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5.4. Static Versus Dynamic RAM

As computers are built faster and faster, all of the components must get faster and faster. Design a system around a 100 MHz chip, and you need a lot of 100 MHz components including RAM. Unfortunately, about the fastest type of common (that is, cheap) RAM doesn't come any faster than 20 MHz.

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5.4.1. Static RAM

The simplest kind of memory to understand is called static RAM (SRAM). It's called "static" RAM because when you put data into it, the data stays there.

To build that kind of memory, you've got to build about six transistors into each bit storage location. That kind of memory-SRAM-can bc quite fast, but also quite expensive. If we used SRAM for our PC memory, then there'd be no trouble with getting memories that kept up with our CPUs. But, on the other hand, we would be able to afford those computers, as SRAM is about 10 times more expensive than the DRAM that we're used to buying.

5.4.2. Dynamic RAM

The economical answer to SRAM was Dynamic RAM (DRAM). Each DRAM built of a single transistor and a capacitor, in contrast to SRAM's six transistor. DRAM has two problems from the point of view of a PC designer. First, the "dynamic" in its name means that it forgets the data that you give it almost as you can give it the data. That means that DRAM-based systems require refresh circuitry to get around this problem. Second, the way that DRAM is built to be cheaper is that DRAM is organized not simply into a set of addresses; rather, each bit in a DRAM has a row address and a column address.

5.4.3. SDRAM

Synchronous Dynamic Random Access Memory, (SDRAM) is a new variant of DRAM that includes an on-chip burst counter. This burst counter can be used to increment column addresses and helps increase SDRAM the speed of burst accesses. Aside from the facts that faster is always better and speed is pursued for its own sake, the reason behind the SDRAM is that CPUs are getting faster. With the increasing of speed, they demand faster memory, in order to function at Its maximum potential. With SDRAM, the CPU and RAM are locked together same clock. Thus, the speed of the RAM and the CPU are linked, or synchronized.