1. I/O SYSTEM AND CASE STUDY
By
Hariharan.A
14BIT025
III B.Sc (IT)
2. One of the important jobs of an Operating System is to manage various I/O
devices including mouse, keyboards, touch pad, disk drives, display adapters,
USB devices, Bit-mapped screen, LED, Analog-to-digital converter, On/off
switch, network connections, audio I/O, printers etc.
An I/O system is required to take an application I/O request and send it to the
physical device, then take whatever response comes back from the device and
send it to the application. I/O devices can be divided into two categories −
Block devices − A block device is one with which the driver communicates by
sending entire blocks of data. For example, Hard disks, USB cameras, Disk-
On-Key etc.
Character devices − A character device is one with which the driver
communicates by sending and receiving single characters (bytes, octets). For
example, serial ports, parallel ports, sounds cards etc
3. DEVICE CONTROLLERS:
Device drivers are software modules that can be plugged into an OS to handle a
particular device. Operating System takes help from device drivers to handle all
I/O devices.
The Device Controller works like an interface between a device and a device
driver. I/O units (Keyboard, mouse, printer, etc.) typically consist of a
mechanical component and an electronic component where electronic
component is called the device controller.
There is always a device controller and a device driver for each device to
communicate with the Operating Systems. A device controller may be able to
handle multiple devices. As an interface its main task is to convert serial bit
stream to block of bytes, perform error correction as necessary.
Any device connected to the computer is connected by a plug and socket, and
the socket is connected to a device controller. Following is a model for
connecting the CPU, memory, controllers, and I/O devices where CPU and
device controllers all use a common bus for communication.
4. Magnetic disks provide bulk of secondary storage of modern computers
Drives rotate at 60 to 250 times per second
Transfer rate is rate at which data flow 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 results from disk head making contact with the disk
surface -- That’s bad
Disks can be removable
Drive attached to computer via I/O bus
Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel,
SCSI, SAS, Firewire
Host controller in computer uses bus to talk to disk controller built
into drive or storage array
OVERVIEW OF MASS STORAGE STRUCTURE
6. Can just attach disks, or arrays of disks
Storage Array has controller(s), provides features to attached
host(s)
Ports to connect hosts to array
Memory, controlling software (sometimes NVRAM, etc)
A few to thousands of disks
RAID, hot spares, hot swap (discussed later)
Shared storage -> more efficiency
Features found in some file systems
Snaphots, clones, thin provisioning, replication,
deduplication, etc
STORAGE ARRAY
7. MEMORY-MAPPED I/O
When using memory-mapped I/O, the same address space is shared by memory and I/O devices. The device is
connected directly to certain main memory locations so that I/O device can transfer block of data to/from memory
without going through CPU.
While using memory mapped IO, OS allocates buffer in memory and informs I/O device to use that buffer to send
data to the CPU. I/O device operates asynchronously with CPU, interrupts CPU when finished.
The advantage to this method is that every instruction which can access memory can be used to manipulate an
I/O device. Memory mapped IO is used for most high-speed I/O devices like disks, communication interfaces.
8. Synchronous vs asynchronous I/O
Synchronous I/O − In this scheme CPU execution waits while I/O
proceeds
Asynchronous I/O − I/O proceeds concurrently with CPU execution
Communication to I/O Devices:
The CPU must have a way to pass information to and from an
I/O device. There are three approaches available to communicate with the
CPU and Device.
Special Instruction I/O
Memory-mapped I/O
Direct memory access (DMA)
Special Instruction I/O:
This uses CPU instructions that are specifically made for controlling I/O
devices. These instructions typically allow data to be sent to an I/O device
or read from an I/O device.
9. Swap-space — Virtual memory uses disk space as an extension of main memory
Less common now due to memory capacity increases
Swap-space can be carved out of the normal file system, or, more commonly, it can
be in a separate disk partition (raw)
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 dirty page is forced out of
physical memory, not when the virtual memory page is first created
File data written to swap space until write to file system requested
Other dirty pages go to swap space due to no other home
Text segment pages thrown out and reread from the file system as
needed
What if a system runs out of swap space?
Some systems allow multiple swap spaces
SWAP-SPACE MANAGEMENT
10. DIRECT MEMORY ACCESS (DMA)
Slow devices like keyboards will generate an interrupt to the main CPU
after each byte is transferred. If a fast device such as a disk generated an
interrupt for each byte, the operating system would spend most of its time
handling these interrupts. So a typical computer uses direct memory access
(DMA) hardware to reduce this overhead.
Direct Memory Access (DMA) means CPU grants I/O module authority to
read from or write to memory without involvement. DMA module itself
controls exchange of data between main memory and the I/O device. CPU
is only involved at the beginning and end of the transfer and interrupted
only after entire block has been transferred.
