computer Science 1

2 Objectives After studying this chapter, students will be able to: Describe the main categories of system tasks Compare the virtual machine created for the user by system software with the naked machine Explain the benefits of writing system software in assembly language, rather than machine language Read and write short assembly language programs Describe how an assembler translates assembly language programs into machine instructions Invitation to Computer Science, 6th Edition 2

3 Objectives (continued) After studying this chapter, students will be able to: List five key tasks of an operating system, and explain what each is and why it is critical to modern systems Describe the different generations of operating systems, what the features of each generation were, and how each generation solved a drawback of the previous generation Invitation to Computer Science, 6th Edition 3

4 Introduction A naked machine has no tools or programs to help the user Write instructions in binary Write data in binary Load instructions into memory one cell at a time Initiate program run Too difficult for humans to do We must build an interface to hide the details and make the computer easier to build Invitation to Computer Science, 6th Edition 4

5 System Software System software is a collection of programs to manage resources of the computer serve as intermediary between user and hardware System software creates a virtual machine (or virtual environment) that user sees Invitation to Computer Science, 6th Edition 5

6 System Software (continued) Operating system: central system software Communicates with users Starts up other system software and applications as needed Graphical user interface (GUI), often pronounced gooey visual interface to operating system (or other system software/applications) I/O systems: communicate with various devices Invitation to Computer Science, 6th Edition 6

7 System Software (continued) Language services: support high level languages Memory managers: allocate memory to programs Information managers: organize mass storage Scheduler: manage programs waiting to run Utilities: tools, including program librariesi Invitation to Computer Science, 6th Edition 7

8 System Software (continued) Naked machine: 1. Write program in binary 2. Load instructions one-by- one into memory 3. Insert start into memory address 0 and push go button 4. Read results from memory one-by-one, in binary Virtual machine: 1. Write program using text editor in high-level language 2. Save program to folder 3. Use translator t to convert to binary 4. Use scheduler to load and run 5. Use I/O system to print results Invitation to Computer Science, 6th Edition 8

9 Assemblers and Assembly Language Assembly language Instructions map one-to-one to machine language Symbolic op codes (not binary) Symbolic addresses for instructions and data Pseudo-ops for data generation and more (data in human-friendly terms) Assembly = a low-level l l programming language Java, C, Python = high-level programming languages Invitation to Computer Science, 6th Edition 9

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11 Assemblers and Assembly Language (continued) Assembly language process: Source program, in assembly language Translated by the assembler to Object program, in machine language Loader places in memory Hardware runs Invitation to Computer Science, 6th Edition 11

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13 Assemblers and Assembly Language (continued) Example Assembly language: NEXTSTEP: LOAD X -- Put X into reg. R label: opcode address field -- comment Label is optional name for this instruction s s location Op code mnemonic and address field translate to machine language Comments are ignored by assembler, just for human use Invitation to Computer Science, 6th Edition 13

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15 Assemblers and Assembly Language (continued) Advantages over machine code: Clarity, readability, and maintainability Can be placed at different locations in memory Pseudo-op: commands in the program directed to the assembler, not converted to machine instructions.begin and.end to mark where instructions are.data to mark memory location as holding data: COUNTER:.DATA 0 X:.DATA 12 Invitation to Computer Science, 6th Edition 15

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17 Assemblers and Assembly Language (continued) Simple examples: x = y + 3 input a and b while a < b do print a a = a 2 LOAD Y ADD THREE STORE X -- Data comes after.end X:.DATA 0 -- X is initially 0 Y:.DATA 5 Y is initially 5 THREE:.DATA 3 The constant 3 IN A IN B LOOP1: LOAD A A:.DATA 0 COMPARE B B:.DATA 0 JUMPLT LOOP1END TWO:.DATA 2 JUMPEQ LOOP1END OUT A LOAD A SUBTRACT TWO STORE A JUMP LOOP1 LOOP1END: Invitation to Computer Science, 6th Edition 17

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20 Assemblers and Assembly Language Translation and loading: (continued) Assembler translates to machine language Convert symbolic op codes to binary equivalents Convert symbolic labels to memory addresses Perform pseudo-op actions Write object file containing machine instructions Loader gets program ready to run Places instructions in memory Triggers the hardware to run the program Invitation to Computer Science, 6th Edition 20

21 Assemblers and Assembly Language (continued) Converting symbolic op codes to binary ones Assembler maintains a table Assembler looks up symbolic op codes in the table and substitutes the binary analogue Use binary search to optimize table lookups 21

22 Assemblers and Assembly Language (continued) Converting symbolic labels to memory addresses Assembler needs two passes looks over assembly code two times First pass: Keep a count of how many instructions from the start Collect symbolic labels and add to symbol table along with location counter Invitation to Computer Science, 6th Edition 22

