Increasing the system clock frequency to 100 MHz promises huge performance gain for socket 7 systems
A quick look at Intel’s latest road map and AMD’s promises at Cebit 97 leave no doubts that the times of 66 MHz system clock frequency, introduced with the birth of the Pentium processor, are going to be over soon. A memory throughput of only 528 MB/s will put the breaks on the Advanced Graphics Bus AGP soon, and as long as the processor accesses memory at a width of 64 bits instead of 128 bits the only feasible solution is an increase in the system clock frequency. The tinkers and overclockers among the readers of my website know since November 1996 what kind of a substantial performance gain can be had from an increase of the system clock from 66 to 75 or even 83 MHz; however a system clock of 100 MHz has so far be unattainable. Actually, a jump in the system clock frequency from 66 to 100 MHz doesn’t seem that impressive, it being equivalent to a mere 50% increase, as opposed to the 100% increase from 33 to 66 MHz at the time of the switch over between the 486 and the Pentium processor. While the type of RAM in use at that time could easily be adapted to this doubling of the clock frequency, the increase to 100 MHz approaches the technical limits of currently used RAM, and a doubling to 133 MHz will probably have to wait for some time. In all likelihood a system clock frequency of 100 MHz will mean the death of FPM (Fast Page Mode) and EDO (Enhanced Data Out) RAM, which already has trouble with a system clock frequency slightly in excess of 66 MHz. Until Intel presents us with their nDRAM, which is currently under development (based on Rambus-RAM technology) and allows for extremely high clock frequencies, systems with 100 MHz system frequency will primarily be based on SDRAM. However, the quality requirements of this type of RAM will have to be raised as well before this happens. Right now there is a pretty big chaos in the SDRAM market and it is not all rare occurrence that a given type of SDRAM works only in few motherboards, or even none at all. This has to change, because systems running at a 100 MHz need cleaner timing than the 66 MHz systems in use today.
L2 cache speed is the key
Addressing the question why higher system clock frequencies lead to impressive performance gains even with no change to the core speed of the processor, we find the answer in two important components of a typical computer. One is the L2 cache frequency, which is tightly coupled to the system frequency in Socket 7 systems. The other is the frequency at which the main memory operates. Both components benefit from an increase in the system clock frequency and the acceleration of the L2 cache by 50% especially has a very significant impact on the overall performance of the system. This explains why the performance gain (from increasing the system clock frequency) is small or even unmeasurable in systems based on the Pentium Pro or Pentium II processors, because in these system the L2 cache is always running at the core frequency or at least one half the core frequency.
So far no 100 MHz motherboards available
While AMD already announced 100 MHz system clock speed for Socket 7 in spring of this year and while we also know that Intel will introduce their second generation AGP chip set, 440BX, with support for 100 MHz system clock frequency in the first half of 1998, motherboards with support for 100 MHz system frequency are so far not even sparingly, in fact, not at all available. So imagine my joy when I received a motherboard that doesn’t only support 100 MHz system frequency, but allows the user to select system frequency in a large range below as well as beyond 100 MHz in small steps. This is a very special «hand crafted» board which was never intended to leave the lab. The main purpose of this main board was to study system behavior at very high system frequencies, and thus searching for the best design of a motherboard that can run stably at a system frequency of 100 MHz or more. It features redundant clock generators (PLLs) and power supplies (voltage regulators), and is obviously of the highest imaginable quality. The manufacturer of this board would like to remain unknown since this is a motherboard that will never be shipped to customers. Nonetheless this motherboard is of course eminently qualified for first tests with 100 MHz system frequency, although the high quality requirements mentioned above caused only 1 out of 5 brands of SDRAM memory to enable stable test runs, three brands wouldn’t even boot at all.
Intel Pentium has difficulties at 100 MHz system frequency
The BIOS of the test motherboard could not at all deal with the latest Cyrix and IBM 6x86MX processor, thus only AMD’s K6 and Intel’s Pentium Classic and Pentium MMX could be used for the tests. The K6 passed the test runs with no problems whatsoever, while tries to get the Pentium running at 100 MHz system clock where utterly unsuccessful. Admittedly Intel never designed this processor for such a high system clock frequency, although it does run fine at 75 and even 83 MHz. AMD’s K6 was obviously designed off the bat for this faster system frequency, since AMD had already announced the future move to 100 MHz system frequency when introducing the K6. This left only the K6 for comparisons between 66 and 100 MHz system clock frequency.
AMD K6: 10% performance gain at same core clock frequency
The results speak for themselves. Comparing an AMD K6 with 200 MHz and 66 MHz system clock to the same processor with 100 MHz system clock we find an impressive performance gain of 10% in both the Business and Highend parts of Winstone. The well known benchmark based on the action game Quake does even better and is accelerated by almost 20%, although floating point computation does not benefit from the higher system clock frequency as can be seen by the almost identical results in the 3D Studio Max measurements. To demonstrate the performance gain for experienced «overclockers» a AMD K6 was overclocked once to 3×83 MHz and the other time to 2.5×100 MHz. The gain at 100 MHz clock frequency is smaller at about 4%, but it is still significant. In case you aren’t impressed by the 10% gain seen, please compare the results with the currently fastest AMD K6 processor running at 233 MHz. The 200 MHz processor running in a 100 MHz system is already 5% faster than the «older brother» running at 233 MHz and achieves a performance that can otherwise only be expected from the 266 MHz variant. It is faster than an Intel Pentium Pro at 200 MHz in both Windows 95 and Windows NT. The K6 with 250 MHz and 100 MHz system clock frequency achieves the Windows-95 scores of a Pentium II 266 and the Windows NT scores of a Pentium II 233.
100 MHz system clock frequency will primarily benefit Socket 7
The performance gains achieved through 100 MHz system frequency for Socket 7 systems is impressive and offers an easy upgrade path for consumers. As opposed to other scenarios, the consumer for once has to purchase only a new motherboard and not a new processor and therefore achieves a performance gain in a comparatively low cost fashion. Pentium II systems which are also scheduled to go to 100 MHz system clock frequency next year unfortunately can not expect a comparable performance gain. Increasing just the system clock frequency without the L2 cache speed is not sufficient for that.
AMD 640 AGP chipset possibly first in the field to support 100 MHz
The first chipsets announced for 100 MHz are Intel’s 440BX for the Pentium II platform, and VIA’s Apollo VP4 for Socket 7 systems. Both sport AGP support of course. Little has been heard so far about AMD’s 640 AGP chipset. Taking into account AMD’s 100 MHz announcement in spring of this year, this seems to indicate that AMD doesn’t want to take a detour by first introducing AGP support at 66 MHz, as Intel does with the 440LX and VIA does with the VP3 chipset, but instead wants to go straight to AGP and 100 MHz system clock. If on top of this it should become clear that AMD’s K6 is the only processor ready for 100 MHz system clock frequency, Intel’s market share in the Socket 7 market will finally be reduced drastically. And AMD’s road map already talks about 300 and 400 MHz variants of the K6 processor which, working off a 100 MHz system bus will mean serious competition for Intel’s Pentium II.
Benchmark Results of 66 and 83 MHz vs 100 MHz System Bus
* Processor ran at higher clock frequency than specified.
