1. Review of system architecture
USER
Shell
System and application programs
systems calls | API | middleware
user mode
Kernel access points
file sys. memory process
support mgt mgt
kernel mode
I/O subsystem
Device drivers
Hardware ports | buses | controllers
DEVICES
Chapter 11 I/O Systems

2. Review of File System
File interface User friendly, abstractions, e.g.,
(user mode) GUI, folders, standard interface,
portability/transparency/virtual
services
File implementation Resource manager, including
(kernel mode) efficiency (e.g., clusters for CPU)
(defragmentation, compression)
security (e.g., encryption)
availability
reliability
fault tolerance
Chapter 11 I/O Systems

3. I/O management as compared to file management
File management is divided into file interface (services provided to application layer, user) and file implementation
File interface designed for user-friendliness, e.g.virtual file systems give consistent interface to users
File implementation handles implementation
I/O also divided into interface and implementation
I/O interfaces are standardized, simplified for services to other OS services, application layer
I/O implementation is becoming more and more complex
More devices
Compression and redundancy added to devices
Much of this is now in hardware (controllers)
Chapter 11 I/O Systems

4. I/O Systems Computing versus I/O COBOL for business processing (1960s)
Early systems had minimal user interface
I/O was most tedious and repetitive part – beginning of OS
Early programming languages (FORTRAN, etc.) designed for computation
COBOL for business processing (1960s)
Lots of I/O
Systems integrated I/O and process-bound jobs
Current Applications (games, video, etc.)
“Challenge to incorporate them into our computers”
Chapter 11 I/O Systems

5. I/O layers
I/O subsystem provides device independent interface to applications and OS
Device drivers – software that encapsulates devices for uniform device-access interface to I/O subsystem
Basic I/O hardware
Ports, buses, device controllers
Chapter 11 I/O Systems

6. Question Why are i/o functions divided into separate layers?
a.To handle the increased complexity of devices
b.To provide a uniform interface for application programs and OS using so many different types of devices
ans: a and b
Chapter 11 I/O Systems

7. Some Types of I/O Devices
Machine readable
Disk drives, sensors, controllers
Transmission
NICs, modems, FAX
Human-readable
Keyboard, mouse, screen, joystick, printer
Robotics
Chapter 11 I/O Systems

8. Some differences between devices
Data rate
Keyboard 100bps; Gigabit ethernet Gbps
Application
Software and policies vary for applications
Priorities; access rights
Complexity
Unit of transfer
Data representation
Error conditions
Chapter 11 I/O Systems

9. Device communication Bus
Set of wires and circuitry, with functional information on each wire (data, address, control) and protocol for exchanging information (send, receive) on channel
Discuss PCI bus (next slide)
Discuss daisy chain
Port- Connection point
Serial (USB, firewire), parallel (IDE)
Timer (typically at port hex)
Controller-firmware (microcode on chips) operates port, bus, or device
Host adapter (NIC) and SCSI controllers typically are on circuit boards
Chapter 11 I/O Systems

10. PCI bus monitor Processor(s)
SCSI controller to SCSI bus and multiple disks
caches
Graphics
controller
controller
memory
USB controller to USB bus
ISA bridge
mouse
ISA bus
keyboard
Serial printers, cameras, etc.
IDE controller to disks
Parallel printers
modem
Chapter 11 I/O Systems

11. PCI bus http://www.tech-pro.net/intro_pci.html
This is an edited version of an article that appeared a few years ago in PC Support Advisor. Although it provides a good general introduction to PCI bus concepts, further improvements have been made since the article was written.)
The acronym PCI stands for Peripheral Component Interconnect, which aptly describes what it does. PCI was designed to satisfy the requirement for a standard interface for connecting peripherals to a PC, capable of sustaining the high data transfer rates needed by modern graphics controllers, storage media, network interface cards and other devices.
Earlier bus designs were all lacking in one respect or another. The IBM PC-AT standard ISA (Industry Standard Architecture) bus, for example, can manage a data transfer rate of 8MB/sec at best. In practice, the throughput that can be sustained is much less than that. Other factors, like the 16-bit data bandwidth and 24 bit wide address bus - which restricts memory mapped peripherals to the first 16MB of memory address space - made the ISA bus seem increasingly outmoded .
Chapter 11 I/O Systems

