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What Does Mem ratios mean?
- Thread starter stan03
- Start date
The ratio that people talk about most of the time is the FSB:MEM ratio. That ratio relates the FSB clock speed (not quad-pumped) to the memory bus speed (not double-pumped).
If you have a P4-C, the default FSB is 200Mhz. The default memory bus speed for PC3200 RAM is 200Mhz. So you would want a 1:1 ratio at default.
When you start overclocking, every 1 Mhz increase in the FSB speed increases the memory bus speed through that ratio. Let's say you increase the FSB to 250Mhz. At 1:1 the memory would be running at 250Mhz (500Mhz DDR) -- which is asking a bit much of PC3200-rated RAM. If you change the ratio to 5:4, the memory bus returns to 200Mhz (400Mhz DDR).
If you have a P4-C, the default FSB is 200Mhz. The default memory bus speed for PC3200 RAM is 200Mhz. So you would want a 1:1 ratio at default.
When you start overclocking, every 1 Mhz increase in the FSB speed increases the memory bus speed through that ratio. Let's say you increase the FSB to 250Mhz. At 1:1 the memory would be running at 250Mhz (500Mhz DDR) -- which is asking a bit much of PC3200-rated RAM. If you change the ratio to 5:4, the memory bus returns to 200Mhz (400Mhz DDR).
- Joined
- Sep 9, 2002
- Location
- Bartlesville,OK
What if the numbers don't match any of the ratio's that are in your bios, then how do you deside what the divider? Do you just allow it to change accordingly?
Yeah that can be a problem. Basically, you want to get the fastest stable combination of FSB speed and memory speed. FSB speed (i.e. CPU speed) is always more important.
So for example let's say your CPU can do 275Mhz FSB, but your memory can only do 420Mhz. You could either go with 262Mhz FSB and 420Mhz memory, or 275Mhz FSB and 367 Mhz memory. The 275 Mhz FSB will almost certainly provide a faster system (because it means an extra 150Mhz for a 2.4C, more for a 2.6 or a 2.8), even though the memory is somewhat slower.
Here's a table of FSB vs memory speeds for 1:1, 5:4, and 3:2. It should give you some idea of when you would need to jump from one ratio to another.
FSB 1:1 5:4 3:2
200 400 320 266.8
205 410 328 273.47
210 420 336 280.14
215 430 344 286.81
220 440 352 293.48
225 450 360 300.15
230 460 368 306.82
235 470 376 313.49
240 480 384 320.16
245 490 392 326.83
250 500 400 333.5
255 510 408 340.17
260 520 416 346.84
265 530 424 353.51
270 540 432 360.18
275 550 440 366.85
280 560 448 373.52
285 570 456 380.19
290 580 464 386.86
295 590 472 393.53
300 600 480 400.2
So for example let's say your CPU can do 275Mhz FSB, but your memory can only do 420Mhz. You could either go with 262Mhz FSB and 420Mhz memory, or 275Mhz FSB and 367 Mhz memory. The 275 Mhz FSB will almost certainly provide a faster system (because it means an extra 150Mhz for a 2.4C, more for a 2.6 or a 2.8), even though the memory is somewhat slower.
Here's a table of FSB vs memory speeds for 1:1, 5:4, and 3:2. It should give you some idea of when you would need to jump from one ratio to another.
FSB 1:1 5:4 3:2
200 400 320 266.8
205 410 328 273.47
210 420 336 280.14
215 430 344 286.81
220 440 352 293.48
225 450 360 300.15
230 460 368 306.82
235 470 376 313.49
240 480 384 320.16
245 490 392 326.83
250 500 400 333.5
255 510 408 340.17
260 520 416 346.84
265 530 424 353.51
270 540 432 360.18
275 550 440 366.85
280 560 448 373.52
285 570 456 380.19
290 580 464 386.86
295 590 472 393.53
300 600 480 400.2
- Joined
- Sep 9, 2002
- Location
- Bartlesville,OK
so
a= cpu fsb ratio
b= mem speed/2 ratio
c=fsb
d=mem speed/2
a/b = c/d
or am I a little weak on my algebra since its summer?
a= cpu fsb ratio
b= mem speed/2 ratio
c=fsb
d=mem speed/2
a/b = c/d
or am I a little weak on my algebra since its summer?
- Joined
- Sep 9, 2002
- Location
- Bartlesville,OK
forget my recent post. Its right but it won't help you find stuff out. I made a nice little spreadsheet to calculate what your memory speed would be. You just put in your cpu fsb (exp. 200) the first number in your divider (what i call the numerator) and the second number (the denominator) and it'll show your the memory speed. I call the numbers numerator and denominator because a ratio is also a fraction. http://www.hpottercr.com/divider_calc.xls
No, the motherboard always enforces "perfect" ratios. But because there are a limited number of ratios, you sometimes can't have both the best FSB and the best MEM speeds. In those cases, it's almost always better to sacrifice some memory bandwidth to get a higher CPU speed.
- Joined
- Sep 9, 2002
- Location
- Bartlesville,OK
so is my spreadsheet right?
- Joined
- Sep 10, 2002
- Location
- Mississauga, Ontario
NookieN great explanation. We should have a sticky, as so many people have asked the very same ratio question before.
