The memory is characterized on the basis of two key factors: capacity and access time. Capacity is the amount of information (in bits) that a memory can store. Access time is the time interval between the read/ write request and the availability of data. The lesser the access time, the faster is the speed of memory. Ideally, we want the memory with fastest speed and largest capacity. However, the cost of fast memory is very high. The computer uses a hierarchy of memory that is organized in a manner to enable the fastest speed and largest capacity of memory.
Fig: Memory Hierarchy
The internal memory and external memory are the two broad categories of memory used in the computer.
Internal memory
It consists of the CPU registers, cache memory and primary memory. The internal memory is used by the CPU to perform the computing tasks. The primary memory is further of two kinds: RAM and ROM. Registers are the fastest and the most expensive among all the memory types. The registers are located inside the CPU, and are directly accessible by the CPU. Cache memory is next in the hierarchy and is placed between the CPU and the main memory. Any program or data that has to be executed must be brought into RAM from the secondary memory. Primary memory is relatively slower than the cache memory.
External memory
It is also called the secondary memory. The secondary memory is used to store the large amount of data and the software. It stores data and instructions that are not currently being used by CPU but may be required later for processing, Magnetic disk, optical disk, and magnetic tape constitute the internal memory.
Internal Memory | External Memory |
limited storage capacity | Very high storage capacity |
Temporary storage | Permanent storage (non-volatile), unless erased by user |
Fast access | Relatively slower access |
High cost | Cheapest among all memory |
