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Chapter 1 Computer System Overview

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Operating System Exploits the hardware resources of one or more processors Provides a set of services to system users Manages secondary memory and I/O devices

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A Computer’s Basic Elements Processor Main Memory I/O Modules System Bus

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Processor Controls operation, performs data processing Two internal registers Memory address resister (MAR) Memory buffer register (MBR) I/O address register I/O buffer register

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Main Memory Volatile Data is typically lost when power is removed Referred to as real memory or primary memory Consists of a set of locations defined by sequentially numbers addresses Containing either data or instructions

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I/O Modules Moves data between the computer and the external environment such as: Storage (e.g. hard drive) Communications equipment Terminals Specified by an I/O Address Register (I/OAR)

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System Bus Communication among processors, main memory, and I/O modules

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Top-Level View

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Processor Registers Faster and smaller than main memory User-visible registers Enable programmer to minimize main memory references by optimizing register use Control and status registers Used by processor to control operating of the processor Used by privileged OS routines to control the execution of programs

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User-Visible Registers May be referenced by machine language Available to all programs – application programs and system programs Types of registers typically available are: data, address, condition code registers.

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Data and Address Registers Data Often general purpose But some restrictions may apply Address Index Register Segment pointer Stack pointer

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Control and Status Registers Program counter (PC) Contains the address of an instruction to be fetched Instruction register (IR) Contains the instruction most recently fetched Program status word (PSW) Contains status information

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Condition codes Usually part of the control register Also called flags Bits set by processor hardware as a result of operations Read only, intended for feedback regarding the results of instruction execution.

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Instruction Execution A program consists of a set of instructions stored in memory Two steps Processor reads (fetches) instructions from memory Processor executes each instruction

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Basic Instruction Cycle

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Instruction Fetch and Execute The processor fetches the instruction from memory Program counter (PC) holds address of the instruction to be fetched next PC is incremented after each fetch

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Instruction Register Fetched instruction loaded into instruction register Categories Processor-memory, processor-I/O, Data processing, Control

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Characteristics of a Hypothetical Machine

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Example of Program Execution

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Interrupts Interrupt the normal sequencing of the processor Provided to improve processor utilization Most I/O devices are slower than the processor Processor must pause to wait for device

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Common Classes of Interrupts

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Flow of Control without Interrupts

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Interrupts and the Instruction Cycle

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Transfer of Control via Interrupts

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Instruction Cycle with Interrupts

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Short I/O Wait

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Long I/O wait

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Simple Interrupt Processing

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Changes in Memory and Registers for an Interrupt

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Multiple Interrupts Suppose an interrupt occurs while another interrupt is being processed. E.g. printing data being received via communications line. Two approaches: Disable interrupts during interrupt processing Use a priority scheme.

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Sequential Interrupt Processing

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Nested Interrupt Processing

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Example of Nested Interrupts

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Multiprogramming Processor has more than one program to execute The sequence the programs are executed depend on their relative priority and whether they are waiting for I/O After an interrupt handler completes, control may not return to the program that was executing at the time of the interrupt

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Memory Hierarchy Major constraints in memory Amount Speed Expense Faster access time, greater cost per bit Greater capacity, smaller cost per bit Greater capacity, slower access speed

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The Memory Hierarchy Going down the hierarchy Decreasing cost per bit Increasing capacity Increasing access time Decreasing frequency of access to the memory by the processor

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Secondary Memory Auxiliary memory External Nonvolatile Used to store program and data files

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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Cache Memory Invisible to the OS Interacts with other memory management hardware Processor must access memory at least once per instruction cycle Processor speed faster than memory access speed Exploit the principle of locality with a small fast memory

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Principal of Locality More details later but in short … Data which is required soon is often close to the current data If data is referenced, then it’s neighbour might be needed soon.

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Cache and Main Memory

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Cache Principles Contains copy of a portion of main memory Processor first checks cache If not found, block of memory read into cache Because of locality of reference, likely future memory references are in that block

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Cache/Main-Memory Structure

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Cache Read Operation

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Cache Design Issues Main categories are: Cache size Block size Mapping function Replacement algorithm Write policy

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Size issues Cache size Small caches have significant impact on performance Block size The unit of data exchanged between cache and main memory Larger block size means more hits But too large reduces chance of reuse.

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Mapping function Determines which cache location the block will occupy Two constraints: When one block read in, another may need replaced Complexity of mapping function increases circuitry costs for searching.

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Replacement Algorithm Chooses which block to replace when a new block is to be loaded into the cache. Ideally replacing a block that isn’t likely to be needed again Impossible to guarantee Effective strategy is to replace a block that has been used less than others Least Recently Used (LRU)

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Write policy Dictates when the memory write operation takes place Can occur every time the block is updated Can occur when the block is replaced Minimize write operations Leave main memory in an obsolete state

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Roadmap Basic Elements Processor Registers Instruction Execution Interrupts The Memory Hierarchy Cache Memory I/O Communication Techniques

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I/O Techniques When the processor encounters an instruction relating to I/O, it executes that instruction by issuing a command to the appropriate I/O module. Three techniques are possible for I/O operations: Programmed I/O Interrupt-driven I/O Direct memory access (DMA)

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Programmed I/O The I/O module performs the requested action then sets the appropriate bits in the I/O status register but takes no further action to alert the processor. As there are no interrupts, the processor must determine when the instruction is complete

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Programmed I/O Instruction Set Control Used to activate and instruct device Status Tests status conditions Transfer Read/write between process register and device

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Programmed I/O Example Data read in a word at a time Processor remains in status-checking look while reading

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Interrupt-Driven I/O Processor issues an I/O command to a module and then goes on to do some other useful work. The I/O module will then interrupt the processor to request service when it is ready to exchange data with the processor.

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Interrupt- Driven I/O Eliminates needless waiting But everything passes through processor.

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Direct Memory Access Performed by a separate module on the system When needing to read/write processor issues a command to DMA module with: Whether a read or write is requested The address of the I/O device involved The starting location in memory to read/write The number of words to be read/written