Post on 05-Apr-2018
7/31/2019 System Bus Functions and Features
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System Bus Functions and
Features
7/31/2019 System Bus Functions and Features
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Bus Hierarchy • The Processor Bus: This is the highest-level bus that the chipset uses to
send information to and from the processor.
• The Cache Bus: Higher-level architectures, such as those used by the
Pentium Pro and Pentium II, employ a dedicated bus for accessing the
system cache
• The Memory Bus: This is a second-level system bus that connects the
memory subsystem to the chipset and the processor
• The Local I/O Bus: This is a high-speed input/output bus used for
connecting performance-critical peripherals to the memory, chipset, and
processor. For example, video cards, disk storage devices, high-speed
networks interfaces generally use a bus of this sort. The two most
common local I/O buses are the VESA Local Bus (VLB) and the Peripheral
Component Interconnect Bus (PCI).
• The Standard I/O Bus: Connecting to the above three buses is the "good
old" standard I/O bus, used for slower peripherals (mice, modems, regular
sound cards, low-speed networking) and also for compatibility with older
devices. On almost all modern PCs this is the Industry Standard
Architecture (ISA) bus.
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System Chipset
• The system chipset is the conductor that
controls this orchestra of communication, and
makes sure that every device in the system is
talking properly to every other one.
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Intermission…
• What’s the difference between port and bus?
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Data and Address Buses
• Every bus is composed of two distinct parts:
– data bus
• these are the lines that actually carry the data beingtransferred
– address bus
• is the set of lines that carry information about where inmemory the data is to be transferred to or from
• Control Bus
– there are a number of control lines that control howthe bus functions, and allow users of the bus to signalwhen data is available
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Bus Width
• We know that bus is a channel over whichinformation flows
• The wider the bus, the more information can
flow over the channel• The address bus width can be specified
independently of the data bus width. The
width of the address bus dictates how manydifferent memory locations that bus cantransfer information to or from.
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Bus Speed
• The speed of the bus reflects how many bits of
information can be sent across each wire each
second.
• Going back to highway analogy: This could be
likened to how fast a car runs
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Bus Bandwidth
• Bandwidth/throughput, refers to the total
amount of data that can theoretically be
transferred on the bus in a given unit of time.
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Bus Interfacing
• On a system that has multiple buses, circuitry
must be provided by the chipset to connect
the buses and allow devices on one to talk to
devices on the other
• This device/circuitry is called a "bridge”
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Bus Mastering
• On the higher-bandwidth buses, a great deal of information is flowing through the channel everysecond
• Processor is required to control the transfer of this information – Processor can be thought of as a "middleman",
– it is far more efficient to "cut out" the middleman andperform the transfer directly.
– This is done by having capable devices take control of the bus and do the work themselves; devices that cando this are called bus masters
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The Local Bus Concept• The switch from character-based applications to graphics-based
caused the increase in the amount of information that must bemoved among the processor, memory, video and hard disks
• The transformation of the software world from text to graphics
also meant much larger programs and more storage requirements.
•
The solution was to create a new, faster bus, that would augmentthe ISA bus and be used especially for high-bandwidth devices
such as video cards.
• This new bus would be put on (or near) the processor's much
faster memory bus, to let it run at or near the external speed of
the processor, and to allow data to flow between these devices
and the processor without having to go through the much slower
ISA bus.
• By placing these devices "local" to the processor, the local bus was
born.
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System Cache
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System Cache
• The cache is a buffer between the very fastprocessor and the relatively slow memory thatserves it. – The memory is not really slow , only that the processor
is faster – The presence of the cache allows the processor to do
its work while waiting for memory far less often thanit otherwise would.
•
There are several different "layers" of cache in amodern PC, each acting as a buffer for recently-used information to improve performance.
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Role of Cache in the PC
• One of the key goals in modern system design
is to ensure that to whatever extent possible,
the processor is not slowed down by the
storage devices it works with
– Slowdowns mean wasted processor cycles, where
the CPU can't do anything or is IDLE because it is
sitting and waiting for information it needs.
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Role of Cache in the PC
• Using this small cache to hold the information
most recently used by the processor
• In general, a processor is much more likely to
need again information it has recently used,
compared to a random piece of information in
memory
– This is the principle behind caching.
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Layers of Cache
• Each layer is closer to the processor and faster
than the layer below it. Each layer also caches
the layers below it, due to its increased speed
relative to the lower levels:
Level Devices Cached
Level 1 Cache Level 2 Cache, System RAM, Hard
Disk / CD-ROMLevel 2 Cache System RAM, Hard Disk / CD-ROM
System RAM Hard Disk / CD-ROM
Hard Disk / CD-ROM --
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What happens:
1. The processor requests a piece of information
2. The first place it looks is in the level 1 cache
3. If it finds it there (called a hit on the cache) it uses itwith no performance delay.
4. If not, (called a miss on the cache) and the level 2cache is searched
5. If it finds it there (level 2 "hit"), it is able to carry onwith relatively little delay.
6. Otherwise, it must issue a request to read it from thesystem RAM. The system RAM may in turn either havethe information available or have to get it from thestill slower hard disk or CD-ROM.
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Level 1 (Primary) Cache
• Level 1 or primary cache is the fastest memory onthe PC.
• It is in fact, built directly into the processor itself.
• This cache is very small, generally from 8 KB to 64KB, but it is extremely fast; it runs at the samespeed as the processor.
• If the processor requests information and canfind it in the level 1 cache,
– the best case because the information is thereimmediately and the system does not have to wait
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Level 2 (Secondary) Cache
• The level 2 cache is a secondary cache to the
level 1 cache
• Is larger and slightly slower
• It is used to catch recent accesses that are not
caught by the level 1 cache
• Is usually found either on the motherboard
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Disk Cache
• A disk cache is a portion of system memory
used to cache reads and writes to the hard
disk.
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Peripheral Cache
• Other devices can be cached using the system
RAM as well.
• CD-ROMs are the most common device
cached other than hard disks