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Operating System. Exploits hardware resources one or more processors main memory, disk and other I/O devices Provides a set of services to system users program development, program execution, access to I/O devices, controlled access to files and other resources etc. .

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Operating System

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Operating system

Operating System

  • Exploits hardware resources

    • one or more processors

    • main memory, disk and other I/O devices

  • Provides a set of services to system users

    • program development, program execution, access to I/O devices, controlled access to files and other resources etc.

Chapter 1 computer system overview

Operating Systems:Internals and Design Principles, 6/EWilliam Stallings

Chapter 1Computer System Overview

Given credits

Given Credits

  • Most of the lecture notes are based on the slides from the Textbook’s companion website: http://williamstallings.com/OS/OS6e.html

  • Some of the slides are from Dr. David Tarnoff in East Tennessee State University

    • I have modified them and added new slides

  • Computer components top level view

    Computer Components: Top-Level View

    Processor registers

    Processor Registers

    • User-visible registers

      • Enable programmer to minimize main memory references by optimizing register use

    • Control and status registers

      • Used by processor to control operation of the processor

      • Used by privileged OS routines to control the execution of programs

    Control and status registers

    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)

      • Condition codes

      • Interrupt enable/disable

      • Kernel/user mode

    Control and status registers1

    Control and Status Registers

    • Condition codes or flags

      • Bits set by processor hardware as a result of operations

      • Can be accessed by a program but not altered

      • Example

        • Condition code bit set following the execution of arithmetic instruction: positive, negative, zero, or overflow

    Instruction execution

    Instruction Execution

    • Two steps

      • Processor reads (fetches) instructions from memory

      • Processor executes each instruction

    Basic instruction cycle

    Basic Instruction Cycle

    Instruction fetch and execute

    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

    Instruction register

    Instruction Register

    • Fetched instruction loaded into instruction register

    • An instruction contains bits that specify the action the processor is to take

    • Categories of actions:

      • Processor-memory, processor-I/O, data processing, control

    Characteristics of a hypothetical machine

    Characteristics of a Hypothetical Machine

    Example of program execution

    Example of Program Execution



    • Interrupt the normal sequencing of the processor

    • Why do we need interrupts

    Classes of interrupts

    Classes of Interrupts



    • Most I/O devices are slower than the processor

      • Without interrupts, processor has to pause to wait for device

    Program flow of control

    Program Flow of Control

    Program flow of control1

    Program Flow of Control

    Interrupt stage

    Interrupt Stage

    • Processor checks for interrupts

    • If interrupt

      • Suspend execution of program

      • Execute interrupt-handler routine

    Transfer of control via interrupts

    Transfer of Control via Interrupts

    Instruction cycle with interrupts

    Instruction Cycle with Interrupts

    Simple interrupt processing

    Simple Interrupt Processing

    Changes in memory and registers for an interrupt

    Changes in Memory and Registers for an Interrupt

    Changes in memory and registers for an interrupt1

    Changes in Memory and Registers for an Interrupt

    Multiple interrupts

    Multiple Interrupts

    • What to do if another interrupt happens when we are handling one interrupt?

    Sequential interrupt processing

    Sequential Interrupt Processing

    Nested interrupt processing

    Nested Interrupt Processing



    • 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

    Input output techniques

    Input/Output Techniques

    • Programmed I/O

    • Interrupt driven – I/O

    • Direct Memory Access (DMA)

    • What are they & the ranking of their efficiencies

    Input output techniques1

    Input/Output Techniques

    • Programmed I/O – poll and response

    • Interrupt driven – I/O module calls for CPU when needed

    • Direct Memory Access (DMA) – module has direct access to specified block of memory

    I o module structure

    I/O Module Structure

    Programmed i o cpu has direct control over i o

    Programmed I/O – CPU has direct control over I/O

    • Processor requests operation with commands sent to I/O module

      • Control – telling a peripheral what to do

      • Test – used to check condition of I/O module or device

      • Read – obtains data from peripheral so processor can read it from the data bus

      • Write – sends data using the data bus to the peripheral

    • I/O module performs operation

    • When completed, I/O module updates its status registers

    • Sensing status – involves polling the I/O module's status registers

    Programmed i o continued

    Programmed I/O (continued)

    • I/O module does not inform CPU directly

    • CPU may wait or do something and come back later

    • Wastes CPU time because

      • CPU acts as a bridge for moving data between I/O module and main memory, i.e., every piece of data goes through CPU

