Types of DRAM

DRAM is the most popular type of memory used in systems today. It is also the most popular type of memory that computer users are adding to their computers for the purpose of upgrading memory. Therefore, you must understand the different types of DRAM and what types of DRAM outperform others.

Standard DRAM

Memory is organized into rows and columns like a spreadsheet. The information is stored in the different cells or blocks that make up these rows and columns. With standard RAM, the CPU requests data by sending the address of the row and the address of the column for every block of data that needs to be read. The memory controller then fetches the information from that memory location.

Fast Page Mode

Fast Page Mode (FPM) improves on the performance of standard DRAM by not requiring a row address for each request to memory, assuming that the next block of data is on the same row (which in most cases will be true). The following list outlines the steps to access the same two blocks of data, shown in

Extended Data Output

Extended Data Output (EDO) memory is about 10 to 15 percent faster than FPM memory and is usually found on 66 MHz motherboards. With EDO memory, the memory controller can read data from a memory block while listening for the next instruction. This capability increases performance, because the memory controller does not have to wait for the next instruction after reading a block of memory; while it is reading one block of memory, it is receiving the next instruction. With FPM RAM, the reading of one memory block and listening for the next instruction would be done in multiple steps.

Burst Extended Data Output

Burst Extended Data Output (BEDO) is a bursting-type technology. The word burst refers to the fact that when one memory address is requested and that address is retrieved, the system bursts into the next couple of blocks and reads those as well. The theory behind BEDO is that the system has already gone through the trouble of locating that block, and chances are that the next request will be for the next block, so why not take it while we are there? If that extra block is the next requested block from the CPU, the memory controller already has the data and can pass it to the CPU immediately.

BEDO is 50 percent faster than EDO. Because of lack of support from computer manufacturers, however, BEDO has not been used in many systems. It seems that PC manufacturers are using SDRAM instead.

Synchronous DRAM

Synchronous DRAM (SDRAM) is memory synchronized at system speed. This synchronized speed means that the data stored in memory is refreshed at the system speed and data is accessed in memory at the system speed. SDRAM is the popular RAM in systems today. Because you are running at the system speed, however, you must match the RAM with the motherboard speed. Thus, if you have a 100 MHz motherboard, you need 100 MHz SDRAM. If you have a 133 MHz motherboard, you need 133 MHz SDRAM.