-Device. -Physical or virtual thing that does something -Software + hardware to operate a device (Controller runs port, Bus, device)

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1 Devices -Host -CPU -Device -Controller device) +memory +OS -Physical or virtual thing that does something -Software + hardware to operate a device (Controller runs port, Bus, Communication -Registers -Control Status ß From Device Control àto Device -Data -Data in -Data Out -Memory-mapped I/O -Map device info into memory -Abstract -Encapsulate -Layer Doing I/O -Polling -Interrupt -DMA

2 Polling: Controller indicates ready in status register. Host issues instruction through control register. Host loops (busy waits) and checks all devices. Controller finishes and indicates ready. Interrupts Direct Memory Access -Can usually only copy to/from memory with the CPU -Expensive if we want to copy Kilobytes (megabytes, gigabytes) -DMA controller avoids the CPU accesses memory directly. -OS sets a control block with source, destination & amount to copy -DMA starts transferring words across the bus

3 -cycle stealing -When finished DMA controller interrupts host. Device Behavior Block -Addressable array of blocks -Read get some bytes -Write set some bytes -Seek define where we are looking Character -Spontaneous Data -get read a character (libraries) -put write a character (libraries) Sockets Networking -Create a local resource connected to a remote resource (with hardware support) -Keep buffers of incoming and outgoing messages (packets) -Periodically check for new packets Non-blocking & Asynchronous I/O Non-blocking do I/O fast. -get minimum amount of data as quickly as possible Asynchronous- return as soon as data is requested -agree on where data will be Kernel and I/O -Tries to make it FASTER Scheduling -keep a list of all pending I/O requests -Resort the list to maximize throughput Buffering -Memory to store data being transferred between devices or processes -Speed mismatches (modem vs. disk) -read a buffer full of slow data and then handle quickly ( or vice versa ) Size Mismatch -write large data in small pieces -assemble small pieces into larger data Consistency -Make sure data is written in the right order.

4 Efficiency -Double buffering. 2 buffers one being written -one being read Cacheing -keep a copy of data closer to where it will be used. -Directory system -Need to manage when the cache is updated Spooling -Keep a buffer on the host to manage devices that cant manage their own. Error Handling -Try to recover -repeat I/O -Manage Failure -Clean up & Kill -Log File Systems -How do we think about files? - a document with data in it -text source file -binary files executable sequence of instructions -video sequence of frames. What do we do with files? Create Read Write Delete Truncate What does the OS know about a file? -Attributes name -owner -location -permissions -size - -type -Extension magic number Organization Directories -Logical construct to organize system data -contains files -link to the physical file system

5 -mount points. ( let a directory represent a volume) No requirement for uniformity -files -mount point temporary file system Let us search for and manage files find,read,create Tree Structure -Keep directories in directories -access control system files, personal files. Paths -absolute file s location from some fixed point -relative file s location from the current working directory. Links -Some times we want a file in the directories at once -Soft link -Add tiny reference to a file in another directory. -Hard link -Add real reference to a file in directory Directories are files -we can link them. -soft links are easy -scanning we ignore soft links -no hard links File System Implementation -Structure: Applications à Logical file system Manage file meta data à File organization module links logical to physical, - manages free space. Basic File System -allow OS to act on disks (read, cylinder >3, track 2, block 57) Device Driver -Send the controls to disk1 to read. File System Information On Disk: Boot Control Block (per volume) -usually at block 0. Volume Control Block

6 -Keep details about the volume, (# of Blocks, size of blocks, free space, bad blocks). Directory Structure -To organize files File Control Blocks -Unique identifier for each file and associated to a director -Records the files physical location. In Memory Mount Table- where are the volumes in the logical file system. Directory Structure Cache copy of physical System wide open files Table: Copy of FCD for open files + list of associated processes. Per process Open File Tables: point to entries in the system table. Actions on Files Create: -Process requests new file of logical file system. -New FCB is created/ allocated -Associated with a directory -Has some size Open -Process requests file by name -Logical file system checks the system wide table If file isn t in table, LFS looks in the directory structure finds FCB and puts in in the system wide table. LFS returns a reference to the file to the process ( file handler) and increment the open count. Closing a file: Pointer is removed from the process table Counter is decremented If counter hits zero, FCB is removed.