L2 latency

The L2 Cache latency is the amount of time that it takes to communicate with the L2 Cache after a miss with the L1 Cache. The L2 Cache latency depends on the motherboard (specifically the RAM and chipset) and processor. The default way to get the correct value is to run the BIOS defaults settings (not the Power-on Defaults) and use that number. If you use a number that is lower than that, you may get memory errors or additional L2 Cache misses which slow the computer. If you use a number that is larger, the system will be slower. The default setting for L2 Cache latency is generally at 5. Faster settings (1-4) decrease the wait states, while slower settings (6-10) increase the wait states; greater wait states allow for better error detection.

After extensive testing on Abit BH6, Abit BX6.2 and Abit BE6 regarding these settings I have found out that a setting off 2 is highest. L2 latency set at 1 is lower than 3 and a setting of 15 is lowest.  Well here comes the numbers - starting with the highest :   2 - 3 - 1 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13 - 14 -15     Confusing - ain't it ??? So how does L2 latency impact performance ???   It has tremendous impact on performance in testprograms .....  If you can run your PIII450 at 129MHz FSB (580MHz) with L2 set to 2 this gives better performance than 133MHz FSB (600MHz) with L2 set to default .....  CpuMark99 results : On AbitBH6 MoBo (133FSB/ 600 MHz)  default L2 setting gives a score of 46.0 , with L2 set to 3 it gives 47.3. This clearly indicates the L2 importance  To lower L2 latency is often necessary to force a stubborn CPU up to desired speed - but if you have to use the lowest settings it might not be worth it at all. Disabling L2 is not recommended at all because of nearly 50% performance drop - see the proof here . Overclocking your CPU also overclocks L2 - so that a PIII450 at 600MHz acttually outperforms a stock PIII600MHz with good margin !

Memory Explained :

RAM

Random Access Memory.  All memory accessible at any instant (randomly) by a microprocessor.  Sometimes referred as "System Memory".  Eventhough Microsoft has set minimum requirement or the amount of RAM for their operating systems, for DOS/Win3.1 you will need 8 MB or more and for Windows95 16 MB or more in order to run efficiently.  The maximum amount of RAM installed is limited by your motherboard and in some instance the motherboards chipset (and your wallet).

Conventional Memory

The original PC/XT-type system was designed to use 1 MB of memory workspace, sometimes called RAM.  This 1 MB of RAM is divided into several sections, some of which have special uses.  DOS can read and write to the entire megabyte, but can manage the loading of programs only in the portion of RAM space called conventional memory, whichat the time the first PC was introduced was 512K and later revised to 640K.
 

Upper Memory Area/Blocks (UMA/UMB)

The term Upper Memory Area describes the reserved 384K at the top of the first megabyte of system memory of a PC computer.  This memory has the addresses from A0000 through FFFFF.  The UMA is divided into three equal parts of 128K for Video RAM (A0000-BFFFF), Adapter BIOS (C0000-DFFFF) and Motherboard BIOS (E0000-FFFFF).  Not all the 384K of UMA is fully used on most AT-type systems, and therefore in DOS 5.0 (and after), a feature is added to the operating system so that some TSR programs and part of the DOS kernel can be loaded into the high memory area.  This way more conventional memory can be reserved for loading programs.

Extended Memory

The memory map on a system based on the 286 or higher processor can extend beyond the 1 megabyte boundary.  On a 286 or 386SX system, the extended memory limit is 16 MB; on a 386DX, 486 or pentium system, the extended memory limit is 4 GB (4,096 MB).  Systems based on Pentium Pro or Pentium II /Pentium III  can go as high as 64 GB (65,536 MB).  Both Windows 3.1 and Windows95 require extended memory in order to load in Enhanced Mode.  Gerenally, the more extended memory you have, the more efficient Windows can perform.

Expanded Memory

Some older programs (especially games) can use a type of memory called Expanded Memory Specification or EMS memory.  unlike conventional or extended memory, expanded memory is not directly addressable by the processor.  Instead it can only be accessed through a small 64K window established in the UMA.  Expanded memory is a seagment or bank-switching scheme in which a custom memory adapter has a large number of 64K segments onboard combined with special switching and mapping hardware.  DOS' EMM386 driver can emulated EMS using extended memory therefore no one is producing this type of memory cards anymore.

SIMMs (Single In-line Memory Modules)

For memory storage, most modern systems have adopted the SIMM as an alternative to individual memory chips.  These small boards plug into special connectors on a motherboard or memory card.  The individual memory chips are soldered to the SIMM, hence simplify the add or remove RAM procedures.  There are two major standards, 30-pin (3.5" long) and 72-pin (4.25" long).  30-pin SIMMs are used mainly in 286/386 and some early 486 motherboards.  30-pin SIMMs usually are used 4 in a bank, meaning if you want 4 MB you will need to install 4 pieces of 1 MB modules.  72-pin SIMMs are used commonly in 486, Pentium and Pentium Pro computers.  486 motherboards can use one 72-pin SIMM at a time, and you can use one 4 MB and one 16 module to coombine for 20 MB.  For Pentium or Pentium Pro motherboards, you will have to use a pair in a bank, meaning if you want 16 MB total you will need to install two pieces of 8 MB 72-pin SIMMs.

EDO RAM

EDO (Extended Data Out) RAM.  These are 72-pin SIMMs with specially manufacturerd chips that allow for a timing overlap between successive accesses.  This allows for a tighter coupled access cycle and a performance improvement of 20% or so over regular non-EDO SIMMs. 

DIMMs (Dual in-Line Memory Modules)

These are 168-pin memory modules designed for with bus speeds ranging from 66MHz and upwards and are todays most used type - you can also get 256MB modules today - and the architecture can also differ from the table below.

SPD DIMMs (or DIMMs with EEPROM)

SPD (Serial Presence Detect) is a  feature available on a number of SDRAM DIMM modules that solves industry-wide compatibility problems by making it easier for the BIOS to properly configure the system to fit SDRAM performance profiles.

The SPD device is an 8-pin serial EEPROM chip that stores information about the DIMM module’s size, speed, voltage, drive strength, and number of row and column addresses. When the BIOS reads these parameters during POST, it automatically adjusts values in the CMOS Chipset Features screen for maximum reliability and performance.

Without SPD, the BIOS (or user) must make assumptions about the DIMM’s parameters. While this does not usually cause problems when EDO DIMMs are used, many users have found that their system will not boot if they are using non-SPD SDRAM DIMMs. As SDRAM is capable of running at twice the speed of EDO RAM, there is less room for error. An incorrect BIOS assumption about an SDRAM DIMM’s parameters can have dramatic consequences (e.g. failure to boot or fatal exception errors).

See also Craig Taylor's :

Everything You Wanted to Know
About SDRAM But Were Afraid to Ask