RAM stands for Random Access Memory. It randomly selects and uses any location of the memory directly to store and retrieve data. It takes same time to retrieve any address of the memory as it takes for the first address. It is also called read/write memory. The storage of data and instructions inside the primary storage is temporary, i.e. data or instruction stored in RAM disappears from RAM as soon as the power to the computer is switched off. So now we can say that RAM is a volatile memory. You can store and retrieve data much faster with primary memory compared to secondary memory. Secondary memory such as floppy disk, magnetic disk, etc., are external and located outside the motherboard whereas RAM is located on the motherboard.
RAM is also referred to as Dynamic Random Access Memory (DRAM). It consists of chips of memory, either in short chips (Dual In-line Pin (DIP) chips) or in memory modules. Memory modules are either “Single Inline Memory Modules” SIMMs or “Dual In-Line Memory Modules” DIMMs. A single 30-pin SIMM can hold up to 4MB and a 72- pin SIMM can hold up to 32 MB of memory. Nowadays, DRAM comes in either DDR, DDR2 or DDR3 form.
The term “DDR” stands for Double Data Rate. DDRSDRAM was capable of two data transfers per clock cycle, giving it twice the theoretical peak bandwidth of previous SDRAM (Synchronous Dynamic Random Access Memory or Synchronous DRAM is a type of memory that synchronizes itself with the computer’s system clock). DDR2 RAM provides 4 data transfers per cycle, while DDR3 increases the number to 8. Assuming a base clock speed of 100Mhz, DDR RAM will provide 1600 MB/s of Bandwidth, DDR2 provides 3200 MB/s, and DDR3 provides 6400 MB/s.
These are also known as Magnetic Disk. It is an external storage unit that is located inside the system box. It looks like as given in Fig. 4.6. You might have seen the gramophone record, which is circular like a disk. Magnetic disks used in computer are made on the same principle. It rotates with very high speed inside the disk drive. Data are stored on both the surface of the disk. Each disk consists of a number of invisible concentric circles called tracks. Information is recorded on tracks of a disk surface in the form of tiny magnetic spots. The presence of a magnetic spot represents one bit (1) and its absence represents zero bit (0). The information stored in a disk can be read many times without affecting the stored data.