If I have a CPU with a 66Mhz FSB, is there any advantage to using faster PC100 or PC133 memory? In this case I'm using a Pentium 2 with a i440EX chipset.
Is the memory speed locked to the FSB?
Kahenraz wrote on 2021-08-19, 11:19:If I have a CPU with a 66Mhz FSB, is there any advantage to using faster PC100 or PC133 memory?
Almost certainly not, although it depends on chipset.
In this case I'm using a Pentium 2 with a i440EX chipset.
Is the memory speed locked to the FSB?
In the i440EX (and all Intel's i440-series chipsets) it is. The only advantage to faster-rated RAM than the FSB is that you can set the tightest timings which gives a (very) small performance boost. As a rule of thumb, if you go up a speed tier (i.e. PC100 instead of PC66) you can set CAS latency on CL3-rated RAM to 2.
Some chipsets (eg. Via's ApolloPro series) are capable of running RAM asynchronously, so if you have 66MHz FSB, you can run RAM at 100MHz for slightly more performance (marred by slight increase in latency due to no longer being in sync). Here again you can go up one speed grade for lower latencies, so PC133 CL3 RAM can be run at CL2 at 100MHz with ApolloPro+ and 66MHz FSB. Unfortunately, base RAM performance of Via chipsets is poorer so you are better off with an i440EX and synchronous RAM at 66MHz than same CPU on an ApolloPro+ chipset and RAM at 100MHz.
Inversely, Intel did a nasty thing with i810 chipset and capped RAM speed at 100MHz, even if FSB ran at 133MHz. This was obviously terrible for performance, all the more so as the integrated graphics core shared the same memory bandwidth. With a 133MHz FSB P3 on i810 using integrated graphics, you would have just 3/8 of nominal memory bandwidth available for CPU under heavy CPU+video load (i.e. in 3D games). It sucked.
yes, higher spec'd memory tends to have lower CAS/RAS access times, which you can use on your slower speed memory bus if your bios allows you to.
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Not really. For example 133MHz modules has the SPD on the modules has multiple timings for 66, 100, and 133 fsb.
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Only, if you can tighten the timings on memory because it runs at lower speed, it does make a difference.
But if you just stick it in and don't play with memory timings, it doesn't matter.
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Memory prices are about the same between PC66 and PC133 so this is helpful as a purchasing decision.
Theoretically if I'm using a motherboard with a 133Mhz FSB, what happens if I add a PC66 or PC133 memory stick? If the memory is synchronous, will it refuse to POST?
Kahenraz wrote on 2021-08-19, 11:19:If I have a CPU with a 66Mhz FSB, is there any advantage to using faster PC100 or PC133 memory? In this case I'm using a Pentium 2 with a i440EX chipset.
Is the memory speed locked to the FSB?
answer is no.
SDRAM clocked at 100MHZ or 133Mhz on a 66Mhz will clock down to the 66 Mhz FSB bus you are using.
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