Bandwidth: Bus Speed and Cache
Let's talk about the other part of the performance equation: bandwidth. The processor transfers data back and forth to other components such as the memory (for storing temporary data) and the hard drive (for reading / saving permanent data - like your files). These components don't work at the same speed as the processor. For example, a PC3200 DDR-SDRAM memory module works at 200 MHz. An Athlon 64 motherboard also works at 200 MHz (remember a hard drive is connected to the motherboard which handles all data transfers to your processor). So, the processor is actually working faster than the other components (9 times 200 MHz for an Athlon 64 running at 1,8 GHz). This speed (200 MHz) is what we call the bus speed or Front Side Bus (FSB) speed to some.One of the reasons AMD and Intel uses different sockets is related to how the bus (or connection) for each processor works. AMD uses a double data rate bus (DDR bus for short, don't confuse this with DDR-SDRAM) which means that a bus working at 200 MHz effectively transfers data at double the rate (400 MHz). So, when describing an AMD bus, people use words like '200 MHz FSB (effective 400 MHz)' or something similar. When speaking of bus speed on AMD processors, a processor using 100 MHz bus is effectively transferring data at 200 MHz and so on (266 MHz is 133 MHz, 400 MHz is 200 MHz). Intel chooses a slightly different route. In Intel's processors, a bus working at 200 MHz transfers data four times as much with a technique they call QuadPump. Hence the term '800 MHz QuadPumped bus'. This is one of the reasons why Intel processors are faster when processing multimedia data than AMD processors. At the same bus speed, the Intel processor theoretically transfers more data. When speaking of bus speed on Intel processors, a processor using 400 MHz bus is effectively using bus speed of 100 MHz and so on (533 MHz is 133 MHz, 800 MHz is 200 MHz, 1066 MHz is 266 MHz).
Bus speed and cache sizes of different processors
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Bus Speed | L2 Cache Size |
| Pentium 4 | 100, 133, 200, 266 MHz | 512 KB, 1 MB, 2 MB |
| Celeron | 100, 133, 200 MHz | 256 KB |
| Athlon 64 | 200 MHz | 512 KB, 1 MB |
| Athlon XP | 100, 133, 166, 200 MHz | 256 KB, 512 KB |
| Sempron | 100, 133, 166 MHz | 192 KB |
| Duron | 100, 133, 166 MHz | 192 KB |
Both AMD and Intel makes processor not just using different speeds, but also different bus speeds (although they're using the same socket). For example, Intel's Celeron uses a bus speed of 100 or 133 MHz while their Pentium 4 uses 133 or 200 MHz. AMD does the same thing with their Athlon XPs and Duron / Sempron processors. Remember that a higher bus speed means higher bandwidth and higher bandwidth means the processor is more likely to reach its optimal or maximum performance. The other difference between these processors is cache size. Cache is small amount of memory inside your processor that acts as a buffer for your PC's main memory (RAM). Cache really helps out the processor in the bandwidth department. Since cache runs at the same speed as the processor and located inside of it, your processor can get the data much faster than if the data is stored in RAM. The larger the cache, the higher the possibility the processor can find the data in the cache. Intel's Celeron processors uses a smaller cache (256 KB) compared to their Pentium 4s (512 or 1024 KB). AMD's Duron and Sempron processor also uses a smaller cache of 128 KB, while their Athlon XP and Athlon 64 uses 512 KB or 1024 KB. Not surprisingly, these lesser bandwidth processors (with lower bus speed and smaller cache) are priced lower than the higher bandwidth ones.
Bandwidth: AMD's Unique Solution
Beginning with Athlon 64, AMD choose a different approach to bandwidth. Instead of using the memory controller outside the processor (in the north bridge chipset connected through the bus), AMD integrates the memory controller into processor. Just like cache, the memory controller runs at the same speed so overall data transfer are quicker.There are several interesting points that make this move a very 'smart' solution. Since the main memory now has a direct line to the processor, the processor don't have to use the bus to transfer data when accessing memory. So in effect, AMD's Athlon 64 didn't just transfer data faster from memory, but also from other components (by using the bandwidth that's previously used to access data in RAM). They also solve most memory compatibility and performance problems. Data access is still faster, even with slower memory. If you look at the various Athlon 64 benchmarks, you'll notice that the performance stays relatively the same regardless of the motherboard and chipset. The downside to this solution is that to support newer types of memory, AMD will have to revise the memory controller. This means you have to replace your still working processor. But don't worry too much, these processors still delivers very high performance with the current memory standard - Double Data Rate (DDR) SDRAM.
