Chapter 7Input/Output

Continued

Interrupt Physical Model

• CPU

• Memory

• Device

Interrupt Physical Model

• CPU— General Purpose Registers

– PC Storage – R7– Stack Pointer R6

— Program Status Word (PSW) – Includes – State– Program Priority– Condition Codes (CC)

— User stack Pointer Storage – USP.saved— Supervisor Stack Pointer Storage – SSP.saved— Hardware to communicate over the BUS

• Memory— User program— Interrupt Service Routine— Operating System— Interrupt Vector Table

– Includes an entry that points to the Interrupt Service Routine (Interrupt vector #)

• Device— Status/Control Register(s) – Includes:

– Interrupt Enable bit– Interrupt bit (sometimes called ready or done)

— Priority Level for Interrupt Service Routine (In hardware or firmware)— Interrupt vector number (In hardware or firmware)— Hardware to communicate with CPU over the BUS

Interrupt Sequence

1) What does the programmer do?

2) What does the computer do?

Interrupt Sequence

1) Programmer Action: Enable Interrupts by setting “intr enable” bit in Device Status Reg

2) Enabling Mechanism for device: When device wants service, and its enable bit is set (The I/O device has the right to request service), and its priority is higher than the priority of the presently running program, and execution of an instruction is complete, then The processor initiates the interrupt

4) Process to service the interrupt: The Processor saves the “state” of the program (has to be able to return)

The Processor goes into Privileged Mode (PSR bit 15 cleared) Priority level is set (established by the interrupting device)The (USP), (R6) USP.saved register (UserStackPointer.saved)The (SSP.saved) R6 (SupervisorStackPointer)The (PC) and the (PSR) are PUSHED onto the Supervisor StackThe contents of the other registers are not saved. Why? The CC’s are cleared

5) The Processor Loads the PC from the Interrupt vector (vectors in 0100:01FF)

6) Interrupt Service Routine is executedEnds with an RTI

7) The stored user PSR (POP into PSR), PC (POP into PC), (R6)SSP.saved, (USP.savedR6), and the next instruction fetched

DMA Function

• DMA controller(s) takes over from CPU for I/O

• Additional Module(s) attached to bus to control DMA operation

Typical DMA Module Diagram

DMA Operation

• CPU tells DMA controller:-—Read/Write—Device address—Starting address of memory block for data—Amount of data to be transferred

• CPU carries on with other work

• DMA controller deals with transfer

• DMA controller sends interrupt when finished

DMA TransferCycle Stealing

• DMA controller takes over bus for a cycle

• Transfer of one word of data

• Not an interrupt—CPU does not switch context

• CPU suspended between bus cycles—i.e. before an operand or data fetch or a data

write

• Slows down CPU but not as much as CPU doing transfer

DMA and Interrupt Breakpoints During an Instruction Cycle

What could be wrong with this?

Aside

• What effect does caching memory have on DMA?

• What effect does use of DRAMs have on DMA ?

DMA Configurations (1)

• Single Bus, Detached DMA controller

• Each transfer uses bus twice—I/O to DMA then DMA to memory

• CPU is suspended twice

DMA Configurations (2)

• Single Bus, Integrated DMA controller• Controller may support >1 device• Each transfer uses bus once

—DMA to memory

• CPU is suspended once

DMA Configurations (3)

• Separate I/O Bus• Bus supports all DMA enabled devices• Each transfer uses bus once

—DMA to memory

• CPU is suspended once

I/O Channels

• I/O channels are processors dedicated to I/O e.g. 3D graphics cards

• CPU instructs I/O controller to do transfer

• I/O controller does entire transfer from one or many devices

• Makes transfers less visible to CPU

• Improves speed—Takes load off CPU

I/O Channel Architecture

Interfacing Options• Parallel - PCI - SCSI• Serial - RS 232• Local Networks - Ethernet

• Newer technologies- FireWire

- InfiniBand- USB

• Wireless - BlueTooth - WiFi• Automation - CAN

Intel 82C55A Programmable Peripheral Interface

Keyboard/Display Interfaces to 82C55A

Layering – Example: OSI Network Layers

•The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals.

Example: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.

•The Data Link Layer describes the logical organization of data bits transmitted on a particular medium.

Example: this layer defines the framing, addressing and check-summing of Ethernet packets.

•The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network.

Example: this layer defines the addressing and routing structure of the Internet.

•The Transport Layer describes the quality and nature of the data delivery.

Example: this layer defines if and how retransmissions will be used to ensure data delivery.

•The Session Layer describes the organization of data sequences larger than the packets handled by lower layers.

Example: this layer describes how request and reply packets are paired in a remote procedure call.

