Internal storage is also known as main, primary, or (for historical reasons) core storage, or, memory. It is used to hold those instructions and data required at any moment while a program is running, which must be available instantaneously. Silicon semiconductor chips are now invariably used for internal storage; these are categorised as either RAM (random access memory) or ROM (read only memory) chips. ROMs are used for data which is never altered for example, a computer's operating system instructions, while RAM is used for data which is liable to change often, and instructions used in the execution of a program. Internal storage capacity is measured in. bits or bytes and multiples thereof. The size of individual chips comprising the internal memory has rapidly increased from 4 KB (Kilo Byte chip to 16 MB (Mega Byte) chips.