Packaging
Memory chips are called DIPs which stands for Dual Inline Packages. They are black chips with pins on both sides. Some say they look like black bugs. To make memory installation easier than it was in the past, these DIP chips were places on modules. There are two main module types that memory comes packaged on today.
SIMM - Single Inline Memory Module. They may have DIPs on one or both sides and will have 30 or 72 pins. Today, they normally are available in the 72 pin size which supports a 32 bit data transfer between the microprocessor and the memory.
DIMM - Double Inline Memory Module. The modules have 168 pins and support a 64 bit data transfer between the microprocessor and the memory. Synchronous Dynamic Access Memory (SDRAM) is the type of memory that is found on DIMM packages. The term SDRAM describes the memory type, and the term DIMM describes the package. These modules are available in 3.3 or 5 volt types and buffered or unbuffered memory. This allows four choices of DIMM types. You should check your motherboard manual to determine the type of memory required. You should be able to find this information on the motherboard manufacturers website before buying the motherboard. The most common choice for todays motherboards is 3.3 volt unbuffered DIMMs.
To install these packages, you press them into the socket on the motherboard and latch them in with a plastic latch on both sides. Normally as the memory module is pressed into place the latch will automatically latch the module in place. This is the essential knowledge required to understand enough to buy and install memory on your motherboard. The following sections give further technical details.
Types of DRAM
The term DRAM stands for Dynamic Random Access Memory. There are three common types of DRAM today.
FPM DRAM - Fast Page Mode DRAM. When the first memory access is done, the row or page of the memory is specified. Once this is done, FPO DRAM allows any other row of memory to be accessed without specifying the row number. This speeds up access time.
EDO DRAM - Extended Data Out DRAM. This works like FPO DRAM but it holds the data valid even after strobe signals have gone inactive. This allows the microprocessor to request memory, and it does not need to wait for the memory to become valid. It can do other tasking, then come back later to get the data.
SDRAM - Synchronized DRAM inputs and outputs its data synchronized to a clock that runs at some fraction of the microprocessor speed. SDRAM is the fastest of these three types of DRAM. There is a new SDRAM called DDR (Double Data Rate) SDRAM which allows data reads on both the rising and falling edge of the synchronized clock.
Another new type of DRAM is called RDRAM developed by Rambus, Inc. It uses a high bandwidth channel to transmit data at very high rates. It attempts to eliminate the time it takes to access memory. Synclink DRAM (SLDRAM) competes with RDRAM and uses 16 bank architecture rather than 4 along with other performance enhancing improvements.
Cache Memory
Cache memory is special memory that operates much faster than SDRAM memory. It is also more expensive. It would be impractical to use this memory for the entire system both for reasons of expense and physical board and bus channel design requirements. Cache memory lies between the microprocessor and the system RAM. It is used as a buffer to reduce the time of memory access. There are two levels to this memory called L1 (level 1) and L2 (level 2). The level 1 memory is a part of the microprocessor, and the level 2 memory is just outside the microprocessor.
