Study Guide to Accompany Operating Systems Concepts 8th Ed by Silberschatz, Galvin and Gagne
By Andrew DeNicola, BU ECE Class of 2012
Figures Copyright (c) John Wiley & Sons 2012

Ch.1 – Introduction
* An OS is a program that acts as an intermediary between a user of a computer and the computer hardware
* Goals: Execute user programs, make the comp. system easy to use, utilize hardware efficiently
* Computer system: Hardware ? OS ? Applications ? Users (? = ‘uses’)
* OS is:
? Resource allocator: decides between conflicting requests for efficient and fair resource use
? Control program: controls execution of programs to prevent errors and improper use of computer
* Kernel: the one program running at all times on the computer
* Bootstrap program: loaded at power-up or reboot
? Stored in ROM or EPROM (known as firmware), Initializes all aspects of system, loads OS kernel and starts execution
* I/O and CPU can execute concurrently
* Device controllers inform CPU that it is finished w/ operation by causing an interrupt
? Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines
? Incoming interrupts are disabled while another interrupt is being processed
? Trap is a software generated interrupt caused by error or user request
? OS determines which type of interrupt has occurred by polling or the vectored interrupt system
* System call: request to the operating system to allow user to wait for I/O completion
* Device-status table: contains entry for each I/O device indicating its type, address, and state
? OS indexes into the I/O device table to determine device status and to modify the table entry to include interrupt
* Storage structure:
? Main memory – random access, volatile
? Secondary storage – extension of main memory That provides large non-volatile storage
? Disk – divided into tracks which are subdivided into sectors. Disk controller determines logical interaction between the device and the computer.
* Caching – copying information into faster storage system
* Multiprocessor Systems: Increased throughput, economy of
scale, increased reliability
? Can be asymmetric or symmetric
? Clustered systems – Linked multiprocessor systems
* Multiprogramming – Provides efficiency via job scheduling
? When OS has to wait (ex: for I/O), switches to another job
* Timesharing – CPU switches jobs so frequently that each user
can interact with each job while it is running (interactive computing)
* Dual-mode operation allows OS to protect itself and other system components – User mode and kernel mode
? Some instructions are only executable in kernel mode, these are privileged
* Single-threaded processes have one program counter, multi-threaded processes have one PC per thread
* Protection – mechanism for controlling access of processes or users to resources defined by the OS
* Security – defense of a system against attacks
* User IDs (UID), one per user, and Group IDs, determine which users and groups of users have which privileges

Ch.2 – OS Structures
* User Interface (UI) – Can be Command-Line (CLI) or Graphics User Interface (GUI) or Batch
? These allow for the user to interact with the system services via system calls (typically written in C/C++)
* Other system services that a helpful to the user include: program execution, I/O operations, file-system manipulation, communications, and error detection
* Services that exist to ensure efficient OS operation are: resource allocation, accounting, protection and security
* Most system calls are accessed by Application Program Interface (API) such as Win32, POSIX, Java
* Usually there is a number associated with each system call
? System call interface maintains a table indexed according to these numbers
* Parameters may need to be passed to the OS during a system call, may be done by:
? Passing in registers, address of parameter stored in a block, pushed onto the stack by the program and popped off by the OS
? Block and stack methods do not limit the number or length of parameters being passed
* Process control system calls include: end, abort, load, execute, create/terminate process, wait, allocate/free memory
* File management system calls include: create/delete file, open/close file, read, write, get/set attributes
* Device management system calls: request/release device, read, write, logically attach/detach devices
* Information maintenance system calls: get/set time, get/set system data, get/set process/file/device attributes
* Communications system calls: create/delete communication connection, send/receive, transfer status information
* OS Layered approach:
? The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface
? With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers
* Virtual machine: uses layered approach, treats hardware and the OS kernel as though they were all hardware.