•The Presentation Layer describes the syntax of data being transferred.

Example: this layer describes how floating point numbers can be exchanged between hosts with different math formats.

•The Application Layer describes how real work actually gets done.

Example: this layer would implement file system operations.

International Standards Organization’s (ISO) Open Systems Interconnection (ISO) Model:

Simple Example OF 7 Layer OSI Model

Application Layer: Set of C Instructions, Set of Data{I0 I1 I2 …. IN Do D1 D2 … Dm}

Presentation Layer: ASCII Coding {ASC {I0 I1 I2 …. IN Do D1 D2 … Dm}}

Session Layer: What process at computer x is communicating with what process at computer y {X4 Y6 {ASC {I0 I1 I2 …. IN Do D1 D2 … Dm}}}

Transport Layer: Guaranteed Transmission, sequentially numbered packets of 4096 bytes {GT4 P34 {X4 Y6 {ASC {I0 I1 I2 …. IN Do D1 D2 … Dm}}} PCKSUM}

Network Layer: Path through Network {N23 N3 N53 {GT4 P34 {X4 Y6 {ASC {I0 I1 I2 …. IN Do D1 D2 … Dm}}} PCKSUM}}

Data Link Layer: Serial 256 bytes per frame {STRT T{N23 N3 N53 {GT4 P34 {X4 Y6 {ASC {I0 I1 I2 …. IN Do D1 D2 … Dm}}} PCKSUM}}CHKSM}

Physical Layer: 9600Baud, Coax cable - {Start {….}Parity Stop Stop}

Serial - RS 232

• UART (Universal Asynchronous Receiver & Transmitter)

• Serial interface on a chip• Historically very significant• After 30 years, still a standard

RS232 Character transmission

UART Block Diagram

Connector Wiring – Null Modem

UART Application

Ethernet• CSMA/CD (Carrier Sense Multiple Access/Collision

Detection) • A local area network access method in which contention

between two or more stations is resolved by collision detection.

• When two stations transmit at the same time, they both stop and signal a collision has occurred. Each then tries again after waiting a predetermined time period. To avoid another collision, the stations involved each choose a random time interval to schedule the retransmission of the collided frame.

• To make sure that the collision is recognized, Ethernet requires that a station must continue transmitting until the 50 microsecond period has ended. If the station has less than 64 bytes of data to send, then it must pad the data by adding zeros at the end.

Bob Metcalf’s Ethernet Concept - 1976

Network Reference model - Ethernet

Ethernet packet

Ethernet block diagram

IEEE 1394 FireWire (Competitor to USB)

• High performance serial bus• Fast• Low cost• Easy to implement• Also being used in digital cameras, VCRs

and TV

FireWire Configuration

• Daisy chain• Up to 63 devices on single port

—Really 64 of which one is the interface itself

• Up to 1022 buses can be connected with bridges

• Automatic configuration• No bus terminators• May be tree structure

Simple FireWire Configuration

FireWire 3 Layer Stack

• Physical—Transmission medium, electrical and signaling

characteristics

• Link—Transmission of data in packets

• Transaction—Request-response protocol

FireWire Protocol Stack

FireWire - Physical Layer

• Data rates from 25 to 400Mbps• Two forms of arbitration

—Based on tree structure—Root acts as arbiter—First come first served—Natural priority controls simultaneous requests

– i.e. who is nearest to root

—Fair arbitration—Urgent arbitration

FireWire - Link Layer

• Two transmission types—Asynchronous

– Variable amount of data and several bytes of transaction data transferred as a packet

– To explicit address– Acknowledgement returned

—Isochronous– Variable amount of data in sequence of fixed size

packets at regular intervals– Simplified addressing– No acknowledgement

FireWire Subactions

InfiniBand

• I/O specification aimed at high end servers—Merger of Future I/O (Cisco, HP, Compaq, IBM)

and Next Generation I/O (Intel)

• Version 1 released early 2001• Architecture and spec. for data flow

between processor and intelligent I/O devices

• Intended to replace PCI in servers• Increased capacity, expandability,

flexibility

InfiniBand Architecture

• Remote storage, networking and connection between servers

• Attach servers, remote storage, network devices to central fabric of switches and links

• Greater server density• Scalable data centre• Independent nodes added as required• I/O distance from server up to

—17m using copper—300m multimode fibre optic—10km single mode fibre

• Up to 30Gbps

InfiniBand Switch Fabric

InfiniBand Operation

• 16 logical channels (virtual lanes) per physical link

• One lane for management, rest for data• Data in stream of packets• Virtual lane dedicated temporarily to end

to end transfer• Switch maps traffic from incoming to

outgoing lane

InfiniBand Protocol Stack