      • CPU waits for I/O module to complete operation

    Interrupt driven i o

    Interrupt Driven I/O

    • Overcomes CPU waiting

    • Requires interrupt service routine

    • No repeated CPU checking of device

    • I/O module interrupts when ready

    • Still requires CPU to go between for moving data between I/O module and main memory

    Interrupt driven i o1

    Interrupt-Driven I/O

    • Consumes a lot of processor time because every word read or written passes through the processor

    Direct memory access dma

    Direct Memory Access (DMA)

    • Impetus behind DMA –Interrupt driven and programmed I/O require active CPU intervention (all data must pass through CPU)

    • Transfer rate is limited by processor's ability to service the device

    • CPU is tied up managing I/O transfer

    Dma continued

    DMA (continued)

    • Additional Module (hardware) on bus

    • DMA controller takes over bus from CPU for I/O

      • Waiting for a time when the processor doesn't need bus

      • Cycle stealing – seizing bus from CPU (more common)

    Dma operation

    DMA Operation

    • CPU tells DMA controller:

      • whether it will be a read or write operation

      • the address of device to transfer data from or to

      • the starting address of memory block for the data transfer

      • the amount of data to be transferred

    • DMA performs transfer while CPU does other processing

    • DMA sends interrupt when completes

    Cycle stealing

    Cycle Stealing

    • DMA controller takes over bus for a cycle

    • Transfer of one word of data

    • Not an interrupt to CPU operations

    • CPU suspended just before it accesses bus– i.e. before an operand or data fetch or a data write

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

    Direct memory access

    Direct Memory Access

    • Transfers a block of data directly to or from memory

    • An interrupt is sent when the transfer is complete

    • Most efficient

    The memory hierarchy

    The Memory Hierarchy

    Going down the hierarchy

    Going Down the Hierarchy

    • Decreasing cost per bit

    • Increasing capacity

    • Increasing access time

    • Decreasing frequency of access to the memory by the processor

    Cache memory

    Cache Memory

    • Processor speed faster than memory access speed

    • Exploit the principle of locality with a small fast memory

    Cache and main memory

    Cache and Main Memory

    Cache principles

    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

    Cache main memory structure

    Cache/Main-Memory Structure

    Cache read operation

    Cache Read Operation

    Cache principles1

    Cache Principles

    • Cache size

      • Small caches have significant impact on performance

    • Block size

      • The unit of data exchanged between cache and main memory

      • Larger block size more hits until probability of using newly fetched data becomes less than the probability of reusing data that have to be moved out of cache

    Cache principles2

    Cache Principles

    • Mapping function

      • Determines which cache location the block will occupy

      • Direct Mapped Cache, Fully Associative Cache, N-Way Set Associative Cache

    • Replacement algorithm

      • Chooses which block to replace

      • Least-recently-used (LRU) algorithm

    Cache principles3

    Cache Principles

    • 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

    Operating system

    • 2. [25 pts]This problem concerns the performance of the cache memory in web applications that play media files. Consider a video streaming workload that accesses working sets of size 256KB sequentially with the following byte-address stream:

    • 0, 2, 4, 6, 8, 10, …

    • Suppose the computer that processes the above stream has a 32 KB direct-mapped L1 cache. The cache block size is 32 bytes.

    • a) What would be the cache miss rate of the address stream above? Show all calculations.

    • Every 16th access would be a miss, hence, miss ratio is 1/16 = 6.25%

    Operating system

    • b) If the cache size were changed to 64 KB, what would be the change in the miss rate? Justify your answer.

    • There would be no change; the miss rate depends only on the block size.

    • c) If the cache organization were changed to two-way set associative, without changing the block size, would the cache miss rate change? Justify your answer.

    • There would be no change; the data is fetched in units of blocks from the memory, therefore the miss rate will be the same.

    Operating system

    • d) If the cache block size is changed to 16B would the miss rate change? If so, what would be the new value?

    • Now, every 8th access would be a miss, hence the rate is 1/8 = 12.5%

    Operating system

    • e) Prefetching is a technique that can be used effectively in streaming applications, such as the one described above. Describe how prefetching works and how it impacts the cache miss rate.

    • In prefetching, cache lines are brought in speculatively, in anticipation of future accesses. This works very well in streaming applications, where memory accesses are in sequential order, and the preloading for a new block can be overlapped with the consumption of the current block. This effectively reduces the miss rate to zero.

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