[xylon]

In his Macdates, Anand points out the macs use the slowest DDR400 RAM. I don't get it. There are speed differences within RAM of a certain Mhz? Someone care to enlighten me?


Link to article

From the above linked article:

I mentioned that the very first upgrade I tried on the G5 was to stick a full 4GB of some of the fastest OCZ memory I had laying around. I was met with failure at that attempt thanks to Apple's motherboard not playing too well with aggressively timed DDR400. OCZ sent over 8 - 512MB sticks of their G5 DDR400 modules which are rated at 3-3-3-8, the slowest DDR400 I've ever used. Unfortunately it is the only stuff that the G5 will work with. I will admit that for my work machine I never really tweaked memory timings, I just left everything at SPD but in most cases SPD was at least 2-2-3-7. I'd like to see Apple migrate to some faster memory, especially considering the price premium these machines are going for, but that'll most likely have to wait for the next revision of the G5 systems. Lower latency memory will also give more of a benefit on the higher clocked G5s in any case.

[Catfish_Man]

The CAS latency of ram measures the number of cycles it takes to do a certain operation (specifically the time for a column access strobe). The click frequency (MHz) measures the number of cycles per second.

In plain English, all DDR400 transfers at the same speed, but some get started quicker.

[Dr Reducto]

If you have low latency RAM, you can drop the timings and run the RAM faster. I have crappy RAM on my PC, so I can't push the RAM fast, so to overclock, I have to drop the RAM clock to DDR266, and increase the FSB to 266mHz. This brings the RAM back up to DDR400. If I was able to push the RAM like you can push Corsair XMS RAM (DDR434 @ 3-4-4-7), you get loads of extra bandwidth on your FSB and from your RAM. I only get extra bandwidth on my FSB.

[Scotttheking]

Dr Reducto, your post makes no sense. FYI.


As for ram speeds, Macs don't use aggressive ram timings. I think part of the reason is the apple memory controller. It does some neat stuff to get 4 ram slots. Someone more knowledgeable will have to post more details.

[djohnson]

I have to agree there. I dont know many PC DDR400 boards, if any, that have 4 ram slots. Let alone 4 ram slots per processor... Also, if you want to increase the bandwidth for the ram, you have to do a little more then drop the latency so minutable that it doesnt show in real world results. "Hey look, I can get 0.0005 more FPS playing UT!!!" If you want to increase the bandwidth, look into the dual channel ddr stuff...

[saru boy]

Uh, FYI pretty much every Intel 865/875 chipset based motherboard out there has 4 RAM slots (which would be 2 banks of dual channel DDR400 memory). If you want to increase the memory bandwidth, you can overclock the memory. I have cheapo DDR400 RAM (paid $70 each for 2 512MB dimms) in my PC running at 220mhz (which translates into an FSB of 880 mhz). From overclocking it 10% the memory bandwidth is increased from 6.4 GB/s to slighty over 7 GB/s (theoretical - in actual use you may see only about 60% of this bandwidth).