The 64 bit Evolution
If you haven't been following modern processors developments, you might be wondering what the '64' in Athlon 64 stands for. Quite simply, it indicates that Athlon 64 is a 64 bit processor. In fact, Athlon 64 is the first 64 bit desktop processor for the PC that's fully compatible with current 32 bit operating systems and applications. This may not seem much, but it really is.What's with the bit and numbers? Well, a lot actually. In case you don't know, processors (and all PCs for that matter) handle data as combinations of 1s and 0s. Each one of these 1s and 0s are called bit(s). Using more bit, we can store more data. For example, 8 of these bits (together they formed a byte) can hold any number from 0 to 255. If we add more bits to this, we can use it to store even greater numbers. So a 32 bit data can hold any number from 0 to 4.228.250.625 and a 64 bit data can hold any number from 0 to 17.878.103.347.812.890.625. Well, that's quite a lot. A 64 bit processor is capable of processing a lot more range of data than a 32 bit processor. Remember that we're talking about the range of data and not the data itself, a 64 bit processor will not be twice as fast as a 32 bit processor.
Well, if that's good than why didn't anybody do it sooner? Well, they actually did, but earlier 64 bit processor uses a totally different architecture from PC processors. PC processors are CISC processors, while 64 bit processors are RISC processors. They handle different types of data but more importantly, they use different instructions that are not compatible with each other. The only way to run 32 bit PC processor's applications and operating systems on these 64 bit processors is through a process called emulation. Emulation is slow, tricky and problematic. Slow because the 64 bit processor must translate all the 32 bit instructions and reformat the 32 bit data into a format that it can understand and process. Tricky and problematic because a 32 bit application expects to see a 32 bit processor so the 64 bit processor must 'fool' that application. It will also still be 32 bit application and can not take full advantage of the 64 bit processor's features. Simply put, we never want to emulate anything and emulation should always be a last resort.
AMD's engineer figured out a way to make 64 bit processors that still run current 32 bit PC applications and operating systems without emulation. Quite simply, they made their Athlon 64 processors share the same features with their 32 bit processors, but can switch to 64 bit mode if the software tells it to do so. To really take full advantage of 64 bit computing on Athlon 64, you have to use a 64 bit operating system, drivers and application, but fortunately that's not far down the road. A 64 bit version of Windows is scheduled to launch in April 2005 and 64 bit versions of your favorite applications are soon to follow. The 64 bit drivers are being finished and should offer the same (or faster) performance as the 32 bit ones. Since you can run your 32 bit applications just as well (and more importantly, just as fast), there's no reason not to go 64 bit.
As of 2004, Intel have also supports this 64 bit evolution. Their new processors, while using a slightly different instruction set, will still be compatible with 64 bit version of Windows - and that means it's compatible with AMD's Athlon 64 as well, software wise.
Remember these facts when choosing a processor:
- Intel uses two sockets: socket 478 and 775, while AMD uses socket 462 (or socket A), socket 754 and socket 939. You need to have the processor and the socket share the same number.
- Clock speed isn't everything. As an example, an AMD processor working at 1,8 GHz could process data just as fast or faster than an Intel processor working at 3 GHz.
- Higher bus speeds mean higher bandwidth and higher bandwidth means the processor is more likely to reach its optimal or maximum performance. Intel's Pentium 4 runs at bus speed of 133 or 200 MHz while their Celeron runs at a bus speed of 100 or 133 MHz. AMD's Duron and Sempron processor runs at bus speed of 133 or 200 MHz while their Athlon XP uses 133, 166 and 200 MHz and Athlon 64 uses 200 MHz.
- The larger the cache, the more likely your processor can get the data faster than if the data is stored in RAM. Intel's Celeron processors use 256 KB compared to Pentium 4's 512 or 1024 KB. AMD's Duron and Sempron processor uses 128 KB cache while their Athlon XP uses 512 KB and Athlon 64 uses 512 KB or 1024 KB.
- If you're concerned with performance in games and applications, go with Athlon 64 or Pentium 4. Pentium 4 has a slight edge in multimedia (producing and editing), while Athlon 64 is generally faster in games. For office work and multimedia playback, Celeron and Duron / Sempron are fast enough. Upgrading your processor won't do much anyway.
- If you're more concerned with upgrades, choose socket 939 or 775. Those who don't, should choose socket 462 / 754 or 478, these are cheaper and fast enough.
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