? Host creates the illusion that a process has its own processor and own virtual memory
? Each guest provided with a ‘virtual’ copy of the underlying computer
* Application failures can generate core dump file capturing memory of the process
* Operating system failure can generate crash dump file containing kernel memory

Ch.3 – Processes
* Process contains a program counter, stack, and data section.
? Text section: program code itself
? Stack: temporary data (function parameters, return addresses, local variables)
? Data section: global variables
? Heap: contains memory dynamically allocated during run-time
* Process Control Block (PCB): contains information associated with each process: process state, PC, CPU registers, scheduling information, accounting information, I/O status information
* Types of processes:
? I/O Bound: spends more time doing I/O than computations, many short CPU bursts
? CPU Bound: spends more time doing computations, few very long CPU bursts
* When CPU switches to another process, the system must save the state of the old process (to PCB) and load the saved state (from PCB) for the new process via a context switch
? Time of a context switch is dependent on hardware
* Parent processes create children processes (form a tree)
? PID allows for process management
? Parents and children can share all/some/none resources
? Parents can execute concurrently with children or wait until children terminate
? fork() system call creates new process
? exec() system call used after a fork to replace the processes’ memory space with a new program
* Cooperating processes need interprocess communication (IPC): shared memory or message passing
* Message passing may be blocking or non-blocking
? Blocking is considered synchronous
? Blocking send has the sender block until the message is received
? Blocking receive has the receiver block until a message is available
? Non-blocking is considered asynchronous
? Non-blocking send has the sender send the message and continue
? Non-blocking receive has the receiver receive a valid message or null

Ch.4 – Threads
* Threads are fundamental unit of CPU utilization that forms the basis of multi-threaded computer systems
* Process creation is heavy-weight while thread creation is light-weight
? Can simplify code and increase efficiency
* Kernels are generally multi-threaded
* Multi-threading models include: Many-to-One, One-to-One, Many-to-Many
? Many-to-One: Many user-level threads mapped to single kernel thread
? One-to-One: Each user-level thread maps to kernel thread
? Many-to-Many: Many user-level threads mapped to many kernel threads
* Thread library provides programmer with API for creating and managing threads
* Issues include: thread cancellation, signal handling (synchronous/asynchronous), handling thread-specific data, and scheduler activations.
? Cancellation:
? Asynchronous cancellation terminates the target thread immediately
? Deferred cancellation allows the target thread to periodically check if it should be canceled
? Signal handler processes signals generated by a particular event, delivered to a process, handled
? Scheduler activations provide upcalls – a communication mechanism from the kernel to the thread library.
? Allows application to maintain the correct number of kernel threads
Ch.5 – CPU Scheduling
* Process execution consists of a cycle of CPU execution and I/O wait
* CPU scheduling decisions take place when a process:
? Switches from running to waiting (nonpreemptive)
? Switches from running to ready (preemptive)
? Switches from waiting to ready (preemptive)
? Terminates (nonpreemptive)
* The dispatcher module gives control of the CPU to the process selected by the short-term scheduler
? Dispatch latency- the time it takes for the dispatcher to stop one process and start another
* Scheduling algorithms are chosen based on optimization criteria (ex: throughput, turnaround time, etc.)
