System Bus Functions and Features

7/31/2019 System Bus Functions and Features

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System Bus Functions and

Features



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.



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.



Intermission…

• What’s the difference between port and bus?



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



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.



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



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.



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”



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



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.



System Cache



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.



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.



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.



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 --



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.



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



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



Disk Cache

• A disk cache is a portion of system memory

used to cache reads and writes to the hard

disk.



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

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