Introduction
One of the most important topics beside the processor speed is the bus clock of a CPU. The clock speed at which the Front Side Bus (that's how Intel calls it) and the system is working has reached 133 MHz today. Some time ago, Tom already dealt with this topic and proved impressively that rising the bus speed can lead to better performance numbers than just using higher clocked CPUs (The Bus Speed Guide ). As a byproduct of processor development, L2 caches have been added to the CPUs to increase performance and to gain more independence from the main memory.
Bus Speed History
In the beginning of computer history, there was no difference between the internal and external clock speed. A 386 DX 25 worked at 25 MHz processor clock and was able to communicate at the same 25 MHz with its page mode DRAM. In the beginning of the 90s it became obvious that processor speeds would rise faster than clock speeds of the main memory (best example was the 486 DX 50) and upcoming synchronous bus systems. Since the performance losses at lower clocked memory were tolerable, Intel introduced a new series of processors: The 486 DX2. The "2" means that the processor is clocked twice as high as the memory/bus. A 486 DX2 - 66 worked at 33 MHz memory/bus and 66 MHz processor clock.
Thus it was possible to use less expensive memory chips. Thanks to the increasing use of L2 caches, performance drops caused by the lower memory clock were small. The main reason for this "clock splitting" was the introduction of the VESA Local Bus and the first PCI systems, offering much faster bus performance for graphics and I/O components. While PCI was specified for only 33 MHz, VLB was specified for 40 MHz. To obey it, processors were clocked at double (486 DX2-50, DX2-66) or triple bus speed (486 DX4-100).
With the introduction of the first Pentium computers, the internal and external clock speeds had been equalized for the last time: Pentium 60, Pentium 66. Following processors (Pentium 75, 90, 100) worked with a multiplier of x1.5 and different bus speeds (50, 60, 66 MHz).
Today's computer architectures have been optimized, so that some computers do already work with multipliers of up to x8 (Pentium III 800 at 100 MHz bus or Celeron 533 at 66 MHz bus clock). However, performance increases cannot hold up to the clock speed rising as long as the bus speed is not pushed up as well.
Common Bus Speeds
What exactly does happen when we chose a higher clock speed? As you know there is the system clock speed and the processor clock, which can be "produced" by multiplying the system speed with numbers between 1.5 (2.0, 2.5, 3.0...) and 8.0. All other clock speeds depend on the system clock. The PCI bus always runs at 1/2, 1/3 or 1/4 of the external clock just like the AGP runs at 2/3 (100 MHz bus) or 1/2 (133 MHz bus) of the system clock. This table will give you a little more explanation:
| System clock | PCI clock | AGP clock | Processors (Multiplier) |
|---|---|---|---|
| 60 MHz | 30 MHz (1/2) | 60 MHz (1/1) 1 | Pentium 60 1Pentium 90 (1.5) 1Pentium 120 (2.0) 1K5-90 1K5-120 (2.0) 1 |
| 66 MHz | 33 MHz (1/2) | 66 MHz (1/1) | Pentium 66 1Pentium 100 (1.5) 1Pentium 133 (2.0) 1Pentium 166 (2.5) 1Pentium MMX 166 (2.5) 1Pentium MMX 200 (3.0) 1Pentium MMX 233 (3.5) 1K5-100 (1.5) 1K5-133 (1.5) 1,2K5-166 (2.5) 1,2K6-166 (2.5) 1K6-200 (3.0) 1K6-233 (3.5) 1K6-266 (4.0)K6-300 (4.5)Pentium II 233 (3.5)Pentium II 266 (4.0)Pentium II 300 (4.5)Pentium II 333 (5.0)Celeron 266 (4.0)Celeron 300 (4.5)Celeron 333 (5.0)Celeron 366 (5.5)Celeron 400 (6.0)Celeron 433 (6.5)Celeron 466 (7.0)Celeron 500 (7.5)Celeron 533 (8.0) |