? FCFS, SJF, Shortest-Remaining-Time-First (preemptive SJF), Round Robin, Priority
* Determining length of next CPU burst: Exponential Averaging:
1. tn = actual length of nth CPU burst
2. tn+1 = predicted value for the next CPU burst
3. a, 0 = a = 1 (commonly a set to 1/2)
4. Define: tn+1 = a*tn + (1-a)tn
* Priority Scheduling can result in starvation, which can be solved by aging a process (as time progresses, increase the priority)
* In Round Robin, small time quantums can result in large amounts of context switches
? Time quantum should be chosen so that 80% of processes have shorter burst times that the time quantum
* Multilevel Queues and Multilevel Feedback Queues have multiple process queues that have different priority levels
? In the Feedback queue, priority is not fixed ? Processes can be promoted and demoted to different queues
? Feedback queues can have different scheduling algorithms at different levels
* Multiprocessor Scheduling is done in several different ways:
? Asymmetric multiprocessing: only one processor accesses system data structures ? no need to data share
? Symmetric multiprocessing: each processor is self-scheduling (currently the most common method)
? Processor affinity: a process running on one processor is more likely to continue to run on the same processor (so that the processor’s memory still contains data specific to that specific process)
* Little’s Formula can help determine average wait time per process in any scheduling algorithm:
? n = ? x W
? n = avg queue length; W = avg waiting time in queue; ? = average arrival rate into queue
* Simulations are programmed models of a computer system with variable clocks
? Used to gather statistics indicating algorithm performance
? Running simulations is more accurate than queuing models (like Little’s Law)
? Although more accurate, high cost and high risk

Ch.6 – Process Synchronization
* Race Condition: several processes access and manipulate the same data concurrently, outcome depends on which order each access takes place.
* Each process has critical section of code, where it is manipulating data
? To solve critical section problem each process must ask permission to enter critical section in entry section, follow critical section with exit section and then execute the remainder section
? Especially difficult to solve this problem in preemptive kernels
* Peterson’s Solution: solution for two processes
? Two processes share two variables: int turn and Boolean flag[2]
? turn: whose turn it is to enter the critical section
? flag: indication of whether or not a process is ready to enter critical section
? flag[i] = true indicates that process Pi is ready
? Algorithm for process Pi:
do {
flag[i] = TRUE;
turn = j;
while (flag[j] && turn == j)
critical section
flag[i] = FALSE;
remainder section
} while (TRUE);
* Modern machines provide atomic hardware instructions: Atomic = non-interruptable
* Solution using Locks:
do {
acquire lock
critical section
release lock
remainder section
} while (TRUE);
* Solution using Test-And-Set: Shared boolean variable lock, initialized to FALSE
* Solution using Swap: Shared bool variable lock initialized to FALSE; Each process has local bool variable key

* Semaphore: Synchronization tool that does not require busy waiting
? Standard operations: wait() and signal() ? these are the only operations that can access semaphore S
? Can have counting (unrestricted range) and binary (0 or 1) semaphores
* Deadlock: Two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes (most OSes do not prevent or deal with deadlocks)
? Can cause starvation and priority inversion (lower priority process holds lock needed by higher-priority process)
Ch.6 – Process Synchronization Continued
* Other synchronization problems include Bounded-Buffer Problem and Readers-Writers Problem
* Monitor is a high-level abstraction that provides a convenient and effective mechanism for process synchronization
? Only one process may be active within the monitor at a time
? Can utilize condition variables to suspend a resume processes (ex: condition x, y;)
? x.wait() – a process that invokes the operation is suspended until x.signal()
? x.signal() – resumes one of processes (if any) that invoked x.wait()
? Can be implemented with semaphores

Ch.7 – Deadlocks
* Deadlock Characteristics: deadlock can occur if these conditions hold simultaneously
? Mutual Exclusion: only one process at a time can use a resource
? Hold and Wait: process holding one resource is waiting to acquire resource held by another process
? No Preemption: a resource can be released only be the process holding it after the process completed its task
? Circular Wait: set of waiting processes such that Pn-1 is waiting for resource from Pn, and Pn is waiting for P0
? “Dining Philosophers” in deadlock

Ch.8 – Main Memory
* Cache sits between main memory and CPU registers
* Base and limit registers define logical address space usable by a process
* Compiled code addresses bind to relocatable addresses
? Can happen at three different stages
? Compile time: If memory location known a priori, absolute code can be generated
? Load time: Must generate relocatable code if memory location not known at compile time
? Execution time: Binding delayed until run time if the process can be moved during its execution
* Memory-Management Unit (MMU) device that maps virtual to physical address
* Simple scheme uses a relocation register which just adds a base value to address
* Swapping allows total physical memory space of processes to exceed physical memory
? Def: process swapped out temporarily to backing store then brought back in for continued execution
* Backing store: fast disk large enough to accommodate copes of all memory images
* Roll out, roll in: swapping variant for priority-based scheduling.
