a: CAS latency
b: RAS-to-CAS Trcd
c: RAS Precharge Trp
d: Active to Precharge Tras
As the initial CAS latency is fixed for each burst transaction, a longer burst transaction will allow more data to be read or written for less delay than a shorter burst transaction. Therefore, a burst length of 8 will be faster than a burst length of 4.
3)Most manufacturers use a default value of 8T which allows the memory controller to precharge the open pages once eight idle cycles have passed.
There's also an Infinite option as well as an Auto option.
For general desktop use, it is recommended that you choose the Infinite option so that precharging can be delayed for as long as possible. This reduces the number of refreshes and increases the effective memory bandwidth.
4) This BIOS feature allows you to set the refresh interval of the memory chips. There are three different settings as well as an Auto option. If the Auto option is selected, the BIOS will query the memory modules' SPD chips and use the lowest setting found for maximum compatibility.
For better performance, you should consider increasing the DRAM Refresh Rate from the default values up to 128 µsec. Note that if you increase the DRAM Refresh Rate too much, the memory cells may lose their contents.
Therefore, you should start with small increases and test your system after each hike before increasing it further. If you face stability problems upon increasing the refresh interval, reduce the refresh interval step by step until the system is stable.