September 27, 2004
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At a high level memory is just another type of storage medium like paper tape, punched cards, magnetic tape, floppy diskette, hard disk drive, CD, and DVD. These media types are used to store and retrieve data, code, images, audio and visual content and so forth. For each type of media there are corresponding devices for reading/playing and writing/recording. The underlying technologies include mechanical, magnetic, electric, optical and nanotechnology. The combination of storage media and devices are characterized by their
Some of these properties such as shareability are impacted by the operating system and the networking environment. The characteristics listed above make a given media more or less suitable or possibly totally unsuitable for a given application. Any commercial storage product must compete not only within its own type but also across types. Over time most of the storage media-device types have improved in many of these metrics. However, some have become obsolete because they were outperformed by alternatives according to one or more criteria
computer. This drove the need for increased memory capacity for the pc. This combined with the proliferation of personal computers for home and business generated enormous demand for computer memory. The same phenomena were happening in the workstation and computer server arenas.
More recently we have the explosive growth in the market for hand-held, mobile computerized devices such as cell phones, digital cameras, PDAs, and CD and MP3 players. Today end users want to record and play digital images, audio and video files. This translates into the need for storage devices with smaller footprints, greater capacity, lower power consumption and faster speeds than found in personal computers. With the emergence of non-volatile flash memory the previous distinctions between memory and data storage have been blurred.
Memory types can be divided into two main camps: volatile memory that loses its content unless continuously connected to a power source and non-volatile memory that retains its content even after the power source is disconnected or turned off. Volatile memory can be divided into dynamic random access memory (DRAM) and static random access memory (SRAM). Here the term random means that an individual memory cell can be accessed directly rather than sequentially. The most common forms of non-volatile memory are EEPROM (electrically erasable programmable read-only memory) and Flash.
for each memory cell. SRAM does not require refreshing.
described in levels of closeness and accessibility to the microprocessor. The level 1 (L1) cache is the fastest and smallest in capacity, normally between 16KB and 32KB. The L1 or internal cache is built into the same chip as the microprocessor. The level 2 (L2) cache is the second fastest and a bit larger in capacity, normally between 128KB to 512KB for PC usage. Larger computers sometimes have another cache between the L2 cache and main memory called an L3 cache that may range in size from 2 to 256 megabytes.
Extended Data Output RAM (EDO DRAM or EDO RAM) recognizes that most of the time when the CPU requests memory for a particular address, it's going to want some more addresses nearby. Instead of forcing each memory access to start afresh, EDO RAM retains the location of the previous access, thereby speeding access to nearby addresses.
common to all devices and incorporates the command bus, the data bus and a serial control bus for initialization.
SDRAM (Synchronous Dynamic Random Access Memory or Synchronous DRAM) is tied to the system clock and is designed to be able to read or write from memory in burst mode (after the initial read or write latency) at 1 clock cycle per access (zero wait states) at memory bus speeds up to 100 MHz or even higher
DDR SDRAM (Double Data Rate SDRAM) supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput.
DDR SDRAM achieves a data transfer rate that is twice the clock frequency by employing 2-bit prefetch architecture.
DDR2 is nearly identical to DDR. It starts at 400MHz where DDR leaves off. The major improvement include: 4-bit rather than 2-bit prefetch, additive latency, enhanced registers, FBGA packaging, and On Die Termination (ODT) rather than on the motherboard to improve signal integrity in the system. DDR2 has a faster transfer rate, lower supply voltage, lower power consumption, and smaller package size. The 4-bit prefetch effectively doubles the data bus speed while keeping the internal bus speed the same from DDR1. DDR2 requires 240 pins versus 184 pins thereby rendering the two physically incompatible.
and reprogrammed in blocks instead of one byte at a time.
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-- Jack Horgan, EDACafe.com Contributing Editor.
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