Random-access memory (RAM): any location can be accessed in a fixed, short amount of time, independent of which location. The opposite is sequential-access memory (like tape) where reaching a location far down the medium takes longer than one near the start.
The name is misleading. It doesn’t mean access is random, it means access time is the same regardless of location. You can hop around freely without paying a position-dependent cost.
In modern systems “RAM” usually means the volatile semiconductor main memory of a computer, the DRAM chips on a motherboard. But cache, registers, and even some SSDs are also “random access” in the original sense. The narrower usage is just convention.
Properties
A typical RAM chip:
- Word-addressable internally, exposed as byte-addressable to the processor (each address picks one 8-bit byte). The internal access width varies in practice: DRAM rows are hundreds to thousands of bits wide, and SDRAM bursts return multiple words per command, so the word-addressable picture is a simplification.
- Address space of locations for address bits.
- Access time in nanoseconds for current devices, 10 ns for SRAM, 100 ns for DRAM.
- Volatile: data is lost when power is removed.
Memory is organized so that any location is retrieved with the same control sequence: apply the address, assert Read, get the data. No “seek” needed.
Two main flavors of semiconductor RAM
- SRAM — fast (~ ns), uses transistors per cell, expensive per bit. Used for processor registers and cache.
- DRAM — slower (~ ns), uses transistor + capacitor per cell, much cheaper per bit. Used for main memory.
Both are volatile and both offer true random access. The trade-off is speed vs density: more transistors per cell means each cell takes more silicon, so chips of a given size hold less data, but they access it faster.