17 replies to this topic

[Lewis]

Had no other place to put this.


Front side bus
[Categories: Computer buses, Computer terminology]

Front Side Bus (FSB) is the term used to describe the CPU data bus. This bus carries all information that passes from the CPU to other devices within the system such as Quick Facts about: RAM
A tool for driving or forcing something by impactRAM, Quick Facts about: PCI
Quick Summary not found for this subjectPCI expansion cards, Quick Facts about: hard disk
A rigid magnetic disk mounted permanently in a drive unithard disks, etc.

On older computers where the L2 Quick Facts about: cache
(computer science) RAM memory that is set aside as a specialized buffer storage that is continually updated; used to optimize data transfers between system elements with different characteristicscache was not integrated into the CPU, the Quick Facts about: back side bus
Quick Summary not found for this subjectback side bus connects to the L2 cache, and using this was faster than accessing the RAM via the front side bus. With modern CPUs having L2 cache on the processor die itself, this bus no longer exists, as the CPU's cache is simply checked for the appropriate data before a read request is issued to main memory.

Front side bus - History
The frontside bus has been a part of computer architecture since applications first started using more memory than a CPU (a very complex Quick Facts about: integrated circuit
A microelectronic computer circuit incorporated into a chip or semiconductor; a whole system rather than a single componentintegrated circuit) could reasonably hold.

The front side bus as it is traditionally known may be disappearing. Originally, this bus was a central connecting point for all system devices and the CPU. However, in recent years this has been breaking down with the use of more and more individual point-to-point buses.

Three recent bus technologies are GTL+, EV6, and Quick Facts about: HyperTransport
Quick Summary not found for this subjectHyperTransport. Each bus is unique in how it moves data within the system between the CPU and devices.

GTL+/AGTL+
Designed by Intel for the Pentium Pro, Pentium II, and Pentium III CPUs, as well as Xeons based on these cores (GTL+)
Redesigned for the Pentium 4 as well as Xeons on the same cores (AGTL+)
A "shared" bus, meaning that all CPUs compete over the same physical connection for the bus' bandwidth.

EV6 Bus
Designed by Quick Facts about: DEC
The last (12th) month of the yearDEC (now part of Quick Facts about: HP
A unit of power equal to 746 wattsHP) for use with their Alpha EV6 CPUs
Licensed by AMD for their Athlon and Athlon XP line of CPUs
A point-to-point protocol connecting each CPU to the Quick Facts about: northbridge
Quick Summary not found for this subjectnorthbridge, meaning that each CPU has a dedicated connection to the device.

HyperTransport
A point-to-point bus used by AMD for their Athlon64, AthlonFX, and Opteron lines of CPUs.
Not technically a front side bus. The HyperTransport bus is used to connect parts of the system together. This is listed because it is the only bus that allows communication between the processor and the rest of the system, which was traditionally one of the functions of the frontside bus. On AMD-64 CPUs, the frontside bus, which connected the CPU to the northbridge, has been removed in favor of an on-die memory controller.

Front side bus - Current usage
Most modern buses (both GTL+ and EV6) serve as a backbone between the CPU and a chipset. This chipset (usually a combination of northbridge and Quick Facts about: southbridge
Quick Summary not found for this subjectsouthbridge) is the connection point for all other buses in the system. The Quick Facts about: PCI
Quick Summary not found for this subjectPCI, Quick Facts about: AGP
Quick Summary not found for this subjectAGP, and Memory buses all connect to the chipset to allow for data to flow between the connected devices.

These secondary system buses usually run at speeds derived from the front side bus' speed. In general, a faster front side bus means higher processing speeds and a faster computer for a number of reasons which are outlined below.

Front side bus - Overclocking and related bus speeds

CPU
The frequency at which a processor (CPU) operates is determined by applying a clock multiplier to the front side bus (FSB) speed. For example, a processor running at 550 MHz might be using a 100 MHz FSB. This means there is an internal clock multiplier setting of 5.5; the CPU is set to run at 5.5 times frequency of the front side bus: 100 MHz x 5.5 = 550 MHz. By varying either the FSB or the multiplier, different CPU speeds can be achieved.

