Random Access Memory: Difference between revisions

While the [[CPU]] is a heart of a computer system, moving the data to and from and processing it as required, this data still needs to be held ''somewhere''. That's where RAM comes into play. RAM stands for '''R'''andom '''A'''ccess '''M'''emory -- any place in the memory can be written at any time without having to wait. This contrasts with ''sequential access memory'', where you have to rewind or fast-forward a tape or wait for a certain time to access data.

Well before modern memory types became available, early machines still needed to store their data -- even the ENIAC, which didn't even had storable program (it was controlled by sequentially wiring all the modules together) had some storage for data. Initially this was the very straightforward and obvious solution -- static memory, that is, keeping the data in the electronic circuits named triggers, or flip-flops, that could remain in one of the two stable states. But because word size in those early machines was somewhere between 20 to 40 bits, and one flip-flop can hold at most two bits of information, while requiring at least four electronic valves at the time when the only available type of valve was a huge and fragile vacuum tube, having more than couple dozens of such "registers" was simply impractical.

That's where everything got interesting. To hold bigger amounts of data several technologies were used, some of them being decidedly odd. Like storing the data as an acoustic waves (yes, bursts of sound) in mercury-filled tubes, or magnetic pulses on a rotating drum. Technically, these types of memory weren't even random-access, they were sequential, but they simulated RAM relatively well. Then there was a technology where the bits were stored as dots on the phosphor surface of a CRT -- which had the advantage that the programmer could literally ''see'' the bits stored, which often helped in the software debugging.

Core memory consisted of thousands of tiny (1-2 mm wide) donut-shaped magnetic cores, set on a grid of metal wires. By manipulating the voltages put on these wires, the state of any individual core could be read or written. Since there were no moving parts, as with a delay line or a drum, access time was much quicker. Core memory was also substantially denser than either delay-line or CRT memory, and used less power as well. It also held its content when the power was off, which was widely used at the time.

* '''Static RAM''', as mentioned above, is a type of memory where each bit is represented by a state of a certain type of circuit called a ''flip-flop''. With one IC replacing several transistors and their attendant circuitry, static memory became much more affordable, and started appearing in larger and larger amounts. The main advantage of static memory is that it's very quick -- basically, its speed is only limited by the speed of the physical processes inside the transistor, and these are ''extremely'' rapid. It also requires power only to write something, and takes only a token amount when reading or storing, so it dissipates almost no heat. But still, each bit of static memory takes two to four transistors to store, so it remains relatively bulky and expensive.

* '''Dynamic RAM''', on the other hand, uses capacitors to store bits (it requires generally one capacitor and, maybe, one diode to store one bit, which takes much less silicon space), so it's much more compact and thus cheap. Unfortunately, capacitors tend to lose charge over time, so they have to be periodically recharged, usually by reading the memory and writing the same data again, called "memory refresh". This process takes either the attention of the CPU, or the additional support circuitry on the memory chip itself, and, to add insult to injury, the need to constantly refresh the memory contents means that when the power gets turned off, all memory gets completely erased -- core, being magnetic, was completely non-volatile, and static RAM required so little power that it could be kept alive with a simple lithium watch battery. Still, the enormous density that DRAM offers makes it the most affordable and used type of the memory ever.