? Lower priority process swapped out so that higher priority process can be loaded
* Solutions to Dynamic Storage-Allocation Problem:
? First-fit: allocate the first hole that is big enough
? Best-fit: allocate the smallest hole that is big enough (must search entire list) ? smallest leftover hole
? Worst-fit: allocate the largest hole (search entire list) ? largest leftover hole
* External Fragmentation: total memory space exists to satisfy request, but is not contiguous
? Reduced by compaction: relocate free memory to be together in one block
? Only possible if relocation is dynamic
* Internal Fragmentation: allocated memory may be slightly larger than requested memory
* Physical memory divided into fixed-sized frames: size is power of 2, between 512 bytes and 16 MB
* Logical memory divided into same sized blocks: pages
* Page table used to translate logical to physical addresses
? Page number (p): used as an index into a page table
? Page offset (d): combined with base address to define the physical memory address
* Free-frame list is maintained to keep track of which frames can be allocated
Ch.8 – Main Memory Continued
* Transition Look-aside Buffer (TLB) is a CPU cache that memory management hardware uses to improve virtual address translation speed
? Typically small – 64 to 1024 entries
? On TLB miss, value loaded to TLB for faster access next time
? TLB is associative – searched in parallel
* Effective Access Time: EAT = (1 + e) a + (2 + e)(1 – a)
? e = time unit, a = hit ratio
* Valid and invalid bits can be used to protect memory
? “Valid” if the associated page is in the process’ logical address space, so it is a legal page
* Can have multilevel page tables (paged page tables)
* Hashed Page Tables: virtual page number hashed into page table
? Page table has chain of elements hashing to the same location
? Each element has (1) virtual page number, (2) value of mapped page frame, (3) a pointer to the next element
? Search through the chain for virtual page number
* Segment table – maps two-dimensional physical addresses
? Entries protected with valid bits and r/w/x privileges
Ch.9 – Virtual Memory
* Virtual memory: separation of user logical memory and physical memory
? Only part of program needs to be in memory for execution ? logical address space > physical address space
? Allows address spaces to be shared by multiple processes ? less swapping
? Allows pages to be shared during fork(), speeding process creation
* Page fault results from the first time there is a reference to a specific page ? traps the OS
? Must decide to abort if the reference is invalid, or if the desired page is just not in memory yet
? If the latter: get empty frame, swap page into frame, reset tables to indicate page now in memory, set validation bit, restart instruction that caused the page fault
? If an instruction accesses multiple pages near each other ? less “pain” because of locality of reference
* Demand Paging only brings a page into memory when it is needed ? less I/O and memory needed
? Lazy swapper – never swaps a page into memory unless page will be needed
? Could result in a lot of page-faults
? Performance: EAT = [(1-p)*memory access + p*(page fault overhead + swap page out + swap page in + restart overhead)]; where Page Fault Rate 0 ? p ? 1
? if p = 0, no page faults; if p = 1, every reference is a fault
? Can optimize demand paging by loading entire process image to swap space at process load time
* Pure Demand Paging: process starts with no pages in memory
* Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory
? If either process modifies a shared page, only then is the page copied
* Modify (dirty) bit can be used to reduce overhead of page transfers ? only modified pages written to disk
* When a page is replaced, write to disk if it has been marked dirty and swap in desired page
* Pages can be replaced using different algorithms: FIFO, LRU (below)
? Stack can be used to record the most recent page references (LRU is a “stack” algorithm)

? Second chance algorithm uses a reference bit
? If 1, decrement and leave in memory
? If 0, replace next page
