Chapter 13+14: I/O Systems and MassStorage Structure
 I/O Hardware
 Application I/O Interface
 Kernel I/O Subsystem
 Disk Structure
 Disk Scheduling
 Disk Management
 Swap-Space Management
Operating System Concepts
13.1
Silberschatz, Galvin and Gagne 2002
I/O Hardware
 Incredible variety of I/O devices
 Common concepts
 Port
 Bus (daisy chain or shared direct access)
 Controller (host adapter)
 I/O instructions control devices
 Devices have addresses, used by
 Direct I/O instructions
 Memory-mapped I/O
Operating System Concepts
13.2
Silberschatz, Galvin and Gagne 2002
A Typical PC Bus Structure
Operating System Concepts
13.3
Silberschatz, Galvin and Gagne 2002
Interrupts
 CPU Interrupt request line triggered by I/O device
 Interrupt handler receives interrupts
 Maskable to ignore or delay some interrupts
 Interrupt vector to dispatch interrupt to correct handler
 Based on priority
 Some unmaskable
 Interrupt mechanism also used for exceptions
Operating System Concepts
13.4
Silberschatz, Galvin and Gagne 2002
Interrupt-Driven I/O Cycle
Operating System Concepts
13.5
Silberschatz, Galvin and Gagne 2002
Intel Pentium Processor Event-Vector Table
Operating System Concepts
13.6
Silberschatz, Galvin and Gagne 2002
Direct Memory Access
 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
Operating System Concepts
13.7
Silberschatz, Galvin and Gagne 2002
Six Step Process to Perform DMA Transfer
Operating System Concepts
13.8
Silberschatz, Galvin and Gagne 2002
Application I/O Interface
 I/O system calls encapsulate device behaviors in generic
classes
 Device-driver layer hides differences among I/O
controllers from kernel
 Devices vary in many dimensions
 Character-stream or block
 Sequential or random-access
 Sharable or dedicated
 Speed of operation
 read-write, read only, or write only
Operating System Concepts
13.9
Silberschatz, Galvin and Gagne 2002
A Kernel I/O Structure
Operating System Concepts
13.10
Silberschatz, Galvin and Gagne 2002
Characteristics of I/O Devices
Operating System Concepts
13.11
Silberschatz, Galvin and Gagne 2002
Block and Character Devices
 Block devices include disk drives
 Commands include read, write, seek
 Raw I/O or file-system access
 Memory-mapped file access possible
 Character devices include keyboards, mice, serial ports
 Commands include get, put
 Libraries layered on top allow line editing
Operating System Concepts
13.12
Silberschatz, Galvin and Gagne 2002
Blocking and Nonblocking I/O
 Blocking - process suspended until I/O completed
 Easy to use and understand
 Insufficient for some needs
 Nonblocking - I/O call returns as much as available
 User interface, data copy (buffered I/O)
 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
Operating System Concepts
13.13
Silberschatz, Galvin and Gagne 2002
Kernel I/O Subsystem
 Scheduling
 Some I/O request ordering via per-device queue
 Some OSs try fairness
 Buffering - store data in memory while transferring
between devices
 To cope with device speed mismatch
 To cope with device transfer size mismatch
 To maintain “copy semantics”
Operating System Concepts
13.14
Silberschatz, Galvin and Gagne 2002
Kernel I/O Subsystem
 Caching - fast memory holding copy of data
 Always just a copy
 Key to performance
 Spooling - hold output for a device
 If device can serve only one request at a time
 i.e., Printing
 Device reservation - provides exclusive access to a
device
 System calls for allocation and deallocation
 Watch out for deadlock
Operating System Concepts
13.15
Silberschatz, Galvin and Gagne 2002
Life Cycle of An I/O Request
Operating System Concepts
13.16
Silberschatz, Galvin and Gagne 2002
Disk Structure
 Disk drives are addressed as large 1-dimensional arrays
of logical blocks, where the logical block is the smallest
unit of transfer.
 The 1-dimensional array of logical blocks is mapped into
the sectors of the disk sequentially.
 Sector 0 is the first sector of the first track on the outermost
cylinder.
 Mapping proceeds in order through that track, then the rest
of the tracks in that cylinder, and then through the rest of the
cylinders from outermost to innermost.
