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Semiconductor memory has many different types of memory technology, one of which is DRAM or dynamic RAM. Even within the DRAM family of memories there are several types including asynchronous, synchronous (which is covered separately), EDO, BEDO, FPM and others.
Apart from the memory technology type, the memory can also be contained within several types of IC packages. DRAM is also available in module formats and there are several types of memory module including DIMM, SIMM, RIMM and the like.
In this way it is necessary to have an idea of all the different DRAM types and formats in which the memory can be obtained, installed and used.
When looking at the memory technology itself, there is a good variety of different types of DRAM. The main DRAM types are summarised below:
- Asynchronous DRAM: Asynchronous DRAM is the basic type of DRAM on which all other types are based. Asynchronous DRAMs have connections for power, address inputs, and bidirectional data lines.
Although this type of DRAM is asynchronous, the system is run by a memory controller which is clocked, and this limits the speed of the system to multiples of the clock rate. Nevertheless the operation of the DRAM itself is not synchronous.
There are various types of asynchronous DRAM within the overall family:
- RAS only Refresh, ROR: This is a classic asynchronous DRAM type and it is refreshed by opening each row in turn. The refresh cycles are spread across the overall refresh interval. An external counter is required to refresh the rows sequentially.
- CAS before RAS refresh, CBR: To reduce the level of external circuitry the counter required for the refresh was incorporated into the main chip. This became the standard format for refresh of an asynchronous DRAM. (It is also the only form generally used with SDRAM).
- FPM DRAM: FPM DRAM or Fast Page Mode DRAM was designed to be faster than conventional types of DRAM. As such it was the main type of DRAM used in PCs, although it is now well out of date as it was only able to support memory bus speeds up to about 66 MHz.
- EDO DRAM: Extended Data Out DRAM, EDO DRAM was a form of DRAM that provided a performance increase over FPM DRAM. Yet this type of DRAM was still only able to operate at speeds of up to about 66 MHz.
EDO DRAM is sometimes referred to as Hyper Page Mode enabled DRAM because it is a development of FPM type of DRAM to which it bears many similarities. The EDO DRAM type has the additional feature that a new access cycle could be started while the data output from the previous cycle was still present. This type of DRAM began its data output on the falling edge of /CAS line. However it did not inhibit the output when /CAS line rises. Instead, it held the output valid until either /RAS was dis-asserted, or a new /CAS falling edge selected a different column address. In some instances it was possible to carry out a memory transaction in one clock cycle, or provide an improvement from using three clock cycles to two dependent upon the scenario and memory used.
This provided the opportunity to considerably increase the level of memory performance while also reducing costs.
- BEDO DRAM: The Burst EDO DRAM was a type of DRAM that gave improved performance of the straight EDO DRAM. The advantage of the BEDO DRAM type is that it could process four memory addresses in one burst saving three clock cycles when compared to EDO memory. This was done by adding an on-chip address counter count the next address.
BEDO DRAM also added a pipelined to enable the page-access cycle to be divided into two components:
- the first component accessed the data from the memory array to the output stage
- the second component drove the data bus from this latch at the appropriate logic level
BEDO DRAM provided a significant improvement over previous types of DRAM, but by the time it was introduced, SDRAM had been launched and took the market. Therefore BEDO DRAM was little used.
- SDRAM: Synchronous DRAM is a type of DRAM that is much faster than previous, conventional forms of RAM and DRAM. It operates in a synchronous mode, synchronising with the bus within the CPU.
- RDRAM: This is Rambus DRAM - a type of DRAM that was developed by Rambus Inc, obviously taking its name from the company. It was a competitor to SDRAM and DDR SDRAM, and was able to operate at much faster speeds than previous versions of DRAM.
DRAM IC package types
DRAM memory chips are available in a variety of IC packages. It is found that the DRAM packages used in computers may be different to those found in other electronics equipment as a result of the different requirements.
- Dual-in-line, DIL package - a traditional leaded package for integrated circuits
- SMT - DRAMs are available in a variety of surface mount packages. These conform to all the usual SMT packages and the size and actual format being dependent upon the silicon chip size, the number of leads required and the application for which it is intended.
DRAM memory module formats
Although DRAM is produced as integrated circuits, typically in a surface mount format for mounting onto printed circuit boards, the memory available for use in PCs and other computer applications is often in the format of small modules containing a number of different ICs. These multi-chip modules are available in a number of formats:
- Single In-line Memory Module, SIMM: This type of DRAM or memory package holds up to eight nine RAM chips (8 in Macs and 9 in PCs where the 9th chip is used for parity checking). Another important factor is the bus width, which for SIMMS is 32 bits.
With growing speed of processors and their increasing power has brought about an increase in the bus width. With later processors, e.g. after Intel Pentium, the 64-bit wide bus width requires SIMMs installed in matched pairs to match the data bus and so that the processor can access the two SIMMs simultaneously.
- Dual In-line Memory Module, DIMM: With the increase in data bus width, DIMMs began to replace SIMMs as the predominant type of memory module. The main difference between a SIMM and a DIMM is that a DIMM has separate electrical contacts on each side of the module, while the contacts on a SIMM are on both sides are redundant. Standard SIMMs also have a 32-bit data bus, while standard DIMMs have a 64-bit data bus.
- Rambus In-line Memory Module, RIMM: This type of DRAM memory package is essentially the same as a DIMM but is referred to as RIMMs because of their manufacturer and proprietary slot required.
- Small outline DIMM, SO-DIMM: This type of DRAM package is about half the size of the standard DIMM. Being smaller they are used in small footprint PCs including laptops, netbooks, etc.,
- Small outline RIMM, SO-RIMM: This type of DRAM package is a small version of the RIMM.
As can be seen, there are many different DRAM types, package and module formats. Selecting the correct type can sometimes be daunting, but by narrowing the various applicable types down, the selection can become much easier.