12. Personal Computer buses
Bus PCI Clock 33MHz - 66MHz Number of bits Data per Clock Cycle 1 1 Maximum Transfer Rate 133MB/s /266MB/s/533MHz Bus PCI -X Clock 66MHz - 133MHz Number of bits 64 Data per Clock Cycle 1, 2, 4 Maximum Transfer Rate 533MB/s to 4266MB/s Bus AGP (accelerated graphics port) Clock 66HMz Number of bits 32/64 Data per Clock Cycle 1-8 Maximum Transfer Rate 266MB/s- 2133MB/s and higher
Chapter 11 I/O Systems

13. Device Controllers
Device (mechanical component) separated from Controller (electronic component)
Controller is sometimes a chip on the motherboard or else a printed circuit card to be inserted into a PCI expansion slot
Controller connected to device(s) by connecting cables
Interface between device and controller is standardized for IDE, SCSI, USB, FireWire, etc.
Chapter 11 I/O Systems

14. Example: device controller for disk
Serial bit stream feeds between disk to controller
Controller organizes it into preamble (cylinder & sector #, sector size, bit and sector synchronization data) ; perhaps 4096 bit data sector; checksum (or ECC)
ECC verified
If everything checks, data is assembled as block and forwarded to memory
Chapter 11 I/O Systems

15. Controller / CPU Communication
Registers on controller written to/ read from
Contain commands, device status, etc.
May include data buffers
Bus contains address, control, data lines
If CPU wants to read/ write data
Places address on bus address line
For a write, it places data on data lines.
Asserts read/write signal on bus control line
Device responds to signal
Chapter 11 I/O Systems

16. Memory-Mapped I/O or Port I/O or hybrids
Port I/O uses dedicated instructions, dedicated addresses
IN Reg1, Port
CPU to get value in controller Port)
and store in CPU register Reg1
OUT Port, Reg1
CPU sends value in Reg1 to Port for controller
Memory-mapped I/O instruction might be
MOV Reg1, 4
data in memory 4 sent to Reg1
Same instructions as for memory; same address notation
Chapter 11 I/O Systems

17. Memory-Mapped I/O
Each control register is assigned a unique memory address (devices and memory share memory address space); uses same instructions as CPU programs
Programmer can use C code (Port I/O requires some assembly code to distinguish between instructions)
Each method has some disadvantages
Chapter 11 I/O Systems

18. Controller/host handshake with interface registers
Status register - set by controller
Completed/failure
Busy bit for availability
Control register – set by host
Control information for speed, mode (read/write), parity checking on/off, word length, full/half duplex
Command ready bit
Data-in register – set by controller; read by host
Data-out register – written by host; read by controller for output to device
Possibly input/output is cached in multiple registers
Chapter 11 I/O Systems

19. Sample Polling Protocol
Host polls for busy bit to be off (busy waiting)
If busy bit it off (device free), host can send signals (on bus lines) to
set mode bit in command register to write
place byte(s) in data-out register (other info possible)
set command-ready bit
Controller sets busy bit
fetches data-out from register
outputs data-out to device
sets status bit for success/ failure ` `
clears command-ready bit, busy bit
Chapter 11 I/O Systems

20. Interrupts for device I/O
In hardware
Device sends signal (IRQ) on interrupt-request line
Interrupt controller asserts signal; identifies interrupt #
Signal causes CPU to interrupt what it is doing
Interrupt # identifies position in interrupt vector that holds address of ISR
CPU saves value of current PC (possibly other values)
Transfers to interrupt vector, indirectly
Location contains (changeable) address of interrupt handler (ISR)
In software
ISR may disable interrupts, save state of registers it needs, enable interrupts
ISR determines nature of interrupt and handles interrupt
ISR may restore registers of interrupted process
Chapter 11 I/O Systems

21. Interrupt-controller
Prevents interruption during critical processing
Prioritizes interrupts
Determines which device raised interrupt
Programmable or Advanced Programmable Interrupt-controllers
Enables different prioritization methods (hard, rotating, cascading), for example
Chapter 11 I/O Systems

22. Question Why are interrupts prioritized? It is faster It is fairer
It is important not to lose data that may be overwritten by fast devices
Prioritization prevents deadlock
Answer: c
Chapter 11 I/O Systems