Operating System Concepts
13.17
Silberschatz, Galvin and Gagne 2002
Disk Scheduling
 The operating system is responsible for using hardware
efficiently — for the disk drives, this means having a fast
access time and disk bandwidth.
 Access time has two major components
 Seek time is the time for the disk are to move the heads to
the cylinder containing the desired sector.
 Rotational latency is the additional time waiting for the disk
to rotate the desired sector to the disk head.
 Minimize seek time
 Seek time  seek distance
 Disk bandwidth is the total number of bytes transferred,
divided by the total time between the first request for
service and the completion of the last transfer.
Operating System Concepts
13.18
Silberschatz, Galvin and Gagne 2002
Disk Scheduling (Cont.)
 Several algorithms exist to schedule the servicing of disk
I/O requests.
 We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
Operating System Concepts
13.19
Silberschatz, Galvin and Gagne 2002
FCFS
Illustration shows total head movement of 640 cylinders.
Operating System Concepts
13.20
Silberschatz, Galvin and Gagne 2002
SSTF
 Selects the request with the minimum seek time from the
current head position.
 SSTF scheduling is a form of SJF scheduling; may cause
starvation of some requests.
 Illustration shows total head movement of 236 cylinders.
Operating System Concepts
13.21
Silberschatz, Galvin and Gagne 2002
SSTF (Cont.)
Operating System Concepts
13.22
Silberschatz, Galvin and Gagne 2002
SCAN
 The disk arm starts at one end of the disk, and moves
toward the other end, servicing requests until it gets to the
other end of the disk, where the head movement is
reversed and servicing continues.
 Sometimes called the elevator algorithm.
 Illustration shows total head movement of 208 cylinders.
Operating System Concepts
13.23
Silberschatz, Galvin and Gagne 2002
SCAN (Cont.)
Operating System Concepts
13.24
Silberschatz, Galvin and Gagne 2002
C-SCAN
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to the other.
servicing requests as it goes. When it reaches the other
end, however, it immediately returns to the beginning of
the disk, without servicing any requests on the return trip.
 Treats the cylinders as a circular list that wraps around
from the last cylinder to the first one.
Operating System Concepts
13.25
Silberschatz, Galvin and Gagne 2002
C-SCAN (Cont.)
Operating System Concepts
13.26
Silberschatz, Galvin and Gagne 2002
C-LOOK
 Version of C-SCAN
 Arm only goes as far as the last request in each direction,
then reverses direction immediately, without first going all
the way to the end of the disk.
Operating System Concepts
13.27
Silberschatz, Galvin and Gagne 2002
C-LOOK (Cont.)
Operating System Concepts
13.28
Silberschatz, Galvin and Gagne 2002
Selecting a Disk-Scheduling Algorithm
 SSTF is common and has a natural appeal
 SCAN and C-SCAN perform better for systems that place



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a heavy load on the disk.
Performance depends on the number and types of
requests.
Requests for disk service can be influenced by the fileallocation method.
The disk-scheduling algorithm should be written as a
separate module of the operating system, allowing it to be
replaced with a different algorithm if necessary.
Either SSTF or LOOK is a reasonable choice for the
default algorithm.
Operating System Concepts
13.29
Silberschatz, Galvin and Gagne 2002
Disk Management
 Low-level formatting, or physical formatting — Dividing a
disk into sectors that the disk controller can read and
write.
 To use a disk to hold files, the operating system still
needs to record its own data structures on the disk.
 Partition the disk into one or more groups of cylinders.
 Logical formatting or “making a file system”.
 Boot block initializes system.
 The bootstrap is stored in ROM.
 Bootstrap loader program.
 Methods such as sector sparing used to handle bad
blocks.
Operating System Concepts
13.30
Silberschatz, Galvin and Gagne 2002
MS-DOS Disk Layout
Operating System Concepts
13.31
Silberschatz, Galvin and Gagne 2002
Swap-Space Management
 Swap-space — Virtual memory uses disk space as an
extension of main memory.
 Swap-space can be carved out of the normal file
system,or, more commonly, it can be in a separate disk
partition.
 Swap-space management
 4.3BSD allocates swap space when process starts; holds
text segment (the program) and data segment.
 Kernel uses swap maps to track swap-space use.
 Solaris 2 allocates swap space only when a page is forced
out of physical memory, not when the virtual memory page
is first created.
Operating System Concepts
13.32
Silberschatz, Galvin and Gagne 2002