Direct Memory Access needs a special hardware called DMA controller
(DMAC) that manages the data transfers and arbitrates access to the system
bus. The controllers are programmed with source and destination pointers
(where to read/write the data), counters to track the number of transferred
bytes, and settings, which includes I/O and memory types, interrupts and
states for the CPU cycles.
12. POLLING VS INTERRUPTS I/O
A computer must have a way of detecting the arrival of any type of input. There
are two ways that this can happen, known as polling and interrupts. Both of
these techniques allow the processor to deal with events that can happen at any
time and that are not related to the process it is currently running.
Polling I/O:
Polling is the simplest way for an I/O device to communicate with the
processor the processor. The process of periodically checking status of the
device to see if it is time for the next I/O operation, is called polling.
The I/O device simply puts the information in a Status register, and the
processor must come and get the information.
13. INTERRUPTS I/O
An alternative scheme for dealing with I/O is the interrupt-driven method. An
interrupt is a signal to the microprocessor from a device that requires attention.
A device controller puts an interrupt signal on the bus when it needs CPU’s
attention when CPU receives an interrupt, It saves its current state and invokes
the appropriate interrupt handler using the interrupt vector (addresses of OS
routines to handle various events). When the interrupting device has been dealt
with, the CPU continues with its original task as if it had never been interrupted.
14. BEGINNING OF WINDOWS
In 1988, Microsoft decided to develop a “new
technology” (NT) portable operating system that
supported both the OS/2 and POSIX APIs
Originally, NT was supposed to use the OS/2 API
as its native environment but during development
NT was changed to use the Win32 API, reflecting
the popularity of Windows 3.0.
15. ADVANTAGES OF WINDOWS
Advantages over Windows
– It's almost free to relatively
inexpensive
– Source code is included
– Bugs are fixed quickly and help is
readily available through the vast
support in Internet
– Linux is more stable than Windows
– Linux is truly multi-user and multi-
tasking
– multiuser: OS that can simultaneously
serve a number of users
– multitasking: OS that can
simultaneously execute a number of
programs
– Linux runs on equipment that other
operating systems consider too
underpowered, e.g. 386 systems,
PDA, etc
Microsoft has made several
advancements and changes that have
made it a much easier to use
Operating System, and although
arguably it may not be the easiest
Operating System
Because of the large amount of
Microsoft Windows users, there is a
much larger selection of available
software programs, utilities, and
games for Windows.
16. DISADVANTAGES OF WINDOWS
Disadvantages of windows
Although Microsoft Windows has made great
improvements in reliability over the last few versions of
Windows, it still cannot match the reliability ofLinux.
to viruses and other attacks. Although Microsoft has made
great improvements over the years with security on their
Operating System,
their Operating System continues to be the most vulnerable
18. STATES OF PROCESS MANAGEMENT
Running: A process will be in either running(current
process in system)or ready to run(waiting to be assigned
to system cpu)
Waiting:The process is waiting for a resource or event.
Interputable waiting process can be interputable by
signals but are waiting directly on hardware conditions
and cannot be interpated
19. STATES OF PROCESS MANAGEMENT (CONTD)
Stopped:The process can be stopped usually by
receiving a signal. A process that is being
debugged can be in a stopped state
Zombie:This is a halted process which for some
reason still has task_struct data structure in task
vector.It is like a dead process
20. PROCESS MANAGEMENT IN WINDOWS
A process contains its own independent virtual address
space with both code and data, protected from other
processes. Each process, in turn, contains one or more
independently executing threads. A thread running within a
process can execute application code, create new threads,
create new independent processes, and manage
communication and synchronization among the threads.
By creating and managing processes, applications can have
multiple, concurrent tasks processing files, performing
computations, or communicating with other networked
systems. It is even possible to improve application
performance by exploiting multiple CPU processors
21. STATES OF PROCESS MANAGEMENT
Ready:May be scheduled for execution. The microkernel dispatcher
keeps track of all ready threads and schedules them in priority order.
Standby:A standby thread has been selected to run next on a particular
processor. The thread waits in this state until that processor is made
available. If the standby thread's priority is high enough, the running
thread on that processor may be preempted in favor of the standby
thread. Otherwise, the standby thread waits until the running thread
blocks orexhausts its time slice.
Running:Once the microkernel performs a thread or process switch, the
standby thread enters the running state and begins execution and
continues execution until it is preempted, exhausts its time slice, blocks,
or terminates. In the first two cases, it goes back to the ready state
23. ADDRESS TYPES IN PHYSICAL MEMEORY
The following is a list of address types used in Linux.