23. Masking and disabling interrupts
Maskable or nonmaskable
Nonmaskable interrupts (power or memory failure)
Maskable interrupts – devices, etc.
PCI bus has 2 interrupt request lines
Maskable interrupt line can be temporarily turned off during execution of critical instructions
Interrupt chaining
Each address in interrupt vector may point to a chain of ISRs
Nested prioritized interrupts
Interrupts stored and handled in priority order
Chapter 11 I/O Systems

24. Precise and Imprecise Interrupts
Pipelining of instructions - multiple instructions are in a state of execution
When an interrupt occurs, multiple instructions may be partially executed
This is called an imprecise interrupt
System must ensure that the instructions previous to the instruction whose PC has been saved are completed
System can roll back those instructions after the one whose PC has been saved (possibly including PC instruction)
Adds complexity to current systems
Chapter 11 I/O Systems

25. Exceptions or traps
Interrupt mechanism is used to handle software traps
Examples: memory access violations, division by zero
Interrupt mechanism may be used for page or segment faults
Interrupt mechanism may be used for calls to the kernel
Threads may communicate thru interrupts
Place common data on system stack
Chapter 11 I/O Systems

26. DMA (Direct Memory Access)
Fast I/O processor
Kernel process writes command block to DMA
Address in memory, address on device, r/w, # of blocks
DMA transfers block to/from memory to device; interrupt upon completion
Memory cycle stealing
DMA “handshake” with device controller (perhaps IDE)
DVMA can transfer block between two devices
Chapter 11 I/O Systems

27. Question DMAs use cycle stealing. What does that mean?
a. CPU cycles are stolen by the DMA controller
b. Memory cycles are stolen from other devices by the DMA controller
c. Memory cycles are stolen from the CPU by the DMA controller
Ans: c (i.e., CPU is prevented from accessing memory while DMA is transferring data in or out)
Chapter 11 I/O Systems

28. Kernel I/O structure
Kernel requests uniform set of services from subsystem
(through kernel-subsystem interface)
Typically- read(), write(), seek()
Subsystem requests services of device drivers, which encapsulate peculiarities of devices
(through subsystem- driver interface)
Example: PCI device driver
Drivers requests services (data, error flags, etc.) of controllers
(driver- controller interface)
Example of controller: PCI bus device controller
Controllers handle I/O devices - ever more different and complex
Speed, sharing, unit of transfer, access method, in sync or not
They all transfer bits
Example of device: PCI bus
Chapter 11 I/O Systems

29. Back door to device driver
Bypasses subsystem
In UNIX, for example, the command ioctl ()
Returns File descriptor
In UNIX, devices are handled as files
Determines command to device driver
Defined integer value
Provides pointer to data structure with control information, data for output if necessary
Chapter 11 I/O Systems

30. Clocks and Timers
Either hardware clock or very fast (high frequency) counter is used for timing services
Generates periodic interrupts (clock ticks)
Programmable clocks control frequency of interrupt
Clock Driver
Maintains time of day
May limit CPU time allowed to process
Handles the alarm system call
Provides software timers for the system
Statistics, monitoring, etc.
Chapter 11 I/O Systems

31. Software Timers Software interrupts are attached to clock ticks
Time slice interrupt
Preemptive priority scheduling
Periodic write-out of cache to disk
Network time-out and resend, break connections, etc.
Can provide services to user programs –
/usr/bin/time (Cshell)
sleep () in programs
Chapter 11 I/O Systems

32. Kernel I/O Subsystem services
Management of name space for files and devices
Access control authentication for files and devices
Operation control (modem cannot do a seek)
File-system space allocation
Device allocation
Buffer, i/o caches, spooling management
I/O scheduling
Device monitoring
Error handling, failure recovery
Device-driver configuration and initialization
Scheduling of disk operations
Table Maintenance
Chapter 11 I/O Systems

33. Performance Issues -Buffers
Buffering ameliorates speed mismatch between producer and consumer
Ex: modem and hard disk
Typically, double buffering is used
Once buffer is filled, disk write is requested
Modem can continue; writes to other buffer until output is completed
Ameliorates differences in data-transfer size
Perhaps holds data until it can be defragmented (reassembled)
Chapter 11 I/O Systems

34. Performance Issues - Caching
Cache is high speed storage that holds, by definition, copy of data from slower storage
Used for efficiency purposes
Cache on CPU is completely hardware controlled
Only issue for OS would be size, replacement
Caches in memory, on devices
Chapter 11 I/O Systems