Most motherboards allow the user to manually set the clock multiplier and FSB settings by changing jumpers or BIOS settings. Many CPU manufacturers now "lock" a preset multiplier setting into the chip, which causes the manually-set multiplier settings to be ignored in favour of the locked-in multiplier. It is possible to unlock some locked CPUs. For instance, some Athlons can be unlocked by connecting electrical contacts across points on the CPU's surface. For all processors, the FSB speed can be increased to boost processing speed (called Quick Facts about: overclocking
Quick Summary not found for this subjectoverclocking).

Memory
Setting a FSB speed is related directly to the speed grade of memory that a system must use. The memory bus connects the northbridge and RAM just like the frontside bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Pushing the front-side bus to 170 MHz means pushing the memory to 170 MHz in most cases.

Similar to the PCI and AGP buses, however, the memory bus can sometimes also be run asynchronously from the front side bus. In Pentium 4 systems, it is possible to see memory ratios of "5:4" and the like. The bus will run 5/4 faster then the memory in this situation, meaning a 200 MHz bus can run with the memory at only 160 MHz. With bus speeds increasing rapidly, it is sometimes necessary to run the RAM at a lower frequency than the system bus in order to stay within the limitations of the DRAM modules on the memory stick. This incurs a performance penalty, but it allows slower RAM to be used with the faster bus speeds that some processors were designed for.

In a complex image, audio, video, scientific, and encryption applications where the data set is large, FSB speed becomes a major performance issue. A slow FSB will cause the CPU to spend significant amounts of time waiting for data to arrive from system memory.

Front side bus - Some sample FSB frequencies and bandwidths











Processor Class FSB Frequency Theoretical Bandwidth
Pentium II 66/100 MHz 533/800 MB/s
Pentium III 100/133 MHz 800/1066 MB/s
Pentium IV* 100/133/200 MHz 3200/4200/6400 MB/s
Athlon** 100/133 MHz 1600/2100 MB/s
Athlon XP** 133/166/200 MHz 2100/2700/3200 MB/s
Athlon 64*** 200 MHz 3200 MB/s
Athlon FX*** 200 MHz 6400 MB/s
Opteron*** 200 MHz 6400 MB/s
Notes:
* - Pentium IV processors use a frontside bus that transfers data four times per cycle
** - Athlon processors use a frontside bus that transfers data twice per cycle
*** - Athlon 64, FX, and Opteron processors have a memory controller on the CPU die, which replaces the traditional FSB

[Lewis]

Backside Bus

A microprocessor bus that connects the CPU to a Level 2 cache. Typically, a backside bus runs at a faster clock speed than the frontside bus that connects the CPU to main memory. For example, the Pentium Pro microprocessor actually consists of two chips -- one contains the CPU and the primary cache, and the second contains the secondary cache. A backside bus connects the two chips at the same clock rate as the CPU itself (at least 200 MHz). In contrast, the frontside bus runs at only a fraction of the CPU clock speed.

[Lewis]

Please mlurp don't make me laugh, I have ice packs down there.

[Lewis]

But I already had some understanding of FSB and BSB, but I see alot here that I did not know at all. Your should be between 100 or 133 how many MHz is your CPU ? Because I forgot ? Was it 800 or what ?

[JohnT]

Lewis you best believe on my new board I will want the best of bus speed front and back. My P4 is not too shabby but want to keep current with latest technology including 64 bit processor for next board.

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[Lewis]

That looks good John. When are you going to get another rig ?

[Lewis]

Looking good mlurp.

[JohnT]

Lewis I am building my next rig. As long as I figure for upgrades and expandability I can configure it just the way I want. Nothing like one of those Extreme Machines or and Alienware special that is primarily a gamers unit, but one that is more Media and graphic related. No more than 3.0ghz, 64 bit Intel with Raid, and PCI Express, with 3gb of DDR ram one 200gb primary hard drive with a secondary same size, 8-10 usb ports for my toys. Latest ATI vid card X800, Creative Sound blaster Audigy sound card 2 ZS. Pretty much a solid performer and like you I want extra cooling if the Coolermaster is not the case I select.

[Lewis]

Sounds good John, and yes extra cooling is key.

[JohnT]

I agree mlurp about time I did the same. I have it just too lazy and left Aida32 on
my machine.

http://www.lavalys.com/products/download.php?pid=1&lang=en

[Lewis]

SisSandra Diagnostic Software, rules.
http://www.sisoftware.net/

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