Review : What is An Operating System? ! Software ( kernel ) that - PDF document

Review : What is An Operating System? ! Software ( ‘ kernel ’ ) that runs at all times » Really, the part of the system that runs in ‘ kernel CSCI 6730 / 4730 mode ’ (or need to). Operating Systems » But note - there are even exceptions to this ‘ rule ’ ! Distinguishing what makes up the OS is challenging (some grey areas) Structures & System Design ! OS performs two unrelated functions: » (1) Provide abstractions of resources to the users or applications programs (extends the machine), and » (2) Manage and coordinate hardware resources (resource manager) – CPU, memory, disk, printer 2 Maria Hybinette, UGA Maria Hybinette, UGA The OS provides an Extended Machine Key Questions in System Design How to provide a beautiful interface, consider: ! What does the OS look like? to the user? ! What services does an operating system provide? • Memory Management • Process Management • File Management • I/O System Management … user � user � user � 1 � 2 � � 3 • Protection & Security … compiler assembler text editor System and Application ! Operating System turn the ugly ugly hardware into Programs Operating System bea eautiful abstractions. Computer Hardware 3 4 Maria Hybinette, UGA Maria Hybinette, UGA Review: Operating System Two Operating System Design Criteria Roles ! How do you hide the complexity and ! Operating System limitations of hardware from application » A machine that (1) emulates the … � programmers? user � user � user � hardware and (2) provides a nice 1 � 2 � � n programming environment for » What is the hardware interface? (the physical [multiple] … reality) ‘ activities ’ (processes) in the compiler assembler text editor » ~~~~~~~ Transformations ~~~~~~~~~~~ � system. System and Application Programs – Definition: A process is an activity » What is the application interface? (what are the in the system – a running program, Operating System nicer and more beautiful abstractions) an activity that may need ‘ services ’ (we will cover this Computer concept in detail next week). Hardware In terms of particular hardware (i.e., CPU, Memory, Network) what criteria does your system need to address (solve). 5 6 Maria Hybinette, UGA Maria Hybinette, UGA

Example Design Questions Summary of OS Key Roles ! How to make multiple CPU appear as one CPU but faster? ! Provide standard services or resources: ! How to make limited memory appear as infinite (e.g., a large array may not fit into memory). » Screen, CPU, I/O, disk, mouse ! How to make a mechanical disk appear to be as fast as » Resource abstraction (extended machine) electronic memory? ! Provide for sharing of resources: ! How to make insecure, unreliable network transmissions » coordinate between multiple applications to work appear to be reliable and secure? together in ! How to make many physical machines appear to be a – safe, efficient, and fair ways (protected) single machine? ! Fairness Fairness » Resource coordination & management. ! Timeliness Timeliness ! Secure Secure ! Reliable Reliable ! Ownership Ownership ! Single Single ‘ machininess machininess ’ 7 8 ! Power-efficient Power-efficient Maria Hybinette, UGA Maria Hybinette, UGA Shell: Another Level of Resource provided to users ! Example: Accessing a raw disk involves ! Provide ‘ users ’ with access to the services of the kernel. » specifying the data, the length of data, the disk drive, the track location(s), and the sector location(s) within the » A ‘ shell ’ of-course,– illusion of a thin layer of corresponding track(s). (150 mph) abstraction to the kernel and its services. write( block, len, device, track, sector ); ! CLI – command line interface to kernel ! Problem: But applications don ’ t want to worry about the services (project 1 focus) complexity of a disk (don ’ t care about tracks or sectors) ! GUI - graphical user interface to the kernel lseek( file, file_size, SEEK_SET ); » ! Project 1 [Check DONE write( file, text, len ); Person -%+'&.$% Shell Abstraction System Calls !"#$%)*&++,'#% OS Abstraction !"#$%&'(% Heads generate a 9 10 magnetic field that Hardware Maria Hybinette, UGA polarize the disk Maria Hybinette, UGA What is the functionality of a [CLI] ‘ shell ’ Review and Moving On… ! Provides two things: ! Looked at the OS role in abstracting the ‘ machine ’ (system calls, and shells). » Interactive Use (IU) » And an environment for ‘ scripting ’ (programmable) ! Moving on to management now. » Project 1 : dealt primarily with IU. » To provide effective resource sharing -> we need ‘ protection ’ (do we?) ! Tcsh – great for IU, not so great for scripting ! Ksh – original version – bulk of code is to provide a great environment for scripting provides powerful programming constructs). » [problem?] Proprietary (until recently -2006) 11 12 Maria Hybinette, UGA Maria Hybinette, UGA

Coordination : Resource Sharing Coordination : Resource Sharing ! Example Goal: Protect the OS from other activities and provide protection across activities. ! Other areas of protection: ! Problem: Activities can crash each other (and crash the OS) » Writing to disk (where) – really any form of I/O. unless there is coordination between them. » Writing / Reading Memory ! General Solution: Constrain an activity so it only runs in its » Creating new processes own memory environment (e.g., in its own sandbox), and make sure the activity cannot access other sandboxes. » Sandbox: Address Space (memory space) ! How do we create (and manage) these – It ’ s others memory spaces that the activity can ’ t touch ‘ areas ’ of protection. including the Operating System ’ s address space 13 14 Maria Hybinette, UGA Maria Hybinette, UGA Hardware: Different modes of Protection Implementation: “ Dual Mode” Operations protection (>2 Intel) How does the OS prevent arbitrary programs (run by arbitrary users) ! Hardware provides different mode ‘ bits ’ of protection – from invoking accidental or malicious calls to halt the operating system where at the lowest level – ring 0 – anything goes, or modify memory such as the master boot sector? unrestricted mode (trusted kernel runs here). ! General Idea: The OS is omnipotent and everything – Intel x86 architecture provides multiple levels of protection: else isn ’ t - as simple as that » Utilize Two modes CPU operation (provided by hardware) – Kernel Mode – Anything goes – access everywhere (unrestricted access) to the underlying hardware. ! In this mode can execute any CPU instruction and reference any memory access – User Mode – Activity can only access state within its own address space (for example - web browsers, calculators, compilers, JVM, word from microsoft, power point, etc run in user mode). 15 16 Maria Hybinette, UGA Maria Hybinette, UGA Hardware: Provides Dual-Mode Operation “System Calls” (e.g., Intel’s trap()) ! Mechanism for user activities (user processes) to ! Mode bit (0 or 1) provided by hardware access kernel functions. » Provides ability to distinguish ! Example: UNIX implements system calls ( ‘ request when system is running user Interrupt/fault, system call � code or kernel code calls ’ ) via the trap() instruction (system call, e.g., read » Mode 1 : normal when address () contains the trap instruction, internally). space is ‘ limited ’ user � kernel � ! Mode bit switch a libc is ;'&4.<%=>7()? % ‘ interrupts ’ (trap) or when !" intermediate 9&@*,% 6,+%3,'4,*%012,% library that calling set user mode trap 2 handles the set user mode � 9'7#+,2%:12,% ‘ packaging ’ A% Trap in Linux is INT 0x80 /#,'%012,% 3,'4,*567),'8"#1'%012,% assembly • Question: What is the mechanism from the point of instruction view of a process to access kernel functions (e.g., it ! When the control returns to the user code the CPU is wants to write to disk)? switched back to User Mode. 17 18 Maria Hybinette, UGA Maria Hybinette, UGA