Computer Memory

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.

1. 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.
2. 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.

DRAM Access

DRAM memory is is accessed in chunks called cells. Every cell contains a certain number of bits or bytes. A row, column scheme is used to specify the section being accessed. The cells are arranged similar to the following table.

ROW 1, COL 1	ROW 1, COL 2	ROW 1, COL 3	ROW 1, COL 4
ROW 2, COL 1	ROW 2, COL 2	ROW 2, COL 3	ROW 2, COL 4
ROW 3, COL 1	ROW 3, COL 2	ROW 3, COL 3	ROW 3, COL 4
ROW 4, COL 1	ROW 4, COL 2	ROW 4, COL 3	ROW 4, COL 4

When the DRAM is accessed, the row, then the column address is specified. A page in memory is considered to be the memory available in the row.

Types of DRAM

The term DRAM stands for Dynamic Random Access Memory. There are three common types of DRAM today.

1. 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.
2. 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.
3. 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.

The summarised overview of different DRAM chips is shown in the table below:

Type	Where Used	Description and Facts
DRAM Dynamic Random Access Memory	Main System Memory and Video Memory	DRAM is the most common type of computer memory. DRAMs are used mostly in main memory systems. DRAM is volatile and slow, but inexpensive. DRAM is used for memory read and writes and the data must be refreshed after each transfer.
FPM DRAM Fast Page Mode DRAM	Main System Memory and Video Memory	FPM is also commonly used as the main system memory. As microprocessor speeds advanced, more memory throughput was required, resulting in the development of the Fast Page Mode DRAM. FPM DRAM is similar to regular DRAM but faster.
EDO RAM Extended Data Output RAM	Main System Memory and Video Memory	EDO is an improvement on the FPM design. Depending on the type of system and the software applications, EDO can provide approximately a 3% performance gain over FPM DRAM if a secondary (L2) cache is in the system, and approximately a 10% to 12% performance gain if the L2 cache is not present.
SDRAM Synchronous DRAM	Main System Memory and Video Memory	Synchronous DRAM is a new memory with improved performance, simpler design, and faster transfer rates than standard DRAM.
SRAM Static RAM	Cache Memory	SRAM is an extremely fast device that does not require periodic refreshing. SRAM is used primarily in cache memories. SRAM is faster, bigger, and generates more heat than DRAM. It is also more expensive than DRAM.
EDRAM Enhanced DRAM	Cache Memory	EDRAM is positioned as a high-performance specialty memory that combines DRAM and SRAM caches on one chip.
VRAM Video RAM	Video Memory	VRAM, specialized RAM for video, offers higher performance than DRAM. VRAM is dual ported, which allows simultaneous reads and writes of data. It requires a larger package than DRAM, and is more expensive than DRAM, too.
WRAM Windows RAM	Video Memory	WRAM technology evolved from its predecessor, VRAM. Named after its ability to offer full-motion video, WRAM comes in a smaller package than VRAM. It has added intelligence that makes it perform up to 50% faster than VRAM.
RDRAM RAMBUS DRAM	Video Memory	Future generation of DRAM with performance increases of up to 20 fold over standard DRAM. With costs just over 10% of standard DRAM, it is gaining acceptance in the industry.

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.