Computer System Overview Chapter 1. Basic computer structure CPU Memory memory bus I/O bus diskNet...
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Transcript of Computer System Overview Chapter 1. Basic computer structure CPU Memory memory bus I/O bus diskNet...
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Computer System Overview
Chapter 1
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Basic computer structure
CPU Memory
memory bus
I/O bus
disk Net interface
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Computer System • Processor: performs data processing• Main memory: stores both data and programs, typically volatile• Disks: secondary memory devices which provide persistent storage• Network interfaces: inter-machine communication• Buses: intra-machine communication
memory bus (processor-memory) I/O bus (disks, network interfaces, other I/O devices,
memory-bus)
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Top-Level Components
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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 operating of
the processor– Used by operating-system 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
– Data – Address
• Index• Segment pointer• Stack pointer
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User-Visible Registers
• Address Registers– Index
• involves adding an index to a base value to get an address
– Segment pointer• when memory is divided into segments,
memory is referenced by a segment and an offset
– Stack pointer• points to top of stack
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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)– condition codes
– Interrupt enable/disable
– Supervisor/user mode
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Control and Status Registers
• Condition Codes or Flags– Bits set by the processor hardware as a
result of operations– Can be accessed by a program but not
altered– Examples
• positive result• negative result• zero• Overflow
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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
• Program counter is incremented after each fetch
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Interrupts
• An interruption of the normal sequence of execution
• Improves processing efficiency• Allows the processor to execute other
instructions while an I/O operation is in progress
• A suspension of a process caused by an event external to that process and performed in such a way that the process can be resumed
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Classes of Interrupts
• Program– arithmetic overflow– division by zero– execute illegal instruction– reference outside user’s memory space
• Timer
• I/O
• Hardware failure
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Interrupt Handler
• A program that determines nature of the interrupt and performs whatever actions are needed
• Control is transferred to this program
• Generally part of the operating system
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Interrupt Cycle
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Interrupt Cycle
• Processor checks for interrupts
• If no interrupts fetch the next instruction for the current program
• If an interrupt is pending, suspend execution of the current program, and execute the interrupt handler
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Multiple Interrupts
• Disable interrupts while an interrupt is being processed– Processor ignores
any new interrupt request signals
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Multiple InterruptsSequential Order
• Disable interrupts so processor can complete task
• Interrupts remain pending until the processor enables interrupts
• After interrupt handler routine completes, the processor checks for additional interrupts
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Multiple InterruptsPriorities
• Higher priority interrupts cause lower-priority interrupts to wait
• Causes a lower-priority interrupt handler to be interrupted
• Example when input arrives from communication line, it needs to be absorbed quickly to make room for more input
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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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Cache Memory
• Contains a portion of main memory
• Processor first checks cache
• If not found in cache, the block of memory containing the needed information is moved to the cache
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Cache Memory• motivated by the mismatch between processor and memory speed
• closer to the processor than the main memory
• smaller and faster than the main memory
• act as “attraction memory”: contains the value of main memory locations which were recently accessed (temporal locality)
• transfer between caches and main memory is performed in units called cache blocks/lines
• caches contain also the value of memory locations which are close to locations which were recently accessed (spatial locality)
• invisible to the OS
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Cache Memory
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Cache/Main Memory System
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Cache Design
• Cache size– small caches have a significant impact on
performance
• Block size– the unit of data exchanged between cache and
main memory– hit means the information was found in the cache– larger block size more hits until probability of
using newly fetched data becomes less than the probability of reusing data that has been moved out of cache
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Cache Design
• Mapping function– determines which cache location the block
will occupy
• Replacement algorithm– determines which block to replace– Least-Recently-Used (LRU) algorithm
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Cache Design
• Write policy– When the memory write operation takes
place– Can occur every time block is updated– Can occur only when block is replaced
• Minimizes memory operations
• Leaves memory in an obsolete state
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Memory Hierarchy
cpu
cache
main memory
word transfer
block transfer
disks
page transfer
• decrease cost per bit• decrease frequency of access• increase capacity• increase access time• increase size of transfer unit
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Data transfer on the busCPU
Memory
memory bus
I/O bus
disk Net interface
cache
• cache-memory: cache misses, write-through/write-back• memory-disk: swapping, paging, file accesses• memory-Network Interface : packet send/receive• I/O devices to the processor: interrupts
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Programmed I/O
• I/O module performs the action, not the processor
• Sets appropriate bits in the I/O status register
• No interrupts occur
• Processor checks status until operation is complete
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Interrupt-Driven I/O
• Processor is interrupted when I/O module ready to exchange data
• Processor is free to do other work
• No needless waiting• Consumes a lot of processor
time because every word read or written passes through the processor
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Direct Memory Access
• Transfers a block of data directly to or from memory
• An interrupt is sent when the task is complete
• The processor is only involved at the beginning and end of the transfer
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Direct Memory Access (DMA)
• I/O exchanges occur directly with memory
• Processor grants I/O module authority to read from or write to memory
• Relieves the processor responsibility for the exchange
• Processor is free to do other things
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Direct Memory Access (DMA)
address of the I/O device starting location in memory number of bytes direction of transfer (read/write from/to
memory)
bus arbitration between cache-memory and DMA transfers
memory cache must be consistent with DMA
Programming a DMA transfer
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MultiprocessorsCPU
Memory
memory bus
I/O bus
disk Net interface
cache
• simple scheme: more than one processor on the same bus
• memory is shared among processors-- cache consistency
• bus contention increases -- does not scale
• alternative (non-bus) system interconnect -- expensive
• single-image operating systems
CPU
cache
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Network of Computers
• network of computers: “share-nothing” -- cheap• communication through message-passing: difficult to program• challenge: build efficient shared memory abstraction in software• each system runs its own operating system
CPUMemory
memory bus
I/O bus
disk Net interface
cache
CPUMemory
memory bus
I/O bus
diskNet interface
cache
network