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1. Sealed Unit Parts Port Devices Driver Side Mirror
2. Sealed Unit Parts Port Devices Driver
3. Sealed Unit Parts Port Devices Driver Updater

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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.

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.

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.

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.

Sealed Unit Parts Port Devices Driver Side Mirror

The operating system uses the DMA hardware as follows −

Step	Description
1	Device driver is instructed to transfer disk data to a buffer address X.
2	Device driver then instruct disk controller to transfer data to buffer.
3	Disk controller starts DMA transfer.
4	Disk controller sends each byte to DMA controller.
5	DMA controller transfers bytes to buffer, increases the memory address, decreases the counter C until C becomes zero.
6	When C becomes zero, DMA interrupts CPU to signal transfer completion.

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 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.

Most of the time, devices will not require attention and when one does it will have to wait until it is next interrogated by the polling program. This is an inefficient method and much of the processors time is wasted on unnecessary polls.

Compare this method to a teacher continually asking every student in a class, one after another, if they need help. Obviously the more efficient method would be for a student to inform the teacher whenever they require assistance.

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.

Previous Article: Brief USB Overview and USB History

The USB System Architecture consists of the following main components:

• Host Computer

• One or more USB Devices

• A physical bus represented by the USB Cable that links the devices with the host computer

USB Host

The main responsibility of the host computer is to control the communications between the host system and USB devices.

USB Host Computer

The host computer contains two layers: a USB host controller hardware layer, and a software layer, which includes USB device drivers for a wide range of USB devices such as keyboards, mice, digital still cameras, scanners, mass storage devices etc. The drivers convert data between the format that is used by the host computer and the format used by the USB devices.

USB hardware layer is responsible for:

• Detecting the attachment and removal of USB devices

• Monitoring device status and collecting activity statistics

• Providing power to attached USB devices

• Managing control and data flow between the USB host and USB devices

• Checking the basic validity of bus transactions

USB software layer is responsible for:

• Handling USB devices and their connectivity

• USB devices enumeration and configuration

• Loading appropriate device drivers

• Managing the power on the bus and bus bandwidth

• Managing the data transfer between the software and hardware

USB Devices

The USB Devices are peripherals that use the USB protocol for two way communication with the host computer.

USB Flash Drive

USB MP3 Player

USB Image Still Camera

USB Video Camera

The main responsibility of the USB Device is to provide the end user with various specific functions, such as a keyboard, a data/fax modem device, a digital microphone, or a video camera. For this reason the official USB Specification uses the term 'function' to refer to USB peripheral devices. To provide the user with additional attachment points to connect more peripherals there is a special type of USB device called USB Hub.

7-port USB Hub

USB Hub is just a device with multiple USB ports for plugging in the USB devices and other hubs. Internal root hub on the host controller and the external hubs are functionally the same.

USB Cable

USB Cable is a cable to connect between host computer and USB device.

Sealed Unit Parts Port Devices Driver

USB A - USB mini

The USB specification limits the maximum length of a standard USB cable between full- and high-speed devices to 5 meters. For a low-speed device the limit is 3 meters. The primary reason for USB cable's length limit is the maximum allowed delay of a signal. In practice, some USB devices may work with longer than specified cable. However, using a longer cable lowers the signal quality provided by the USB bus below the specification tolerance limit. This may prevent USB devices from working properly or even from working at all. Using USB devices over a greater length require one or several USB hubs or USB Active Extension Cable (USB Repeater Cable) that contains active electronics which regenerate the USB signal for maximum reliability and performance over extended distances. With powered USB hubs or USB active extension cables USB connections can be extended to maximum theoretical distance of about 30 meters.

USB Host Controller

A USB Host Controller is the hardware either on the computer motherboard or PCI card. It provides an interface for transferring streams of data between the host computer and the USB devices. The host computer may have one or multiple USB host controllers with different types of interface.

USB Host Controller Interface

A Host Controller Interface (HCI) is a register level interface which allows host controller hardware to communicate with the operating system of a host computer. There are three types of USB host controller interface:

• Open Host Controller Interface (OHCI)

• Universal Host Controller Interface (UHCI)

• Enhanced Host Controller Interface (EHCI)

• eXtensible Host Controller Interface (xHCI)

Host controller interface standards are not defined in the USB specification itself. There are separate specifications for each type of HCI:

Sealed Unit Parts Port Devices Driver Updater

With USB revision 1.0 and 1.1, there were two HCI specifications, OHCI specification developed by Compaq, Microsoft and National Semiconductor and the UHCI specification developed by Intel. Both HCI standards provide the same capabilities and work with all USB devices but do not support high-speed transfers. The differences however is that the OHCI places more of the burden on a hardware and makes for simpler software and the UHCI's hardware implementation is simpler and cheaper, but requires a more complex software and thus puts more load onto the CPU. The dueling implementations forced vendors to develop and test the hardware and software on both HCI implementations which increased cost. With the introduction of USB revision 2.0 a new Enhanced Host Controller Interface specification was developed by Intel, Compaq, NEC, Lucent and Microsoft. The single integrated EHCI specification eliminates many of the problems that existed because of competing OHCI and UHCI standards. EHCI used by all USB 2.0 devices and the only HCI that can support high-speed transfers. The xHCI specification was defined by Intel with a greatly expanded industry contribution. The xHCI host controller supports super-speed (USB 3.0) devices but is fully backwards compatible with high-speed, full-speed, and low-speed devices.

Using USBlyzer - Identifying the USB Host Controller Type

You can see whether a particular host controller is OHCI, UHCI, EHCI or xHCI compliant by opening USBlyzer and checking for the word 'Open', 'Universal', 'Enhanced' or 'eXtensible' in host controller's description in the Device Tree panel.

USB Host Controllers

Also note that selecting root node in the Device Tree panel brings up the statistics regarding the number and type of installed USB host controllers, hubs and devices on your USB system in USB Properties panel.

USB Host Controller is a Bus Master

The Universal Serial Bus is a host controlled bus. All data transfers are initiated and controlled by the host on a scheduled basis. All USB peripherals are slaves responding to host commands with the only exception of 'remote wakeup' command which USB device can issue to make the suspended host active.

Since only one master controls the bus, two USB devices cannot communicate directly with each other. However, USB On-The-Go (OTG) extension enables direct data exchange between peripheral USB devices, where one USB Device acts as the master and the other as the slave. There is also a device on the market called the 'USB Bridge' that allows you to link two USB devices together to transfer data without the need of a host computer. In this case USB bridge is itself works as a master.

Delkin USB Bridge

Two USB hosts also cannot communicate directly as they both are masters on the bus. To connect two PCs to each other you need a special USB Data Cable (also known as a 'USB to USB Adapter'). There is a device in the cable that acts as a slave for both connected computers.

USB to USB Adapter

The host computer may have one or multiple USB Host Controllers and each host controller contains an implicit USB Hub, called the Root Hub. A USB Root Hub is an internal device that is directly attached to the USB Host Controller and provides a number of initial attachment points called USB Ports for connecting USB devices and external USB hubs to the host computer.

Nowadays typical computer motherboards have many USB ports. Normally two or four of them reside directly on the motherboard, at its back, others are available through connector.

Most of the newer computer cases have frontal USB Ports which can be connected to the motherboard.

Another possibility to extend the number of USB Ports is to install a USB PCI Card.

To connect the USB device to a computer one has to find a free USB port connector on the back or front of the machine and plug the USB device into it.

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Next Article: USB Topology