 Incredible variety of I/O devices
Operating System Concepts 13.1
Silberschatz, Galvin and Gagne

2002

 Devices vary in many dimensions
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Direction
 Read, Write, Read-Write
Character, Block
Speed
 Latency, Transfer rate, Delay between operations
Access
 Sequential, Random
Sharing
 Sharable, Dedicated
 IO subsystem reduces perceived differences for apps, and optimizes performances for apps
13.2
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

 Common concepts (provide abstraction)
 Port (serial, parallel, ethernet)
 Bus (daisy chain or shared direct access)
 Controller (host adapter operates ports/bus/device)
 See my picture
Operating System Concepts 13.3
Silberschatz, Galvin and Gagne

2002

Operating System Concepts 13.4
Silberschatz, Galvin and Gagne

2002

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I/O system calls encapsulate device behaviors in generic classes
Device-driver layer hides differences among I/O controllers from kernel
 Makes OS independent of the IO hardware
 Provided by the device/controller manufacturers
 DDs are part of the OS (not processes) usually
OS defines interface to DDs
 Non-standard across OSs => device manufacturers have to provide a DD for each OS (bugger)
Applications normally reach the DDs via the OS
 Escape entry (e.g., ioctl) allows more direct access
13.5
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

 Scheduling
 To maximize performance
 I/O request ordering via per-device queue
 Some OSs try fairness
 Device reservation - provides exclusive access to a device (e.g., in VMS)
 System calls for allocation and deallocation
 Wait for device on call, e.g., NT
 Watch out for deadlock
13.6
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

 Buffering - store data in memory while transferring between devices
 To cope with device speed mismatch
 E.g., modem to disk (x1000)
 Double buffering
 To cope with device transfer size mismatch
 E.g., keyboard to disk
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 Collating network packets
To maintain “copy semantics”
 Caching - fast memory holding copy of data
 Always just a copy (as opposed to a buffer)
 Often implemented in buffer system
 Key to performance
Silberschatz, Galvin and Gagne

2002 Operating System Concepts 13.7

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OS can recover from transient failures E.g., disk read, device unavailable, network failures
Permanent failures
 OS can make devices unavailable
 Need operator intervention
System calls return an error code when I/O fails
System error logs hold problem reports
 More detailed information than return values
 HW diagnostic information, e.g., from SCSI controllers
13.8
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

 Blocking - process suspended until I/O completed
 Easy to use and understand
 Insufficient for some needs
 Non-blocking - I/O call returns as much as available
 E.g., user interface, data copy (buffered I/O)
 Can be implemented via multi-threading
 Returns quickly with count of bytes read or written
 Asynchronous - process runs while I/O executes
 Difficult to use
 I/O subsystem signals process when I/O completed
13.9
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

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Request may be satisfied immediately, e.g., in cache
The request for the DD may have to be queued
CPU runs async with device
DD waits in sync with device
It’s blocking I/O.
 Nonblocking I/O always “can satisfy request”
 Asynchronous I/O does not block the process
Direct I/O instructions
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Placed in registers
Status, control, data-in, data-out
Memory-mapped I/O
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Maps registers onto RAM
Can be faster than I/O instructions
Silberschatz, Galvin and Gagne

2002 Operating System Concepts 13.10

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Synchronous communication
 Controller waits for command-ready bit in controller status register bit to be set
 Host waits for busy bit in controller status register to be clear
(initially it is clear)
 Host places command in command register, and any required data in data register
 Host sets command-ready bit, and waits for it to clear
 Controller notices command-ready bit, sets busy bit, clears command-ready bit.
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Host loops waiting for busy bit to clear, while controller does IO
Controller clears busy and command-ready bits, and loops
Vantages
 Done once for each byte
 Wasteful of CPU time if IO takes long
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Can use offboard CPU, e.g., in SCSI controller
Useful for fast data streams
Silberschatz, Galvin and Gagne

2002 Operating System Concepts 13.11

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Used to avoid programmed I/O for large data movement
 If device transmits close to memory speeds, little time is left for processing
Requires DMA controller
Bypasses CPU to transfer data directly between I/O device and memory
DMA controller steals RAM cycles from CPU
Operating System Concepts 13.12
Silberschatz, Galvin and Gagne

2002

Operating System Concepts 13.13
Silberschatz, Galvin and Gagne

2002

 Spooling - hold output for a device
 If device can serve only one request at a time
 Provides asynchronous I/O
 i.e., Printing and the lpd
Operating System Concepts 13.14
Silberschatz, Galvin and Gagne

2002

 Approaches vary widely
 Pipes
 FIFOs
 Streams
 Queues
 Mailboxes
 Socket interface
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Separates network protocol from network operation
Includes select functionality
13.15
Silberschatz, Galvin and Gagne

2002 Operating System Concepts

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Kernel keeps state info for I/O components, including open file tables, network connections, character device state
Many, many complex data structures to track buffers, memory allocation, “dirty” blocks
Some use object-oriented methods and message passing to implement I/O, e.g., NT, Nachos, nu
Operating System Concepts 13.16
Silberschatz, Galvin and Gagne
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2002

 Cooked interfaces are buffered
 Block buffers
 C-lists
Raw interfaces are unbuffered 
 Devices have major and minor device numbers.
 Major number used as index into array of DD entry points
 Direct access via ioctl() and /dev files
Operating System Concepts 13.17
Silberschatz, Galvin and Gagne

2002

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Consist of buffer headers, each of which can point to a piece of physical memory, and contain a device number and a block number on the device.
The buffer headers for blocks not currently in use are kept in several linked lists:
 Buffers not recently used, or with invalid contents (AGE list).
 Buffers recently used, linked in LRU order (LRU list).
 Buffers with no associated physical memory (EMPTY list).
On read the cache is searched.
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If the block is found it is used - no I/O transfer is necessary.
If it is not found, a buffer is chosen from the AGE list, or the
LRU list if AGE is empty.
On write, if the block is in the cache, then write and set dirty bit
 Dirty blocks are output by regular sync()
Blocks may be fragmented, and headers are taken from EMPTY
Operating System Concepts 13.18
Silberschatz, Galvin and Gagne

2002

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The raw device interface — unlike the block interface, it bypasses the block buffer cache.
Each disk driver maintains a queue of pending transfers:
 whether it is a read or a write
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 a main memory address for the transfer a device address for the transfer
 a transfer size
Can use user memory for a transfer record
Operating System Concepts 13.19
Silberschatz, Galvin and Gagne

2002

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Terminal drivers use a character buffering system which involves keeping small blocks of characters in linked lists.
A write system call to a terminal enqueues characters on a list for the device. An initial transfer is started, and interrupts cause dequeueing of characters and further transfers, i.e., it’s asynchronous
Input is similarly interrupt driven.
It is also possible to have the device driver bypass the canonical queue and return characters directly from the raw queue — raw mode (used by full-screen editors and other programs that need to react to every keystroke).
13.20
Silberschatz, Galvin and Gagne

2002 Operating System Concepts