Mainstorage is also called memory or internal memory (to distinguish fromexternal memory, such as hard drives).
RAMis Random Access Memory, and is the basic kind of internal memory. RAM iscalled “random access” because the processor or computer can access any location in memory (as contrastedwith sequential access devices, which must be accessed in order). RAM has beenmade from reed relays, transistors, integrated circuits, magnetic core, oranything that can hold and store binary values (one/zero, plus/minus,open/close, positive/negative, high/low, etc.). Most modern RAM is made fromintegrated circuits. At one time the most common kind of memory in mainframeswas magnetic core, so many older programmers will refer to main memory as corememory even when the RAM is made from more modern technology. Static RAMis called static because it will continue to hold and store information evenwhen power is removed. Magnetic core and reed relays are examples of staticmemory. Dynamic RAM is called dynamic because it loses all data whenpower is removed. Transistors and integrated circuits are examples of dynamicmemory. It is possible to have battery back up for devices that are normallydynamic to turn them into static memory.
ROMis Read Only Memory (it is also random access, but only for reads). ROM istypically used to store thigns that will never change for the life of thecomputer, such as low level portions of an operating system. Some processors(or variations within processor families) might have RAM and/or ROM built intothe same chip as the processor (normally used for processors used in standalonedevices, such as arcade video games, ATMs, microwave ovens, car ignitionsystems, etc.). EPROM is Erasable Programmable Read Only Memory, aspecial kind of ROM that can be erased and reprogrammed with specializedequipment (but not by the processor it is connected to). EPROMs allow makers ofindustrial devices (and other similar equipment) to have the benefits of ROM,yet also allow for updating or upgrading the software without having to buy newROM and throw out the old (the EPROMs are collected, erased and rewrittencentrally, then placed back into the machines).
Registersand flags are a special kind of memory that exists inside a processor.Typically a processor will have several internal registers that are much fasterthan main memory. These registers usually have specialized capabilities forarithmetic, logic, and other operations. Registers are usually fairly small (8,16, 32, or 64 bits for integer data, address, and control registers; 32, 64,96, or 128 bits for floating point registers). Some processors separate integerdata and address registers, while other processors have general purposeregisters that can be used for both data and address purposes. A processor willtypically have one to 32 data or general purpose registers (processors withseparate data and address registers typically split the register set in half).Many processors have special floating point registers (and some processors havegeneral purpose registers that can be used for either integer or floating pointarithmetic). Flags are single bit memory used for testing, comparison, andconditional operations (especially conditional branching).