35. Performance issues - Spooling
Disk buffers holding data for output (or input)
Control is handled by system daemon process or kernel thread
Spooling list is available
Job can be removed
Output is delayed until complete (usually)
Can cause deadlock over printer (or other hardware) if a process holds (locks) printer while waiting for other resource
Output process can continue executing if printer is busy
Consistent output as processes complete one at a time
Chapter 11 I/O Systems

36. I/O protection All I/O operations are privileged
User program traps to kernel, kernel suspends process, verifies user rights, completes I/O, signals user
Memory-mapped and i/o port locations are denied to users. Users must trap to kernel
But graphic games need direct access to memory-mapped graphics controller memory
Performance issue – cost of trap to kernel
OS can provide a section of graphics memory to a single user at a time using dedicated bus
Chapter 11 I/O Systems

37. Magnetic Disks “Platters” coated with magnetic material
logically divided into circular tracks
Cylinders are concentric tracks
tracks divided into sectors
sector is smallest unit of access
Access time (about 10ms)
Read-Write head on disk arm positioned over track (seek time – random access)
Rotational latency – disk rotated to proper sector (direct access)
Transfer rate – copying data to/from disk (33MB/s)
Transfer by Disk Controller – one on each disk
Sometimes controller is shared by disks (e.g., SCSI)
Head crash – head damages magnetic surface
Chapter 11 I/O Systems

38. Magnetic Tape
Audio and video recording; archival data storage (disks have taken many of these functions)
Sequential access
“direct” access is achieved by fast wind, rewind
Capacity
the highest capacity Hitachi tape cartridges stored up to 50 TB of data without compression
Chapter 11 I/O Systems

39. Optical Disks Cheaper R or R/W Capacity – 30GB and more
Slower than hard disks (at least currently)
Have replaced floppy disks
Capacity is greater
Cost per byte is less
Reliability is probably better as well
Chapter 11 I/O Systems

40. Disk Structure Sectors, blocks, clusters
A sector is the minimal unit of allocation (hardware determination)
A block is a minimal unit of transfer
OS determines the number of sectors to a block
Blocks (logical) are mapped onto sectors
Mapping difficulties
Number of sectors may decrease for inner cylinders
Bad sectors are mapped onto other positions holding “spare” sectors
In Windows, a cluster is the minimal unit of file allocation
OS determines the number of blocks in cluster to increase contiguity in file storage
Chapter 11 I/O Systems

41. Device Attachment Host-attached storage SCSI – up to 16 devices on bus
Controller card on host; up to 15 storage targets
Polling is used to give each device a turn
Handshakes for connection
IDE-ATA(Advanced Technology Attachment) – parallel interface standards
SATA, Firewire, USB are all serial bus interface standards
Fiber channel (perhaps over Ethernet)
Basis for SANs; switched “fabric” with large address space to connect large number of devices
Alternatively, arbitrated loop (FDDI) is used to connect up to 126 devices
Chapter 11 I/O Systems

42. Device attachment Firewire/ USB
High-speed serial data bus standards to connect a number of different peripheral devices
Have been implemented across different platforms
Firewire developed by Apple; {USB by Intel}
Firewire (speed of version S3200 up to 3.2 Gbps); SCSI (160Mbps, defined for both ATA and SATA); and super speed USB(up to 4.8Gbps); Firewire can daisy-chain 63+ devices
Chapter 11 I/O Systems

43. Device Attachment
USB and firewire standards define the cables and connectors between devices and computers (e.g., keyboards, cameras, printers) as well as the communication protocol used for connection.
Ports, interfaces, power supply, cable connectors are all included in the standard
Chapter 11 I/O Systems

44. Disk Attachment (cont.)
Network-Attached Storage
Network cables rather than SCSI (or other) cables
Location transparency
host does not know whether I/O devices are directly attached or distant
(except for the increased latency (delay) and for outputting a hard copy)
RPC over TCP/IP is commonly used to request service from storage devices
Chapter 11 I/O Systems

45. Disk scheduling
Assume that we have a large number of requests for disk i/o
Scheduling these requests in specific orders can increase throughput and decrease average waiting time
Issues and algorithms are similar to process scheduling although here we try to minimize hardware issues such as seek time (time for disk arm to move the head to the required cylinder).
Chapter 11 I/O Systems

