100MHz Bus Speed & Socket-7 Motherboardsby Anand Lal Shimpi on September 27, 1997 6:46 PM EST
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In most situations, the 100MHz bus speed provided an increase in performance of about 10 - 12% at the same clock speed over the 66MHz bus speed. This can be expected and is a considerable improvement when compared to the virtually non-existent increase the 100MHz bus provides when used with Pentium II systems. The reason for this is quite obvious. As mentioned earlier, Socket-7 systems keep their Level 2 Cache (Cache is basically ultra-fast memory where frequently used data can be stored for easy access) running at the bus speed your motherboard is set to. In most cases, this speed is 66MHz. By pushing the limits of your system using the 100MHz Bus Speed as tested here, your L2 cache can be accessed at a clock speed 50% greater than it could previously (100MHz vs 66MHz).
With the Pentium II's Slot-1 Architecture, things are a little different when it comes to how the L2 Cache speed is derived. You may have heard of the term Dual Independent Bus (DIB for Short) in essence, this DIB architecture the Pentium II uses allows it to have its L2 Cache operate off the CPU, as to reduce costs, while maintaining a direct high speed connection to the L2 cache by keeping it on the CPU Cartridge, as to increase performance. Therefore, the speed the L2 Cache operates at on a Pentium II system is equal to, not the Bus Speed, but one-half of the processor's clock speed. It is for this reason that the Pentium II isn't affected too dramatically by higher bus speeds, because at 100 x 3.0 and 66 x 4.5, the L2 cache of the Pentium II still operates at 150MHz. The only devices receiving the benefit of the 100MHz bus in that case are your main memory and any AGP/PCI devices that you may have.
The best overall solution to extend the life of Socket-7 systems would be to place the L2 cache on the CPU or on a secondary Bus running at half the speed of the CPU (much like the Pentium II) while maintaining backwards compatibility with the Socket-7 architecture. Although changes like this have been proposed for future processors from AMD and Cyrix, we have yet to see any concrete evidence that this has been done. AMD claims that their K6-3D+ will have 256KB of on chip L2 cache running at clock speed, we will just have to wait for its release later this year to confirm that however.
With the results we have here, the applications that received the greatest boost in performance from the 100MHz bus speed were applications that made extensive use of a combination of raw processing power, heavy I/O and Disk Access, and graphics manipulation. The Database 97 tests, Truespace Rendering Tests, and Quake 2 Gaming Tests were the ones the benefited most from the higher bus speed. The Word Processing/Spreadsheet 97 were affected as well, just to a lesser degree, while the Disk Copy Time barely fluctuated at all averaging a 1.8% increase in performance, barely noticeable. When the 100MHz bus does officially hit us, we can expect similar results in our real world usage of systems using the 100MHz bus speed. The tricky part then will be buying SDRAM that will work at the higher bus speed...but is that the only problem?
About that L2 Cache...
Here's an interesting point often overlooked when considering the stability and reliability of a motherboard at the 100MHz bus speed, L2 cache. Since the L2 cache is a component of the motherboard that will be running at this high frequency, it will take some high quality cache modules to keep up with the speed. It is doubtful that we will have to worry much about the quality of the L2 cache used on motherboards that support this setting, since during the design phase of a motherboard the prototype is placed through a rigorous testing process in which the L2 cache remains enabled. If the motherboard does officially support the 100MHz bus speed you can expect it to work properly at the bus speed, provided that you take the initiative to make sure that the components you use with the motherboard also function properly at the 100MHz bus speed. Its not really good practice for a motherboard manufacturer to sell a motherboard that doesn't work at a supported setting, so I wouldn't worry too much about that happening. The question here is, can your peripherals stand the heat at 100MHz?
In the next few months we'll see many motherboards appear with support, either officially or unofficially for the 100MHz Bus Speed. Although you won't have to wait for the AMD K6-3D to take advantage of this bus speed, getting it to work with today's Socket-7 processors may be a bit more tricky than you would imagine. While the 100MHz bus has been highly anticipated for quite some time, now that it is nearly here it is time to look beyond that...can you say 133MHz bus? Who will be the first to once again break Intel's speed limit barrier, as fast as it may be, with another unofficial setting much like the 75MHz bus speed was in comparison to the 66MHz frequency? The race is on once again...