CSC159 Computer Organization: I/O andPeriphials

Introduction to Computer Organization

• Computer Organization refers to the design and structure of a computer system that manages and facilitates communication between the CPU and peripheral devices.

Peripherals

• Peripherals are external devices connected to a computer that provide input, output, or storage functions.
• Examples of peripherals include:
  • Input devices: keyboard, mouse, scanner, microphone
  • Output devices: monitor, printer, speakers, projector
  • Storage devices: external hard drive, USB flash drive, CD/DVD/Blu-ray drives, network attached storage (NAS)

Peripheral Interfacing

• Peripherals connect to the computer through various interfaces and ports, such as:
  • USB (Universal Serial Bus)
  • HDMI (High-Definition Multimedia Interface)
  • Bluetooth
  • Ethernet

Importance of Peripherals

• Peripherals enhance the functionality of a computer beyond its core functions.
• They facilitate user interaction with the computer, making it easier to input and receive data.
• Peripherals enable the computer to perform specialized tasks, such as printing, scanning, and audio recording.

I/O Consideration

• When designing and managing I/O systems, speed and coordination are critical factors to consider.
• Two key I/O considerations are:
  • Speed: ensuring efficient data transfer between the CPU and peripherals
  • Coordination: managing multiple devices performing I/O operations simultaneously

Speed Consideration

• The CPU operates at speeds much faster than the fastest I/O device.
• Techniques such as buffering, caching, and parallel processing can be employed to mitigate speed disparities.
• Managing data transfers efficiently becomes challenging when devices with varying speeds are connected to the same system.
• Implementing adaptive algorithms or prioritization schemes can help manage data transfers based on device speeds.

Coordination Consideration

• Multiple devices perform I/O operations simultaneously, requiring coordination to ensure efficient and error-free data transfer.
• Coordination involves managing unexpected input, various input formats, and status information for each device.

I/O Module

• An I/O module serves as an interface between the CPU and peripheral devices.
• It manages communication and data transfer between the CPU and external devices.
• An I/O module is responsible for:
  • Controlling one or more external devices
  • Exchanging data between devices and main memory and/or CPU

I/O Module Functions

• The I/O module performs two different functions:
  • At the CPU interface, it accepts I/O commands from the CPU and transfers data between the module and the CPU or memory.
  • At the device interface, it supplies control of the device, such as moving the head to the correct track in a disk drive.

Buffering

• Buffering is a method where a buffer holds data and releases it at specific times.
• A buffer is a region of memory used to temporarily hold data while it is being moved from one place to another.
• Buffering is used to manage differences in data transfer rates between devices and the CPU.

Handshaking

• Handshaking is a process that takes place when a computer is about to communicate with a foreign device to establish rules for communication.
• It involves a brief back-and-forth series of requests and acknowledgements to request a connection.

I/O Techniques

• There are three types of I/O communication techniques:
  • Programmed I/O: The CPU initiates a program to transfer data from an I/O device to memory.
  • Interrupt Driven I/O: The CPU offers a read command to the I/O device, and once the device is ready, it sends an interrupt signal to the processor.
  • Direct Memory Access (DMA): The I/O device can send and receive data directly from memory, bypassing the CPU.

Programmed I/O

• Programmed I/O is a method used by the CPU to directly control the data transfer between itself and peripheral devices.

• The CPU is heavily involved in the data transfer process, issuing commands and waiting for operations to complete before moving on to the next task.

• Polling and response are techniques used in programmed I/O, where the CPU repeatedly checks the status of an I/O device to see if it is ready for data transfer.### Programmed I/O

• Initiation: CPU issues a command to the peripheral device by writing to its control register.

• Polling: CPU continuously checks the status of the peripheral device to determine if it is ready for data transfer.

• Data Transfer: CPU reads data from or writes data to the device once it indicates it is ready.

• Completion: CPU completes the data transfer operation and moves on to the next task.

Interrupt-Driven I/O

• CPU initiates an I/O operation with a peripheral device and then continues executing other tasks.
• When the I/O operation is complete or requires attention, the device sends an interrupt request (IRQ) to the CPU.
• CPU interrupts its current task, transfers control to an interrupt handler routine, processes the I/O operation, and then resumes the interrupted task.

Module Calls CPU When Needed

• Module refers to a peripheral device such as a disk drive, network interface, or keyboard.
• Module communicates with the CPU as needed for data transfer or to signal events.
• In interrupt-driven I/O, modules do not constantly communicate with the CPU; instead, they initiate communication or notify the CPU only when necessary.

Interrupt Requests (IRQs)

• When a module needs the CPU's attention, it sends an interrupt request (IRQ) signal.
• IRQ interrupts the CPU's current execution and triggers the CPU to handle the module's request or event.

Interrupt Handler

• CPU responds to the IRQ by transferring control to a specific part of the operating system called an interrupt handler or interrupt service routine (ISR).
• Interrupt handler is responsible for processing the module's request, whether it involves data transfer, error handling, or other actions.

Summary of Interrupt-Driven I/O

• Overcome CPU waiting; no repeated CPU checking of device.
• I/O module interrupts when ready.
• Still requires CPU to be the go-between for moving data between I/O module and main memory.
• Interrupt-driven I/O approach allows the CPU to focus on executing other tasks or processes until a module requires attention.

Interrupt

• A call for the microprocessor to interact or service the interrupting unit.
• Signal that causes the CPU to alter its normal flow of instruction execution.
• Frees CPU from waiting for events; provides control for external input.
• Examples: unexpected input, abnormal situation, illegal instructions, multitasking, multiprocessing.