User virtual addresses:These are the regular addresses seen by user-space programs. User addresses
are either 32 or 64 bits in length, depending on the underlying hardware architecture, and each
process has its own virtual address space.
Physical addresses:The addresses used between the processor and the system's memory. Physical
addresses are 32- or 64-bit quantities; even 32-bit systems can use larger physical addresses in some
situations.
Bus addresses:The addresses used between peripheral buses and memory. Often, they are the same
as the physical addresses used by the processor, but that is not necessarily the case. Some
architectures can provide an I/O memory management unit (IOMMU) that remaps addresses
between a bus and main memory. An IOMMU can make life easier in a number of ways (making a
buffer scattered in memory appear contiguous to the device, for example), but programming the
IOMMU is an extra step that must be performed when setting up DMA operations. Bus addresses
are highly architecture dependent, of course.
24. ADDRESS TYPES IN PHYSICAL MEMEORY
Kernel logical addresses: These make up the normal address space of the kernel.
These addresses map some portion (perhaps all) of main memory and are often
treated as if they were physical addresses. On most architectures, logical addresses
and their associated physical addresses differ only by a constant offset. Logical
addresses use the hardware's native pointer size and, therefore, may be unable to
address all of physical memory on heavily equipped 32-bit systems. Logical
addresses are usually stored in variables of type unsigned long or void *. Memory
returned from kmalloc has a kernel logical address.
Kernel virtual addresses: Kernel virtual addresses are similar to logical addresses in
that they are a mapping from a kernel-space address to a physical address. Kernel
virtual addresses do not necessarily have the linear, one-to-one mapping to physical
addresses that characterize the logical address space, however. All logical addresses
are kernel virtual addresses, but many kernel virtual addresses are not logical
addresses. For example, memory allocated by vmalloc has a virtual address (but no
direct physical mapping). The kmap function (described later in this chapter) also
returns virtual addresses. Virtual addresses are usually stored in pointer variables.
25. MEMORY MANAGEMENT IN WINDOWS
Each process on 32-bit Microsoft Windows has its own virtual address space that enables
addressing up to 4 gigabytes of memory. Each process on 64-bit Windows has a virtual
address space of 8 terabytes. All threads of a process can access its virtual address space.
However, threads cannot access memory that belongs to another process, which protects
a process from being corrupted by another proces
Virtual Address Space
Memory Pools
Memory Performance Information
Virtual Memory Functions
Heap Functions
File Mapping
Large Memory Support
Global and Local Functions
Standard C Library Functions
Comparing Memory Allocation Methods
27. USER INTERFACE OF WINDOWS
The Windows 9x user interface (9x refers to Windows 95
and Windows 98 together) is characterised by a bar at the
bottom of your screen (it may be moved to another edge
by left-mouse-clicking on it and dragging it to the
desired edge while continuing to hold down the mouse
button).
28. USER INTERFACE IN WINDOWS
This bar is called the "taskbar." It includes:
The "Start menu" (which contains shortcuts to
applications stored on your hard drive)
The list of open windows on your desktop (the
Windows desktop is the entire portion of the screen
located above the taskbar, where shortcuts to
applications may be placed in the form of icons)
Icons representing shortcuts to programs loaded in
the computer memory, and which can be configured
29. USER INTERFACE IN WINDOWS
It includes the following shortcuts:
Shut down: To shut down or restart a computer, or restart it in MS-DOS mode
Run: Run a program using a command line (like in DOS). Arguments can be added to an
application (for example, in "dir /s", "/s" is an argument)
Help: Windows online help
Find: Lets you search for a file on the hard drive, or for another computer if you are on a
network
Settings: This is the computer's "control center", where display, hardware, and software
settings can be configured.
Programs: This contains links to your favorite programs. It can be modified by going to
Settings/Taskbar and Start menu, then clicking on Start menu
The menu can include other shortcuts, too, such as: Documents: List of the most recently
opened documents
Favorites: Lets you sort your favorite shortcuts, whether on the Internet or your hard
drive
etc.
30. Write-ahead log scheme requires stable storage
Stable storage means data is never lost (due to failure, etc)
To implement stable storage:
Replicate information on more than one nonvolatile storage media
with independent failure modes
Update information in a controlled manner to ensure that we can
recover the stable data after any failure during data transfer or
recovery
Disk write has 1 of 3 outcomes
1. Successful completion - The data were written correctly on disk
2. Partial failure - A failure occurred in the midst of transfer, so only
some of the sectors were written with the new data, and the sector
being written during the failure may have been corrupted
3. Total failure - The failure occurred before the disk write started, so
the previous data values on the disk remain intact
STABLE-STORAGE IMPLEMENTATION