* Fixed page allocation: Proportional allocation – Allocate according to size of process
? si = size of process Pi, S = Ssi, m = total number of frames, ai – allocation for Pi
? ai = (si/S)*m
* Global replacement: process selects a replacement frame from set of all frames
? One process can take frame from another
? Process execution time can vary greatly
? Greater throughput
* Local replacement: each process selects from only its own set of allocated frames
? More consistent performance
? Possible under-utilization of memory
* Page-fault rate is very high if a process does not have “enough” pages
? Thrashing: a process is busy swapping pages in and out ? minimal work is actually being performed
* Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory
* I/O Interlock: Pages must sometimes be locked into memory
Ch.10 – File-System Interface
* File – Uniform logical view of information storage (no matter the medium)
? Mapped onto physical devices (usually nonvolatile)
? Smallest allotment of nameable storage
? Types: Data (numeric, character, binary), Program, Free form, Structured
? Structure decided by OS and/or program/programmer
* Attributes:
? Name: Only info in human-readable form
? Identifier: Unique tag, identifies file within the file system
? Type, Size
? Location: pointer to file location
? Time, date, user identification
* File is an abstract data type
* Operations: create, write, read, reposition within file, delete, truncate
* Global table maintained containing process-independent open file information: open-file table
? Per-process open file table contains pertinent info, plus pointer to entry in global open file table
* Open file locking: mediates access to a file (shared or exclusive)
? Mandatory – access denied depending on locks held and requested
? Advisory – process can find status of locks and decide what to do
* File type can indicate internal file structure
* Access Methods: Sequential access, direct access
? Sequential Access: tape model of a file
? Direct Access: random access, relative access
* Disk can be subdivided into partitions; disks or partitions can be RAID protected against failure.
? Can be used raw without a file-system or formatted with a file system
? Partitions also knows as minidisks, slices
* Volume contains file system: also tracks file system’s info in device directory or volume table of contents
* File system can be general or special-purpose. Some special purpose FS:
? tmpfs – temporary file system in volatile memory
? objfs – virtual file system that gives debuggers access to kernel symbols
? ctfs – virtual file system that maintains info to manage which processes start when system boots
? lofs – loop back file system allows one file system to be accessed in place of another
? procfs – virtual file system that presents information on all processes as a file system
* Directory is similar to symbol table – translating file names into their directory entries
? Should be efficient, convenient to users, logical grouping
? Tree structured is most popular – allows for grouping
? Commands for manipulating: remove – rm<file-name> ; make new sub directory – mkdir<dir-name>
* Current directory: default location for activities – can also specify a path to perform activities in
* Acyclic-graph directories adds ability to directly share directories between users
? Acyclic can be guaranteed by: only allowing shared files, not shared sub directories; garbage collection; mechanism to check whether new links are OK
* File system must be mounted before it can be accessed – kernel data structure keeps track of mount points
* In a file sharing system User IDs and Group IDs help identify a user’s permissions
* Client-server allows multiple clients to mount remote file systems from servers – NFS (UNIX), CIFS (Windows)
* Consistency semantics specify how multiple users are to access a shared file simultaneously – similar to synchronization algorithms from Ch.7
? One way of protection is Controlled Access: when file created, determine r/w/x access for users/groups
Ch.11 – File System Implementation
* File system resides on secondary storage – disks; file system is organized into layers ?
