I/O SYSTEM AND CASE STUDY

By Hariharan.A14BIT025III B.Sc (IT)

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

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.

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

DIAGRAM:

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

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.

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 executionCommunication 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.

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

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.

DIAGRAM:

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.

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.

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.

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.

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

WINDOWS SYSTEM ARCHITECTURE

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

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

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

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

PROCESS MANAGEMENT WINDOWS

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.

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.

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

MEMORY MANAGEMENT IN WINDOWS

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).

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

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.

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 outcomes1. 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