Servicing the Interrupt

• There are four steps taken by the microprocessor to handle an interrupt:
  1. Finish the current instruction.
  2. Suspend operation and store registers.
  3. Jump to the interrupt routine and execute.
  4. Return to normal operation and restore registers.

Uses of Interrupts

• As an external event notifier.
• As a completion signal.
• As a means of allocating CPU time.
• As an abnormal event indicator.

The Interrupt As An External Event Notifier

• To indicate some external devices require action.
• Frees CPU from polling; a means for the user to control the computer from an input device.
• Example: keyboard input.

The Interrupt As A Completion Signal

• To control the flow of data to an output device.
• Interrupt serves to notify the computer of the completion of a particular course of action.
• Means of controlling the flow of data to an external device.
• Example: printing process and DMA process.

The Interrupt As A Means Of Allocating CPU Time

• Time sharing.
• Used as a method of allocating CPU time to different programs that are sharing the CPU.
• The computer system provides an internal clock that sends an interrupt periodically to the CPU.

The Interrupt As An Abnormal Event Indicator

• Used to handle abnormal events that affect the operation of the computer.
• The events are directly related to problems within the computer system itself.
• Example: execution of an illegal instruction, nonexistent code, and hardware error.

Software Interrupts

• Modern CPU instruction sets include an instruction that simulates an interrupt.
• The interrupt instruction works in the same way as a hardware interrupt, saving appropriate registers and transferring control to an interrupt handling procedure.

Multiple Interrupts

• Multiple interrupts will occur from time to time.
• Two different processing methods used to determine which devices initiated the interrupt:
  1. Vectored interrupt.
  2. Polled interrupt.

Direct Memory Access (DMA)

• A feature in computer systems that allows certain hardware components to directly read from and write to the main memory (RAM) without involving the CPU.

• Method of transferring data from the computer's RAM to another part of the computer without processing it using the CPU.

• Useful when the CPU cannot keep up with the rate of data transfer, or where the CPU needs to perform useful work while waiting for a relatively slow I/O data transfer.### Direct Memory Access (DMA)

• DMA allows I/O modules and main memory to exchange data directly, without CPU involvement.

• DMA enables certain hardware to access main system memory independently of the CPU.

• It enables direct transfer to and from memory.

• DMA is used for large blocks/volumes of data transfer.

• The CPU is not actively involved in the transfer itself.

• In DMA, the data transfer is managed by the DMA controller, which allows the CPU to perform other tasks while the transfer is ongoing.

• When the transfer is complete, the DMA module sends an interrupt signal to the CPU.

DMA Instructions

• The application program requests I/O service from the operating system.
• To initiate DMA, programmed I/O is used to send the following information:
  • The location of the data on the I/O device.
  • The starting location of the block of data in memory.
  • The size of the block to be transferred.
  • The direction of transfer (read or write).
• An interrupt is sent to the CPU upon completion of DMA.

CPU-I/O Interaction

• Conflicts between the CPU and the I/O module must be avoided.
• Bus arbitration is used to handle conflicts, where a priority system determines whether the CPU or the DMA controller gets access to the bus at any given time.

Advantages of DMA

• Reduced CPU overhead, as the DMA controller manages the data transfer.
• Faster data transfer, as DMA can transfer data directly between memory and peripherals at high speeds.

I/O Devices that Require DMA

• Hard Disk Drives (HDDs) and Solid-State Drives (SSDs)
• Network Interface Cards (NICs)
• Graphics Cards (GPUs)
• Sound Cards

Differences between Programmed I/O, Interrupt-Driven I/O, and DMA

• Programmed I/O: The CPU is responsible for directly controlling all data transfers between memory and I/O devices.
• Interrupt-Driven I/O: The CPU initiates a data transfer and then continues with other tasks, while the I/O device interrupts the CPU when it is ready to transfer data.
• DMA: A separate hardware component (the DMA controller) manages the data transfer between memory and I/O devices.

CPU-Memory-I/O Architecture

• Five basic components are involved in the interface between the CPU and the I/O peripheral:
  • The CPU
  • The I/O peripheral device
  • Memory
  • One or more I/O modules
  • Buses

I/O Bus Architecture

• A bus forms the backbone for connection of the various components, memory, and I/O, to the CPU.
• A single system bus connects the CPU to memory and to all the various modules that control I/O devices.
• The system bus organization of a PC consists of a CPU bus, a PCI bus, and sometimes an ISA bus.

Bus Characteristics

• Buses are characterized by their:
  • Configuration
  • Width (bits carried simultaneously)
  • Speed
  • Usage

Categories of Buses

• Parallel bus: A bus in which there is an individual line for each bit of data, address, and control being used.
• Serial bus: A bus in which data is transferred sequentially, one bit at a time, using a single data line pair.

Bus Topology

• Point-to-point bus: A bus that carries signals from a specific source to a specific destination.
• Multipoint bus (multidrop bus/broadcast bus): A bus used to connect several points together.

I/O Channel Architecture

• The channel architecture is based on a separate I/O processor known as a channel subsystem.
• The I/O processor acts as a separate computer, just for I/O operations, thus freeing the CPU for other tasks.
• The channel subsystem executes its own set of instructions, known as channel control words, independent of the CPU.

Comparison of I/O Bus Architecture and I/O Channel Architecture

• I/O Bus Architecture: Connects the CPU, memory, and various peripheral devices via a shared communication pathway called a bus.
• I/O Channel Architecture: Uses dedicated channels to manage I/O operations between the CPU and peripheral devices.
• Each device has its dedicated channel for specific types of I/O operations.