Computer function-and-interconnection 3

Top Level View of Computer Function and Interconnection

Program Concept Hardwired systems are inflexible General purpose hardware can do different tasks, given correct control signals Instead of re-wiring, supply a new set of control signals

What is a program? A sequence of steps For each step, an arithmetic or logical operation is done For each operation, a different set of control signals is needed

Function of Control Unit For each operation a unique code is provided e.g. ADD, MOVE A hardware segment accepts the code and issues the control signals We have a computer!

Components The Control Unit and the Arithmetic and Logic Unit constitute the Central Processing Unit Data and instructions need to get into the system and results out Input/output Temporary storage of code and results is needed Main memory

Computer Components: Top Level View

Instruction Cycle (Processing for a single instruction) Two steps: Fetch Execute Halt if machine is turned off, unrecoverable error

Fetch Cycle Program Counter (PC) holds address of next instruction to fetch Processor fetches instruction from memory location pointed to by PC Increment PC Unless told otherwise Instruction loaded into Instruction Register (IR) Processor interprets instruction and performs required actions

Execute Cycle Processor-memory Data transfer between CPU and main memory Processor I/O Data transfer between CPU and I/O module Data processing Some arithmetic or logical operation on data Control Alteration of sequence of operations e.g. jump Combination of above

Instruction Cycle State Diagram

Interrupts Mechanism by which other modules (e.g. I/O) may interrupt normal sequence of processing Program e.g. overflow, division by zero Timer Generated by internal processor timer Used in pre-emptive multi-tasking I/O from I/O controller Task completion, variety of errors Hardware failure e.g. Power failure, memory parity error

Interrupts A way to improve the processing efficiency Slow peripherals can not comprehend speedy processor Processor remains in stall state until device catches up Different tasks interleaved with WRITE Without interrupts, program would wait for IO to complete May Periodically pole the device. With interrupts, user can execute other instructions while IO is performed OS and Processor manage the suspension and resumption of program

Interrupt Cycle Added to instruction cycle Processor checks for interrupt Indicated by an interrupt signal If no interrupt, fetch next instruction If interrupt pending: Suspend execution of current program Save context (address of next instruction to be executed) Set PC to start address of interrupt handler routine Process interrupt Restore context and continue interrupted program

In the Interrupt handler routine Fetch interrupt handler instructions from memory Interrupt handler routines are part of OS Extra instructions are executed but still save processing power

Transfer of Control via Interrupts

Instruction Cycle with Interrupts Hanggan d2 lng report ko mwaah

Instruction Cycle (with Interrupts) - State Diagram

Multiple Interrupts-I We may have more than one interrupts A program receiving data from communication line and printing data to a printing device Option to handle multiple interrupts

Multiple Interrupts-II Disable interrupts Processor will ignore further interrupts whilst processing one interrupt Interrupts remain pending and are checked after first interrupt has been processed Interrupts handled in sequence as they occur (No Priority task) Data arriving from communication line should be absorbed immediately Define priorities Low priority interrupts can be interrupted by higher priority interrupts When higher priority interrupt has been processed, processor returns to previous interrupt E.g. Printer 2, disk 4, communication line 5

Multiple Interrupts - Sequential

Multiple Interrupts – Nested

Time Sequence of Multiple Interrupts

Connecting All the units must be connected Different type of connection for different type of unit Memory Input/Output CPU Collection of paths connecting various structures are called interconnection structures.

Memory Connection Receives and sends data Receives addresses (of locations) Receives control signals Read Write Timing

Input/Output Connection(1) Similar to memory from computer’s viewpoint Output Receive data from computer Send data to peripheral Input Receive data from peripheral Send data to computer

Input/Output Connection(2) Receive control signals from computer Send control signals to peripherals e.g. spin disk Receive addresses from computer e.g. port number to identify peripheral Send interrupt signals (control)

CPU Connection Reads instruction and data Writes out data (after processing) Sends control signals to other units Receives (& acts on) interrupts

Buses There are a number of possible interconnection systems Single and multiple BUS structures are most common e.g. Control/Address/Data bus (PC) e.g. Unibus (DEC-PDP)

What is a Bus? A communication pathway connecting two or more devices Often grouped A number of channels in one bus e.g. 32 bit data bus is 32 separate single bit channels Power lines may not be shown

Data Bus Carries data Remember that there is no difference between “data” and “instruction” at this level Width is a key determinant of performance 8, 16, 32, 64 bit

Address bus Identify the source or destination of data e.g. CPU needs to read an instruction (data) from a given location in memory Bus width determines maximum memory capacity of system e.g. 8080 has 16 bit address bus giving 64k address space

Control Bus Control and timing information Memory read/write signal IO Read/Write Transfer Ack. Interrupt request (indicates interrupt is pending) Interrupt Ack. (pending intrrupt acknowledged) Sending and requesting data requires: Request use of bus Transfer of data via bus

Arrangement What do buses look like? Parallel lines on circuit boards Ribbon cables Strip connectors on mother boards On Chip and board wires

Physical Realization of Bus Architecture

Single Bus Problems Lots of devices on one bus leads to: Propagation delays Long data paths mean that co-ordination of bus use can adversely affect performance Bus may become a bottleneck as aggregate aggregate data transfer approaches bus capacity Increase the data rate (32-bit,64-bit) Still growing application demands can t be met Most systems use multiple buses to overcome these problems

Devices with high speed demand are closer to processor But independent of processor ( Processor’s Architectural changes have no affect on high speed bus)

Bus Types Dedicated Separate data & address lines Multiplexed (time multiplexing) Shared lines Address valid or data valid control line Advantage - fewer lines, cost benefit Disadvantages More complex control Ultimate performance Physical Dedication (IO bus connects IO modules only)

Bus Arbitration More than one module controlling the bus e.g. CPU and DMA controller Only one module may control bus at one time Arbitration may be centralised or distributed

Centralised or Distributed Arbitration Centralised Single hardware device controlling bus access Bus Controller Arbiter Distributed Each module may claim the bus Control logic on all modules

Timing Co-ordination of events on bus Synchronous Events determined by clock signals Control Bus includes clock line A single 1-0 is a bus cycle All devices can read clock line Usually sync on leading edge Usually a single cycle for an event

PCI Bus Peripheral Component Interconnection Intel released to public domain 32 or 64 bit 50 lines

PCI Bus Lines (required) Systems lines Including clock and reset Address & Data 32 time mux lines for address/data Interrupt & validate lines Interface Control Arbitration Not shared Direct connection to PCI bus arbiter Error lines

PCI Bus Lines (Optional) Interrupt lines Not shared Cache support 64-bit Bus Extension Additional 32 lines Time multiplexed 2 lines to enable devices to agree to use 64-bit transfer JTAG/Boundary Scan For testing procedures

PCI Commands Transaction between initiator (master) and target Master claims bus Determine type of transaction e.g. I/O read/write Address phase One or more data phases

Foreground Reading Stallings, chapter 3 (all of it) www.pcguide.com/ref/mbsys/buses/ In fact, read the whole site! www.pcguide.com/

Computer function-and-interconnection 3

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Computer function-and-interconnection 3