Messages in this thread |  | | Date | Tue, 29 Aug 2000 18:00:13 +0000 | From | Marc Mutz <> | Subject | Re: DRAM to CPU Frequency Ratio (Athlon) |
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Ookhoi wrote: > > Hi Marc, > > > > I have two exactly the same Athlon systems at 750MHz and 512 meg memory > > > (@ 133MHz). In the BIOS there is a option: "DRAM to CPU Frequency > > > Ratio", which can be 3:3 or 4:3. The Help says: "Using this item to set > > > the operating frequency of DRAM". > > > > > > I just installed Debian on the machines, rebooted, changed one machine > > > to 3:3, the other was 4:3, and compiled a kernel on both of them. The > > > time it took was almost exactly the same (about 5 min 32 sec). The 3:3 > > > one was 0.24 seconds faster. > > > > > > Does Linux ignore the DRAM to CPU Frequency Ratio (if at all possible), > > > is it just a useless BIOS option (because of hardware limits or > > > something), or is it not supposed to make the machine faster (if, what > > > _does_ it do then ;-) ? (the book that came with the machine doesn't > > > tell me much about the option). > > > > > <snip> > > > > You have PC133-333 RAM installed (PC133-222 SDRAM is still hard to find) > > which may also be run as PC100-222. So, when you select a ratio of 3:3, > > then the RAM runs in PC100-222 mode, which is not too much slower than > > PC133-333: > > > > 2*1/100e6 = 1/50e6 is actually smaller than 3*1/133e6 = 1/44e6. > > > > There are _many_ factors to SDRAM timing, but the bottomline is that you > > will only see a difference (of then ca. 5%, which sounds tiny, but is > > approximately the same gain as when going from a 600 to a 650 MHz CPU) > > if you use PC133-222 SDRAM. > > Thank you for your reaction. Others on the list also said that with a > kernel compile, disk speed is a bottleneck, not memory. > > What is the difference between PC133-333 and PC133-222, and is that also > the difference between RAM and SDRAM? Do you have an url to more info > about this (and other factors to SDRAM timing)? TIA! >
The full spec: http://developer.intel.com/technology/memory/pcsdram/spec/index.html
The most important parameters (from approx. 30?)
PARAMETER SYMBOL cycle period t_CK CAS Latency CL RAS-to-CAS Delay t_RCD RAS Precharge Time t_RP Output Valid from Clock t_AC RAS Cycle Time t_RC RAS Active Time t_RAS
The PC-blah-blah convention is also spec'ed in the above document, but here comes the short story:
PC<1/t_CK>-<CL><t_RCD><t_RP>
where 1/t_CK is in MHz, CL, t_RCD and t_RP are in units of t_CK.
If you speak German, you can read all that in c't, issue 17/00, pp.166.
Marc
-- Marc Mutz <Marc@Mutz.com> http://marc.mutz.com/Encryption-HOWTO/ University of Bielefeld, Dep. of Mathematics / Dep. of Physics
PGP-keyID's: 0xd46ce9ab (RSA), 0x7ae55b9e (DSS/DH)
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