* File control block: storage structure consisting of information about a file (exist per-file)
* Device driver: controls the physical device; manage I/O devices
* File organization module: understands files, logical addresses, and physical blocks
? Translates logical block number to physical block number
? Manages free space, disk allocation
* Logical file system: manages metadata information – maintains file control blocks
* Boot control block: contains info needed by system to boot OS from volume
* Volume control block: contains volume details; ex: total # blocks, # free blocks, block size, free block pointers
* Root partition: contains OS; mounted at boot time
* For all partitions, system is consistency checked at mount time
? Check metadata for correctness – only allow mount to occur if so
* Virtual file systems provide object-oriented way of implementing file systems
* Directories can be implemented as Linear Lists or Hash Tables
? Linear list of file names with pointer to data blocks – simple but slow
? Hash table – linear list with hash data structure – decreased search time
? Good if entries are fixed size
? Collisions can occur in hash tables when two file names hash to same location
* Contiguous allocation: each file occupies set of contiguous blocks
? Simple, best performance in most cases; problem – finding space for file, external fragmentation
? Extent based file systems are modified contiguous allocation schemes – extent is allocated for file allocation
* Linked Allocation: each file is a linked list of blocks – no external fragmentation
? Locating a block can take many I/Os and disk seeks
* Indexed Allocation: each file has its own index block(s) of pointers to its data blocks
? Need index table; can be random access; dynamic access without external fragmentation but has overhead
* Best methods: linked good for sequential, not random; contiguous good for sequential and random
* File system maintains free-space list to track available blocks/clusters
* Bit vector or bit map (n blocks): block number calculation ? (#bits/word)*(# 0-value words)+(offset for 1st bit)
? Example: block size = 4KB = 212 bytes
disk size = 240 bytes (1 terabyte)
n = 240/212 = 228 bits (or 256 MB)
if clusters of 4 blocks -> 64MB of memory
* Space maps (used in ZFS) divide device space into metaslab units and manages metaslabs
? Each metaslab has associated space map
* Buffer cache – separate section of main memory for frequently used blocks
* Synchronous writes sometimes requested by apps or needed by OS – no buffering
*
? Asynchronous writes are more common, buffer-able, faster
* Free-behind and read-ahead techniques to optimize sequential access
* Page cache caches pages rather than disk blocks using virtual memory techniques and addresses
? Memory mapped I/O uses page cache while routine I/O through file system uses buffer (disk) cache
* Unified buffer cache: uses same page cache to cache both memory-mapped pages and ordinary file system I/O to avoid double caching

Ch.12 – Mass-Storage Systems
* Magnetic disks provide bulk of secondary storage – rotate at 60 to 250 times per second
? Transfer rate: rate at which data flows between drive and computer
? Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)
? Head crash: disk head making contact with disk surface
* Drive attached to computer’s I/O bus – EIDE, ATA, SATA, USB, etc.
? Host controller uses bus to talk to disk controller
* Access latency = Average access time = average seek time + average latency (fast ~5ms, slow ~14.5ms)
* Average I/O time = avg. access time + (amount to transfer / transfer rate) + controller overhead
? Ex: to transfer a 4KB block on a 7200 RPM disk with a 5ms average seek time, 1Gb/sec transfer rate with a .1ms controller overhead = 5ms + 4.17ms + 4KB / 1Gb/sec + 0.1ms = 9.27ms + .12ms = 9.39ms
* Disk drives addressed as 1-dimensional arrays of logical blocks
? 1-dimensional array is mapped into the sectors of the disk sequentially
* Host-attached storage accessed through I/O ports talking to I/O buses
? Storage area network (SAN): many hosts attach to many storage units, common in large storage environments
? Storage made available via LUN masking from specific arrays to specific servers
* Network attached storage (NAS): storage made available over a network rather than local connection
* In disk scheduling, want to minimize seek time; Seek time is proportional to seek distance
* Bandwidth is (total number of bytes transferred) / (total time between first request and completion of last transfer)
* Sources of disk I/O requests: OS, system processes, user processes
? OS maintains queue of requests, per disk or device