| 100 MHz | 33 MHz (1/3) | 66 MHz (2/3) | K6-300 (3.0)K6-2 300 (3.0)K6-2 350 (3.5)K6-2 400 (4.0)K6-2 450 (4.5)K6-2 500 (5.0)K6-2 550 (5.5)K6-3 400 (4.0)K6-3 450 (4.5)Pentium II 350 (3.5)Pentium II 400 (4.0)Pentium II 450 (4.5)Pentium III 450 (4.5)Pentium III 500 (5.0)Pentium III 550 (5.5)Pentium III 600 (6.0)Pentium III 650 (6.5)Pentium III 700 (7.0)Pentium III 750 (7.5)Pentium III 800 (8.0)Athlon 500 (5.0) ³Athlon 550 (5.5) ³Athlon 600 (6.0) ³Athlon 650 (6.5) ³Athlon 700 (7.0) ³Athlon 750 (7.5) ³Athlon 800 (8.0) ³Athlon 850 (8.5) ³Athlon 900 (9.0) ³Athlon 950 (9.5) ³Athlon 1000 (10.0) ³ |
| 133 MHz | 33 MHz (1/4) | 66 MHz (1/2) | Pentium III 533 (4.0)Pentium III 600 (4.5)Pentium III 667 (5.0)Pentium III 733 (5.5)Pentium III 800 (6.0)Pentium III 866 (6.5)Pentium III 933 (7.0)Pentium III 1000 (7.5) |
1 There was no AGP at that time.
² The K5-PR133 works at 100 MHz clock speed. The K5-PR166 as well, but with an internal multiplier of x1.75. You have to set X2.5 on the motherboard.
³ Athlon processors work at 200 MHz FSB and 100 MHz system clock speed. The memory can be clocked at 100 MHz (AMD750/Irongate) or at 133 MHz (VIA KX133).
The table shows all possible processor clockings which are officially specified. Rising the bus clock means overclocking motherboard, controllers and main memory. Of course it's possible to rise the bus speed at all three speed grades, but remember that dependent clock speeds do rise as well.
| 66 MHz | Many boards offer 68, 75 and 83 MHz bus speed. |
| 100 MHz | 103, 112, 124 and 133 MHz are common, but many motherboard companies do offer smaller steps as well. |
| 133 MHz | 140, 150, 155 MHz. |
PCI And AGP Clock At Higher Bus Speeds
Processor overclocking by chosing a higher multiplier can only be done with Athlon CPUs, using a special overclocking board. Intel processors have a fixed multiplier with no option to change it. The only solution for speed freaks and overclockers is of course a higher bus clock. Be careful when rising it! The table above showed the coherencies in standard systems. Never forget the other clock speeds when overclocking:
| 66 MHz ext. clock | PCI clock x1/2 | AGP clock x1/1 |
|---|---|---|
| 68 MHz | 34 MHz | 68 MHz |
| 75 MHz | 37.5 MHz | 75 MHz |
| 83 MHz | 41.5 MHz 1 | 83 MHz 1 |
| 100 MHz ext. clock | PCI clock x1/3 | AGP clock x2/3 |
| 103 MHz | 34.3 MHz | 68.6 MHz |
| 112 MHz | 37.3 MHz | 74.6 MHz |
| 124 MHz | 41.3 MHz 1 | 82.6 MHz 1 |
| 133 MHz | 44.3 MHz 1 | 88.6 MHz 1 |
| 133 MHz ext. clock | PCI clock x 1/4 | AGP clock x y1/2 |
| 133 MHz | 33 MHz | 66 MHz |
| 140 MHz | 35 MHz | 70 MHz |
| 150 MHz | 37.5 MHz | 75 MHz |
1 This setting will most likely fail, since add-on cards and on-board components are overclocked just too much.
Chosing a much higher bus clock than standard will cause instabilities resulting in hang ups or data losses. Thus it's very important to run a reasonable bus clock which most likely won't cause trouble. In the past, 75 instead of 66 MHz have proven to be very reliable (that's the best way to overclock a Celeron processor by the way). For 100 MHz FSB, 110 or 112 MHz seemed to be the best choice.