46. Disk Management Disk formatting
Disk must be formatted before it is used
Low level formatting – usually when manufactured
Divides magnetic recording units into sectors
Each sector contains a header, a trailer (control info for controller) and a data area (usually bytes)
Sector number, error-correcting code
Soft error if ECC can recover from mismatch
Bad sectors are isolated
Spare sectors are set aside for later needs
Chapter 11 I/O Systems

47. OS disk formatting High Level formatting
OS installs multi-level data structures on a disk
Partitions disk into sets of cylinders
Logical disks are mapped onto the physical disk
Can provide a separate logical disk for OS code with limited access rights for users
Separate user data to make backups easier
Logical formatting
Create disk’s file system
Maps of free and allocated space, initial directory of files on disk
Chapter 11 I/O Systems

48. Raw Disks
OS activity is limited for raw disks (except for combining sectors into blocks)
Swap disks
Process data may be stored contiguously
Eliminate cost of i-node directories, linking blocks, etc.
Database systems
Provide their own high level formatting
These services bypass buffer cache, file locking, prefetching, directories, file naming
Chapter 11 I/O Systems

49. Boot block Bootstrap program on system disk in static location
Minimal boot program in ROM executed when machine is powered on
Brings rest of bootstrap program from disk into memory and transfers to it
Bootstrap program stored in “boot blocks” at fixed location on (boot or system) disks
Program does system and register initialization
Copies OS kernel from disk into memory
Transfers control to (jumps to) some address in memory to begin executing kernel
Chapter 11 I/O Systems

50. Swap Space Management Swap space is an extension of memory
Disk access is much slower than memory access
Access to swap space should be minimized for required virtual memory performance
Swap space was typically much larger than main memory until the large increases of main memory today
Swap space may be used for entire process or only for swapping out pages when necessary
Chapter 11 I/O Systems

51. Swap-space location Part of normal file system
Same file-handling routines used as other services
Inefficient, particularly the traversing of file-system data structures
Raw partition or separate disk
Algorithms focus on speed rather than storage efficiency
Linux, Windows may allow raw partitions to be incremented, if necessary, with space in the normal file system
Chapter 11 I/O Systems

52. RAID (redundant arrays of I disks)
I (inexpensive, independent)
Performance and reliability issues by accessing disks in parallel
Reliability
Assume mean time to failure of a single disk is 100,000 hours
If we have 100 disks in our organization, the mean time for one disk to fail is 1000 hours or 42 days!
Necessity of some type of backup
If we want no loss of data at all, then we need parallel redundant disk updates
Chapter 11 I/O Systems

53. RAID -redundancy Mirroring
Two disks, one controller. Every write is to both disks
Issues
Single controller can fail (you can duplex instead with different controllers)
If disks are both old and/or from same manufacturer, probability of both failing is considerably higher
Power failures must also be handled
If power fails in between the two disk writes, which update is correct?
Chapter 11 I/O Systems

54. Performance improvement with RAID
Striping
Bit-level striping
Say bit i is read/written to disk i
We can decrease the time necessary to fetch a block by a factor based on the number of disks.
Block level striping
Each successive block is stored in “next” disk
Byte level striping is also possible
Chapter 11 I/O Systems

55. RAID levels Schemes to balance reliability, efficiency, and cost
RAID level 0 – block striping for efficiency
RAID level 1 – disk mirroring for redundancy (possibly multiple paths for reads for efficiency) 100% overhead for writes; double # of disks
RAID level 2 – ECC organization with bit striping for reliability and efficiency
2-3 error correcting bits stored on extra disk(s) for each byte
3 extra disks must be added for 8 disks
Note that error correcting disks must be updated for each write to a data disk
RAID level 3 – byte-interleaved parity organization
Byte striping, with parity bytes (checksum) on separate disk
Controller will be able to tell which disk was damaged
Parity disk must be updated during each write
Chapter 11 I/O Systems

56. RAID levels RAID level 4 – block-interleaved parity organization
Block-level striping; parity block on an extra disk to be used for reconstruction of failed block
Multiple reads can be done concurrently, but writes must all wait for parity block
Scalable, since new disk can be initialized to all 0s and parity block is still correct
Chapter 11 I/O Systems

