Computer Architecture I/O Module Quiz

Input/Output (I/O)

• I/O involves peripherals with various operation methods
• There's a mismatch between peripheral data transfer rates and memory/processor rates
• Peripherals often use different data formats and word lengths than the computer itself

I/O Module

• Functions as an intermediary between the processor and memory and peripherals
• Interfaces to the processor and memory via the system bus or central switch
• Interfaces to one or more peripheral devices

External Devices

• Provide a means of exchanging data between the external environment and the computer
• An external device attaches to the computer via a link to an I/O module
• The link exchanges control, status, and data between the I/O module and the external device
• Often referred to as a peripheral device

External Device Categories

• Human-readable: Suitable for communication with the user (e.g., screen, printer, keyboard)
• Machine-readable: Suitable for communication with equipment (e.g., magnetic disk, tape system, sensors, actuators)
• Communication: Suitable for communication with remote devices (e.g., modem, network interface card (NIC))

External Device Block Diagram

• Control signals: Determine the device's function (e.g., read/write, report status, etc.)
• Control logic: Processes and interprets control signals to the device
• Buffer: Temporarily holds data during transfer between the I/O module and the environment
• Transducer: Converts data from one form (electrical) to another form (e.g., mechanical, thermal)

I/O Module

• Control & Timing: Coordinates flow between internal and external resources. Handles device communication (commands, status, data).
• Data Buffering: Handles high CPU/memory transfer rates, often vastly slower peripheral rates.
• Error Detection: Monitors peripheral for mechanical/electrical malfunctions

CPU Communication

• Command decoding: I/O module accepts commands from the processor
• Data transfers: Data exchanged between the processor and I/O module via the data bus.
• Status reporting: Key for peripherals that are slow, status information is critical for the I/O module.
• Address recognition: I/O module identifies the device with a unique address for control.

I/O Steps

• CPU checks I/O module's device status
• I/O module returns its status
• CPU requests data
• I/O module gets data from device
• I/O module sends data to CPU

I/O Module Decisions

• Hide or reveal device properties to CPU.
• Support multiple or single devices
• Control device functions or leave for CPU.
• Operating System (O/S) decisions on how devices are treated (e.g. Unix treats peripherals as files).

I/O Operation Techniques

• Programmed I/O: CPU directly controls I/O, waits for each operation to complete

• Interrupt-driven I/O: CPU issues a command, then moves on to other tasks; CPU gets interrupted when device is ready

• Direct Memory Access (DMA): DMA controller handles transfer between memory and peripheral without continuous CPU intervention

I/O Commands

• Issued by the CPU to the I/O module.
• Identifies the module and any specific device as required via addresses
• Includes I/O commands for control (e.g., disc spin-up), test, read, and write processes

I/O Instructions

• Executed by the processor; tightly coupled with the I/O commands.
• The form of the instructions depends on how the external devices are addressed

Addressing I/O Devices

• Programmed I/O data transfers mimic memory access
• Each device gets a unique identifier (address)
• CPU commands contain the identifier for the device

Addressing Modes

• Memory-mapped I/O: Devices and memory share the address space
• Isolated I/O: Separate address spaces, specialized commands

Interrupt-Driven I/O

• Overcomes CPU waiting
• I/O module interrupts CPU when ready.
• No need to repeatedly monitor the device

DMA (Direct Memory Access)

• Interrupt driven and programmed I/O require active CPU intervention
• DMA is the answer to faster transfer speeds, taking over system control from the processor for transfers
• DMA controller uses the bus only when the processor does not, possibly causing temporary suspension (cycle stealing)

DMA Functions

• Additional module on the system bus
• DMA controller takes over I/O functions from the CPU

DMA Operations

• CPU tells the DMA controller on the type of read/write, the device address and the memory block address where to write or read data.
• CPU continues with other tasks whilst DMA manages data transfers between peripheral and memory.
• DMA controller sends interrupt when finished.

DMA Cycle Stealing

• DMA takes control of the bus for a cycle
• Transfer of data without an interrupt
• CPU suspended temporarily

DMA Configurations

• Single-bus: DMA controller detached, using the bus twice for each transfer, CPU is suspended twice
• Single-bus, Integrated: DMA controller integrated with I/O using the bus only once for each transfer, CPU is suspended once.
• Separate I/O bus: Separate I/O bus, the bus is used only once for each transfer, CPU is suspended only once

I/O Channels

• As systems evolve, I/O devices become more complex and I/O channels allow for an improved transfer speed and reduced burden on the CPU.
• This can take over the entire I/O from the CPU.
• I/O controllers respond to I/O instructions and execute them.
• Selector channel: Controls multiple high-speed devices at the same time and on a single transfer.
• Multiplexor channel: Designed for handling multiple low-speed devices; can serve multiple devices rapidly

Interfacing

• Parallel interfaces: Multiple data lines connect the I/O module to the peripheral; bits are transferred simultaneously. (e.g., tape and disc drives)
• Serial interfaces: Only one data line used to transfer data sequentially. (e.g., printers)

Interfacing: I/O Modules and External Devices

• Point-to-point: Dedicated link between the I/O module and the external device (e.g., keyboard, printer).
• Multipoint: Used to support mass storage devices (such as disk and tape drives), and multimedia devices (such as CD-ROMs). Includes examples like FireWire and Infiniband.

External Interconnection Standards

• Various standards exist for connecting external devices, including USB, FireWire, SCSI, Thunderbolt, InfiniBand, PCI Express, SATA, Ethernet, and Wi-Fi

USB

• Widely used for connecting peripherals.
• Multiple generations exist with increasing data rates.

IEEE 1394 FireWire

• High-performance, low-cost serial bus.
• Common in digital cameras, VCRs, and TVs
• Daisy chain or tree-structured configuration, up to 63 devices.
• Layers (physical, link, transaction)

FireWire Configuration

• Daisy-chain up to 63 devices on one port
• Up to 1022 buses via bridges (tree configuration).
• Automatic configuration.

FireWire 3 Layer Stack

• Physical layer: Transmission medium
• Link layer: Packets
• Transaction layer: Requests and responses

FireWire Protocol Stack

• Layers define how data is organized for transmission and handling across the FireWire system

FireWire - Physical Layer

• Data rates range from 25 to 400 Mbps
• Uses tree-based arbitration, with root acting as arbiter
• Arbitration methods include fair, urgent, and first-come, first-served

FireWire - Link Layer

• Two transmission types: Asynchronous and isochronous
• Asynchronous: Variable-length data packets
• Isochronous: Fixed-size data packets at regular intervals

InfiniBand

• High-end server I/O specification
• High capacity, expandability, flexibility
• Replaces PCI in servers.
• Supports up to 30Gbps

InfiniBand Architecture

• Remote storage, network connections between servers.
• Higher server density
• Scalable data centres
• Long distances using fiber optics (up to 10 km)

InfiniBand Operations

• 16 logical channels (virtual lanes) per physical link.
• One channel for management, others for data
• Data transmitted as packets, virtual lane for temporary transfers to end-points.
• Switches map incoming to outgoing lanes.

Summary

• I/O architecture enables interaction with the outside world.
• Three main I/O techniques: programmed, interrupt-driven, and direct memory access (DMA). Each technique is suited to specific needs and workloads relating to the CPU.

Test your knowledge on the role and functioning of I/O modules in computer architecture. This quiz covers their interaction with the CPU, types of commands, and the impact on system design. Dive into the intricacies of programmed I/O and peripheral communication.