What happens to RAM Latency if you leave BIOS at default?

[marlinspike]

I am trying to understand a principle of RAM technology.
Comparing two RAM Modules A and B:
RAM A 1600 MHz CAS 9
RAM B 2600 Mhz CAS 11
Motherboard specifies RAM at 1600 MHz
BIOS at default, no overclocking
Will RAM B, because of its longer CAS Latency, operate more slowly than RAM A?

Or will it get a proportional decrease of CAS Latency?

 

[popatim]

Unable to know the answer without knowing the cas it will operate at when at 1600.

Basically cas is how many clock cycle will pass before it will be ready to be read. So at 1600mhz with cas=9 the system will wait 9/1600mhz.
With RamB it will wait 11/2600Mhz.

If we use easier numbers for cas, lets use 10, its easy to see the difference 10/2600< 10/1600 Ie: ramB is faster.

Honestly though, ramB is very likely to have a cas9 at 1600 and in either case 1 or 2 difference in the cas rating will not be noticable in anything but a benchmark.

 

[Tradesman1]

1600/9 equates to 2666/13 roughly, so 2666/11 is much faster and has the wider bandwidth...in other words, clocked down could run the 2666/11 at about 1600/7 which is much better than 1600/9

 

[Pinhedd]

I am trying to understand a principle of RAM technology.
Comparing two RAM Modules A and B:
RAM A 1600 MHz CAS 9
RAM B 2600 Mhz CAS 11
Motherboard specifies RAM at 1600 MHz
BIOS at default, no overclocking
Will RAM B, because of its longer CAS Latency, operate more slowly than RAM A?

Or will it get a proportional decrease of CAS Latency?

Hi,

All major DRAM manufacturers manufacture their integrated circuits according to a set of backwards compatible JEDEC specifications. When DIMM manufacturers purchase these integrated circuits and assemble them onto DIMMs they encode the DIMM's performance characteristics into one or more profiles that are stored on a small EEPROM on the DIMM itself. This EEPROM is called the Serial Presence Detect (SPD) and follows a standard format established by JEDEC.

The SPD will typically contain several standard profiles, called JEDEC profiles. These profiles correspond to the standards that the integrated circuits were manufactured and tested for, as well as several backwards compatible profiles.

For example, RAM A in your list is advertised as DDR3-1600 CAS 9. Lets say that the DRAM manufacturer is Hynix and the DIMM manufacturer is Corsair. A couple of years ago DDR3-1600 chips were not available commercially from DRAM manufacturers. However, many DIMM manufacturers found that they could take high quality DDR3-1333 chips and overclock them to DDR3-1600 speeds. To prevent compatibility issues, only the manufacturer's standard-adherent specifications would be included in the standard-adherent profiles. Thus, the DIMM's SPD may include JEDEC profiles for DDR3-800, DDR3-1066, and DDR3-1333. The DIMM would then include an XMP profile for the manufactures overclocked settings.

Just as an integrated circuit of a certain rated speed is backwards compatible with slower standard speeds, an integrated circuit of a certain latency is backwards compatible with looser latencies. A DDR3-1333 CL9 module can also operate at DDR3-1333 CL10, DDR3-1333 CL11, etc... as well as DDR3-1066 CL9, DDR3-1066 CL10, etc... If the real time constraints are met, DDR3-1066 CL8 may be permissible as well.

If you were to mix the two RAM modules that you mentioned above, one of two things will happen.

1. It will examine the JEDEC profiles on both modules and compute the highest performance commonly supported configuration. If RAM A is a native DDR3-1600 CL9 module (meaning that it uses DDR3-1600 chips rather than overclocked DDR3-1333 chips for example) and RAM B is a native DDR3-2133 CL12 module then they will both be configured for DDR3-1600. The timings will be computed from the minimums and maximums specified in each SPD.

2. It won't work at all.

 

[Turceops]

Hi from marlinspike--could not recover my account, Thank you for the detailed answer. My interest is not in mixing RAM. I am trying to decide on the best RAM to get for my circumstances. If I understand you, if I choose RAM B, and had it clocked around 1600, could I lower the CAS latency to equal RAM A? Also, where can I find the profile data of a particular RAM I am considering? Such as: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231617
Thanks again

 

[Pinhedd]

Hi from marlinspike--could not recover my account, Thank you for the detailed answer. My interest is not in mixing RAM. I am trying to decide on the best RAM to get for my circumstances. If I understand you, if I choose RAM B, and had it clocked around 1600, could I lower the CAS latency to equal RAM A? Also, where can I find the profile data of a particular RAM I am considering? Such as: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231617
Thanks again

Hi,

Each DIMM contains an EEPROM call the Serial Presence Detect, or SPD. The SPD contains one or more JEDEC standard profiles, and optional XMP non-standard profiles.

Each standard profile corresponds to a standard data rate such as DDR3-800, DDR3-1066, DDR3-1333, and DDR3-1600. Each profile contains a list of timings with which it is compatible. A DDR3-1600 profile may be compatible with CL7, CL8, CL9, CL10, and CL11.
The profile on a different DIMM may be compatible with CL9, CL10, CL11, and CL12. If these two DIMMs are inserted together in a platform that natively supports DDR3-1600, they will be configured with CL9 because that is the fastest commonly supported configuration.