57. RAID levels
RAID level 5 block-level striping; block-interleaved distributed parity (most popular RAID architecture)
Parity bits are spread among all disks but not on disk that holds the data of specified parity
Improves performance, since two different disks are used for all updates
Recovery more complex
RAID level 6 – more redundancy
RAID level 0-1 disks are striped and then mirrored to another identical disk set
Chapter 11 I/O Systems

58. Comments on RAID
Concepts of combination of redundancy and efficiency can be extended to other media
Extra expense of RAID
Not all errors are caught, i.e., corruption of data or metadata
Solaris ZPS file system keeps checksums of data, metadata together with inode of file
Chapter 11 I/O Systems

59. Stable Storage
Definition of stable (persistent) storage – data is never lost
Multiple locations hold B; A overwrites B in one location; now is B or A considered correct?
If failure occurs, protocol must determine which value
Requires redundancy
Multiple disks with independent failure rates
Updating must guarantee that failures will still leave data consistent (and correct)
Chapter 11 I/O Systems

60. Disk write outcomes Write, then read to verify that write is completed
Three possible outcomes
Successful completion
Partial failure
Sector being overwritten is corrupted in midst of transfer
Write, then read, are repeated
Total failure
Failure occurs before the disk write, leaving previous disk data values untouched
Chapter 11 I/O Systems

61. Assume any type of failure occurs
Controller can sense this and sends error message
If both disks hold same data, we assume they were not corrupted
If one block/ disk can be detected as incorrect, replace it with data from the other
If contents are not the same, but both disks appear correct, copy the data from the second disk into the first
Or assume that update to second disk was not complete, and we return to earlier state of no change
Update can be stored in a NVRAM and repeated
Chapter 11 I/O Systems

62. Robotic jukebox Automated hierarchical storage for removal media
May hold tens of drives and thousands of cartridges or disks
Retrieval is typically tens of seconds or minutes
Common for archival storage and infrequently used information
Brought to secondary storage when needed
Chapter 11 I/O Systems

63. Optical Jukebox
Optical Jukebox - An automated storage device that holds many optical disks with a small number of optical read/write drives used to archive and store massive amounts of data. A robotic arm moves optical disks from storage slots to the drives. Also referred to as an optical library.
Phase-Change – A recording process that uses laser light to convert points on an optical disk from an amorphous state to a crystalline state.
Picker – The robotic component of an optical jukebox that moves optical disks to and from storage slots and optical drives. Also referred to as a media picker.
Shelf – See Slot.
Single-Head Drives – Drives that can only read or write to one media surface at a time. The disk must be flipped and re-inserted into the drive in order to read or write to the other side. 5 ¼” drives are single-headed.
Slot – An area inside an optical jukebox where optical disks are stored. A jukebox can store as few as 20 optical disks or as many as 638, depending on the manufacturer and model number of the jukebox.
Time to Deliver Data (TDD) – The total time it takes to deliver requested data back to an application. This includes the time it takes for the picker to fetch an optical disk, move it to and insert it into an optical drive, spin up the drive, locate the requested data, and transfer it back to the application. Measured in seconds.
WORM – Write Once Read Many. A type of optical media that allows data to be written only once. Files cannot be overwritten. Commonly used in regulated industries requiring permanent file archives. Some regulatory agencies include the Securities Exchange Commission, Nuclear Regulatory Commission, and Food and Drug Administration.
Chapter 11 I/O Systems

64. Future mass-storage media
Holographic storage
Laser to record 3-dimensional arrays of pixels
Very fast
Microelectronic mechanical systems
Biological mechanisms
Chapter 11 I/O Systems

65. Question
Name three protocol standards for connecting devices to hosts.
SCSI
firewire
USB
LANs
SANs
Chapter 11 I/O Systems

66. What is a jukebox? Something that holds records.
What is a robotic jukebox?
Chapter 11 I/O Systems

67. Power Management
Most computers consume about 200-watts (200 joules per second) – that is a lot of energy for a company’s computers
Battery-powered computers hold their charge for at most a few hours
Wireless systems use even more power when continually sending/ receiving data
Minimal improvements in battery life
Chapter 11 I/O Systems

68. Operating systems can save power
Devices can be made to sleep/ hibernate
Displays (big users of power) can be lit in zones
Disks (big users of power) powered down when not in use; applications programs delay writing out to disk; RAM disk used
OS uses artificial intelligence to estimate sleep times, etc.
Chapter 11 I/O Systems