* Several algorithms exist to schedule the servicing of disk I/O requests
? FCFS, SSTF (shortest seek time first), SCAN, CSCAN, LOOK, CLOOK
? SCAN/elevator: arm starts at one end and moves towards other end servicing requests as it goes, then reverses direction
? CSCAN: instead of reversing direction, immediately goes back to beginning
? LOOK/CLOOK: Arm only goes as far as the last request in each directions, then reverses immediately
* Low level/physical formatting: dividing a disk into sectors that the disk controller can read and write – usually 512 bytes of data
* Partition: divide disk into one or more groups of cylinders, each treated as logical disk
* Logical formatting: “making a file system”
* Increase efficiency by grouping blocks into clusters – Disk I/O is performed on blocks
? Boot block initializes system – bootstrap loader stored in boot block
* Swap-space: virtual memory uses disk space as an extension of main memory
? Kernel uses swap maps to track swap space use
* RAID: Multiple disk drives provide reliability via redundancy – increases mean time to failure
? Disk striping uses group of disks as one storage unit
? Mirroring/shadowing (RAID 1) – keeps duplicate of each disk
? Striped mirrors (RAID 1+0) or mirrored striped (RAID 0+1) provides high performance/reliability
? Block interleaved parity (RAID 4, 5, 6) uses much less redundancy
* Solaris ZFS adds checksums of all data and metadata – detect if object is the right one and whether it changed
* Tertiary storage is usually built using removable media – can be WORM or Read-only, handled like fixed disks
* Fixed disk usually more reliable than removable disk or tape drive
* Main memory is much more expensive than disk storage
Ch.13 – I/O Systems
* Device drivers encapsulate device details – present uniform device access interface to I/O subsystem
* Port: connection point for device
* Bus: daisy chain or shared direct access
* Controller (host adapter): electronics that operate port, bus, device – sometimes integrated
? Contains processor, microcode, private memory, bus controller
* Memory-mapped I/O: device data and command registers mapped to processor address space
? Especially for large address spaces (graphics)
* Polling for each byte of data – busy-wait for I/O from device
? Reasonable for fast devices, inefficient for slow ones
? Can happen in 3 instruction cycles
* CPU interrupt-request line is triggered by I/O devices – interrupt handler receives interrupts
? Handler is maskable to ignore or delay some interrupts
? Interrupt vector dispatches interrupt to correct handler – based on priority; some nonmaskable
? Interrupt chaining occurs if there is more than one device at the same interrupt number
? Interrupt mechanism is also used for exceptions
* Direct memory access is used to avoid programmed I/O for large data movement
? Requires DMA controller
? Bypasses CPU to transfer data directly between I/O device and memory
* Device driver layer hides differences among I/O controllers from kernel
* Devices vary in many dimensions: character stream/block, sequential/random access, synchronous/asynchronous, sharable/dedicated, speed, rw/ro/wo
* Block devices include disk drives: Raw I/O, Direct I/OU
? Commands include read, write, seek
* Character devices include keyboards, mice, serial ports
? Commands include get(), put()
* Network devices also have their own interface; UNIX and Windows NT/9x/2000 include socket interface
? Approaches include pipes, FIFOs, streams, queues, mailboxes
* Programmable interval timer: used for timings, periodic interrupts
* Blocking I/O: process suspended until I/O completed – easy to use and understand, not always best method
* Nonblocking I/O: I/O call returns as much as available – implemented via multi-threading, returns quickly
* Asynchronous: process runs while I/O executes – difficult to use, process signaled upon I/O completion
* Spooling: hold output for a device – if device can only serve one request at a time (ex: printer)
* Device Reservation: provides exclusive access to a device – must be careful of deadlock
* Kernel keeps state info for I/O components, including open file tables, network connections, character device states
? Complex data structures track buffers, memory allocation, “dirty” blocks
* STREAM: full-duplex communication channel between user-level process and device in UNIX
? Each module contains read queue and write queue
? Message passing used to communicate between queues – Flow control option to indicate available or busy
? Asynchronous internally, synchronous where user process communicates with stream head
* I/O is a major factor in system performance – demand on CPU, context switching, data copying, network traffic

