Types of Rams
Functioning & Different RAM versions
Refers to the delay between the Column Access Strobe (CAS) and the arrival of the data from RAM. It is an indicator of the speed of the memory, and is expressed in the number of clock cycles: a CAS Latency of a means that three clock cycles are needed after the CAS for data to be produced by the RAM.
Double Data Rate SDRAM was an evolutionary improvement over SDRAM. It allowed data to be transferred twice during every cycle. This effectively doubled the frequency of the memory; DDR 266 memory actually works at 133 MHz. It is important to note that the term is “DDR 266” and not “DDR 266 MHz.” Another mode of notation is by referring to the peak data transfer speeds. So a DDR 266 module is also referred to as PC 2100, since it can transfer 2100 Megabytes per second. DDR modules have 184 contacts, and are not backward compatible.
An evolution of DDR SDRAM, DDR2 allows four data transfers per clock cycle, by clocking the internal bus at twice the speed of the memory clock. Therefore the effective frequency of the memory becomes 4 times its actual frequency. A DDR2 800 module operates at 200 MHz. As in the case of DDR SDRAM, the alternate notation relying on the maximum data transfer speeds is also used. PC2 3200 refers to DDR2 400. A DDR2 module has 240 contacts, and is not backward compatible.
This is the latest iteration of SDRAM, and increases the internal bus speed to 8 times the memory clock, effectively operating at 8 times the frequency. DDR3 800 operates at 100 MHz and is also referred to as PC3 6400. DDR3 is still a cutting edge technology, and is supported by very few motherboards and CPUs. DDR3 modules also have 240 pins, but they are keyed differently, so they cannot be inserted into a DDR2 slot.
Dual Inline Memory Module refers to the package in which RAM is available. Unlike the earlier and now obsolete SIMM (Single Inline Memory Module), a DIMM has contacts on both sides of the module.
Dynamic RAM refers to volatile RAM that is constantly refreshed to prevent stored contents from being lost. All modern RAM is of this type, though there have been improvements. Data is stored in the form of rows and columns, with each storage area in RAM having a unique address that is a combination of the row and column number. DRAM was available in different types like SDRAM, EDO (Extended Data Out) RAM, FP (Fast Paging) RAM, and more, all of which are now obsolete except for SDRAM.
An Error Correction Code (ECC) RAM module is a special type of module that includes additional components to verify the integrity of data stored in or transferred by system RAM. This is needed in critical systems that require high levels of data integrity, like servers.
RAS to CAS Delay:
This is the number of the clock cycles that intervene between the identification to the row with RAS and the start of launching the CAS. This is displayed as “tRCD” in the BIOS.
RAS Precharge Time:
This is the number of cycles that are needed to refresh the RAS after the previous access before it can be used for a new access.
RAMBUS Direct RAM was based on a technology developed by RAMBUS. Unlike SDRAM, it used a serial mode of data transfer and though the technology was considered superior to DDR SDRAM, its high cost made it unpopular. RDRAM modules are called RIMMs.
Registered RAM/ Buffered RAM:
This type of RAM module had additional storage areas called buffers or registers where the data is stored temporarily and checked for data integrity before being transferred. Similar in use to ECC RAM, though using a different method, this type of RAM is needed in servers, which require high levels of data integrity.
Synchronous Dynamic RAM was an improved version of DRAM that synchronised all its functions to a single frequency, usually the system’s FSB frequency. The frequency refers to the rate at which the RAM would perform an action, namely refreshing, reading, or storing. SDRAM is available in various frequencies, with the fastest modules capable of running at 266 MHz (High performance modules, targeted at enthusiasts, which perform at higher frequencies, are also available). SD RAM modules have 168 contacts. After the release of DDR SDRAM, the original SDRAM began to be referred to as SDR SDRAM (Single Data Rate SDRAM).