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25 Assemblers and Assembly Language Second pass: (continued) Look up and replace op codes Substitute label references with location from symbol table Set up.data pseudo-ops with location and binary value Write instructions to object file Invitation to Computer Science, 6th Edition 25

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28 Problem 5 on page 328 Exercises Problem 7 on page 328 Problem 9 (a) and (c) on page 328 Solutions are posted in your D2L account. Invitation to Computer Science, 6th Edition 28

29 Homework Assignments Part 1 Problem 4 on page 328 Problem 9 (b), (d) and (e) on page 328 Problem 15 on page 329 Part 2: Lab programming Invitation to Computer Science, 6th Edition 29

30 Operating Systems System commands: user instructions about what the computer should do Demonstrate system commands of the DOS system: type help in the command line. Invitation to Computer Science, 6th Edition 30

31 Operating Systems (continued) User interface: user communicates with operating system Operating system as receptionist and dispatcher User command system software scheduled and run repeat Text-based: System commands typed at a prompt in a terminal Command language must be learned GUI-based: System commands by visual/mouse interface Invitation to Computer Science, 6th Edition 31

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34 Operating Systems (continued) System security and protection: permit only authorized access to resources Operating system as security guard Access protected by usernames and passwords Superusers have more privileges Encrypt data to increase security Folders and files have authorization lists: read a file, add new information to a file, change existing information, delete a file Invitation to Computer Science, 6th Edition 34

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36 Operating Systems (continued) Efficient allocation of resources: keep CPU busy and all programs making progress Operating system as traffic cop Multiple programs active at one time Every yprogram is in one of three states: Running: program currently using the CPU Waiting: programs waiting for I/O operations to complete Ready: programs ready for a turn on CPU Keep a queue of ready programs and share Invitation to Computer Science, 6th Edition 36

37 Operating Systems (continued) Example of sharing CPU: Waiting Ready Running B A C A makes I/O request Waiting Ready Running A C B A finishes with I/O Waiting Ready Running C B A Invitation to Computer Science, 6th Edition 37

38 Operating Systems (continued) Safe use of resources: ensure that computer doesn t get stuck in deadlock Multiple programs requesting access to resources Deadlock occurs when all programs have some resources, and are waiting for resources held by others Deadlock prevention: if you can t get all resources, release all you have and try again later Deadlock recovery: if no acknowledgement, send message again Invitation to Computer Science, 6th Edition 38

39 Deadlock Invitation to Computer Science, 6th Edition 39

40 Operating Systems (continued) Historical development of operating systems First generation: nearly naked computer Programmer hand-loaded programs Second generation: batch operating system Programmers gave programs to operators Operators collects a batch of programs Used I/O computer to translate programs to tape Ran programs as a group Used I/O computer to translate output to text/paper Job control language: instructions to OS Invitation to Computer Science, 6th Edition 40

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43 Operating Systems (continued) Third generation: multiprogrammed operating systems Multiple programs loaded at once Switch between programs when I/O happens Computer security now required! Privileged operations only available to administrator Bound memory a program can legitimately access Invitation to Computer Science, 6th Edition 43

44 Operating Systems (continued) Time-sharing system (also third generation) Multiprogrammed, but users are on system interactively Users need illusion of sole access Allocate run time in time slices. Each program runs until I/O OR time runs out Invitation to Computer Science, 6th Edition 44

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46 Operating Systems (continued) Fourth generation: network operating system Operating system supports all the same local services Also supports services that access resources that are available over a network Shared printer Servers: , data, web Connections to Internet Real-time operating system or embedded system are special-purpose computers in other equipment Invitation to Computer Science, 6th Edition 46

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49 Operating Systems (continued) Fifth generation, the near future Multimedia interfaces (integrate images, speech, video seamlessly) Parallel processing system, to perform multimedia and to permit larger scale tasks Distributed ib t computing environment, users don t know where resources are stored Cloud computing Invitation to Computer Science, 6th Edition 49

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52 Review of Assembly Language Review previous slides on assembly language Problem 10 on page 329 Problem 16 on page 329 Solutions are posted in your D2L account. Invitation to Computer Science, 6th Edition 52

53 Summary System software creates a virtual machine that is easy for users to use Assemblers and loaders are system software: translate human-friendly programs to machine language Assembly language uses symbolic names, symbolic op codes, and pseudo-ops to describe algorithms Assembler translates source programs to object files; loader places object instructions in memory Invitation to Computer Science, 6th Edition 53

54 Summary (continued) Operating systems communicate with users through text or GUI actions The operating system includes key tasks such as: the user interface, system security, scheduling of programs, and system safety Operating systems developed through four generations of improvements, each adding new features and improving efficiency Future operating systems: parallel, distributed, and multimedia, with resources stored remotely Invitation to Computer Science, 6th Edition 54