Ch.14 – Protection
* Principle of least privilege: programs, users, systems should be given just enough privileges to perform their tasks
* Access-right = <obj-name, rights-set> w/ rights-set is subset of all valid operations performable on the object
? Domain: set of access-rights
? UNIX system consists of 2 domains: user, supervisor
? MULTICS domain implementation (domain rings) – if j<i ? Di ? Dj
* Access matrix: rows represent domains, columns represent objects
? Access(i,j) is the set of operations that a process executing in Domaini can invoke on Objectj
? Can be expanded to dynamic protection
* Access matrix design separates mechanism from policy
? Mechanism: OS provides access-matrix and rules – ensures matrix is only manipulated by authorized users
? Policy: User dictates policy – who can access what object and in what mode
* Solaris 10 uses role-based access control (RBAC) to implement least privilege
* Revocation of access rights
? Access list: delete access rights from access list – simple, immediate
? Capability list: required to locate capability in system before capability can be revoked – reacquisition, back-pointers, indirection, keys
* Language-Based Protection: allows high-level description of policies for the allocation and use of resources
? Can provide software for protection enforcement when hardware-supported checking is unavailable

Ch.15 – Security
* System secure when resources used and accessed as intended under all circumstances
* Attacks can be accidental or malicious
? Easier to protect against accidental than malicious misuse
* Security violation categories:
? Breach of confidentiality – unauthorized reading of data
? Breach of integrity – unauthorized modification of data
? Breach of availability – unauthorized destruction of data
? Theft of service – unauthorized use of resources
? Denial of service – prevention of legitimate use
* Methods of violation:
? Masquerading – pretending to be an authorized user
? Man-in-the-middle – intruder sits in data flow, masquerading as sender to receiver and vice versa
? Session hijacking – intercept and already established session to bypass authentication
* Effective security must occur at four levels: physical, human, operating system, network
* Program threats: trojan horse (spyware, pop-up, etc.), trap door, logic bomb, stack and buffer overflow
* Viruses: code fragment embedded in legitimate program; self-replicating
? Specific to CPU architecture, OS, applications
? Virus dropper: inserts virus onto the system
* Windows is the target for most attacks – most common, everyone is administrator
* Worms: use spawn mechanism – standalone program
* Port scanning: automated attempt to connect to a range of ports on one or a range of IP addresses
? Frequently launched from zombie systems to decrease traceability
* Denial of service: overload targeted computer preventing it from doing useful work
* Cryptography: means to constrain potential senders and/or receivers – based on keys
? Allows for confirmation of source, receipt by specified destination, trust relationship
* Encryption: [K of keys], [M of messages], [C of ciphertexts], function E:K to encrypt, function D:K to decrypt
? Can have symmetric and asymmetric (distributes public encryption key, holds private decipher key) encryption
? Asymmetric is much more compute intensive – not used for bulk data transaction
? Keys can be stored on a key ring
Ch.15 – Security Continued
* Authentication: constraining a set of potential senders of a message
? Helps to prove that the message is unmodified
? Hash functions are basis of authentication
? Creates small, fixed-size block of data (message digest, hash value)
* Symmetric encryption used in message-authentication code (MAC)
* Authenticators produced from authentication algorithm are digital signatures
* Authentication requires fewer computations than encryption methods
* Digital Certificates: proof of who or what owns a public key
* Defense in depth: most common security theory – multiple layers of security
* Can attempt to detect intrusion:
? Signature-based: detect “bad patterns”
? Anomaly detection: spots differences from normal behavior
? Both can report false positives or false negatives
? Auditing, accounting, and logging specific system or network activity
* Firewall: placed between trusted and untrusted hosts
? Limits network access between the two domains
? Can be tunneled or spoofed
* Personal firewall is software layer on given host
? Can monitor/limit traffic to/from host
* Application proxy firewall: Understands application protocol and can control them
* System-call firewall: Monitors all important system calls and apply rules and restrictions to them