CSE325 Principles of Operating Systems

Computer System Overview

David Duggan

dduggan@sandia.gov

January 19, 2012

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What is a Computer System?

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Computer System Functional Areas

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Major Computer Components

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Processor

n  Internal registers n  Memory address register (MAR)

n  Specifies the address for the next read or write

n  Memory buffer register (MBR) n  Contains data written into memory or receives data

read from memory

n  I/O address register

n  I/O buffer register

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User-Visible Registers

n  May be read by user processes

n  Available to all programs - application programs and system programs

n  Types of registers n  Data n  Address

n  Index n  Segment pointer n  Stack pointer

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Control and Status Registers

n  Program Counter (PC) n  Contains the address of an instruction to be fetched

n  Instruction Register (IR)

n  Contains the instruction most recently fetched

n  Program Status Word (PSW)

n  Condition codes

n  Interrupt enable/disable

n  Supervisor/user mode

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Simple Instruction Cycle

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Interrupts

n  Suspends the normal sequence of execution

n  Used to improve processor utilization

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Interrupt Cycle

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Interrupt Timeline

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Simple Interrupt Processing

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Multiple Interrupts

n  Disable interrupts while an interrupt is being processed

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Multiple Interrupts (Cont.)

n  Define priorities for interrupts

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Data Transfer on the Bus

CPU!Memory!

memory bus!

I/O bus!

disk! Net interface!

cache!

n  cache-memory: cache misses, write-through/write-back

n  memory-disk: swapping, paging, file accesses

n  memory-network Interface : packet send/receive

n  I/O devices to the processor: interrupts

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Two I/O Methods

Synchronous! Asynchronous!

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I/O Operation: Synchronous vs. Asynchronous

n  After I/O starts, control returns to user program only upon I/O completion n  Wait instruction idles the CPU until operation completes n  Wait loop (contention for memory access?) n  At most one I/O request is outstanding at a time, no

simultaneous I/O processing

n  After I/O starts, control returns to user program without waiting for I/O completion n  System call – request to the operating system to allow user

to wait for I/O completion n  Device-status table contains entry for each I/O device

indicating its type, address, and state n  Operating system indexes into I/O device table to determine

device status and to modify table entry to include interrupt

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Programmed I/O

n  I/O module performs the action, not the processor

n  Sets appropriate bits in the I/O status register

n  No interrupts occur

n  Processor checks status until operation is complete

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Interrupt-Driven I/O

n  Processor is interrupted when I/O module ready to exchange data

n  Processor is free to do other work

n  No needless waiting

n  Consumes a lot of processor time because every byte read or written passes through the processor

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Direct Memory Access (DMA)

n  Used for high-speed I/O devices able to transmit information at close to memory speeds.

n  Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.

n  Only one interrupt is generated per block, rather than the one interrupt per byte.

n  Programming a DMA transfer n  address of the I/O buffer n  starting location in memory n  number of bytes n  direction of transfer (read/write from/to memory)

n  Bus arbitration between cache-memory and DMA transfers

n  Memory cache must be consistent with DMA

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Storage-Device Hierarchy

n  Decreasing cost per bit

n  Increasing capacity

n  Increasing access time

n  Decreasing frequency of access of the memory by the processor n  Locality of reference

n  Increasing size of the transfer unit

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Storage Hierarchy

n  Storage systems organized in a hierarchy. n  Speed

n  Cost

n  Volatility

n  Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

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Performance of Various Levels of Storage

n  Movement between levels of storage hierarchy can be explicit or implicit

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Cache-Memory Transfers

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Cache Memory

n  The mismatch between processor and memory speed

n  closer to the processor than the main memory; smaller and faster than the main memory

n  contains the value of main memory locations that were recently accessed (temporal locality)

n  transfer between caches and main memory is performed in units called cache blocks/lines

n  also contains the value of memory locations that are close to locations which were recently accessed (spatial locality)

n  Cache performance: miss ratio, miss penalty, average access time

n  invisible to the OS, operated by the hardware/firmware CSE325 - Computer System

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Cache/Main Memory System

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Cache Read Operation

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Cache Design

n  Mapping function n  Determines which cache location the block

will occupy

n  Direct-mapped vs. fully-associative vs. set-associative

n  Conflict misses

n  Replacement algorithm n  Determines which block to replace

n  Least-Recently-Used (LRU) algorithm

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Cache Design (Cont.)

n  Write policy n  When the memory write operation takes place

n  Can occur every time block is updated: write through

n  Can occur only when block is replaced: write back n  Minimizes memory write operations

n  Leaves main memory in an obsolete state

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Disk Cache/Buffer Cache

n  A portion of main memory used as a buffer to temporarily to hold data for the disk

n  Disk writes are clustered

n  Some data written out may be referenced again. The data are retrieved rapidly from the software cache instead of slowly from disk

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Multiprocessors

CPU!Memory!

memory bus!

I/O bus!

disk! Net interface!

cache!

n  more than one processor on the same bus n  memory is shared among processors-- cache coherency n  goal: performance speedup n  single-image operating systems n  Multi-core processors (chip-level multiprocessors/CMP)

CPU!cache!

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Clusters of Computers

n  network of computers: “share-nothing”

n  communication through message-passing

n  fast interconnects: memory-to-memory communication

n  goals: performance and availability

n  each system runs its own operating system

CPU!Memory!

memory bus!

I/O bus!

disk! Net interface!

cache!CPU!

Memory!

memory bus!

I/O bus!

disk!Net interface!

cache!

network!

Reading Assignment 2

n  Chapter 2 – By next class

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