Random access

Also called: direct access, random-access

The ability to reach any memory location directly by its address, in about the same time regardless of which location was used before.

Random access means you can jump straight to any address. Give the memory an address, and after the access time you have that word, whether it's next to the previous one or at the other end of the chip.

The opposite is sequential access, like a tape: to reach a spot you must wind past everything in between, so the time depends on where you are and where you're going.

"Random" here doesn't mean unpredictable. It means arbitrary: the order of accesses can be anything, and the memory doesn't care.

Both kinds of semiconductor memory in this course are random-access:

RAM: random-access, read and write
ROM: random-access, read only

The name "RAM" is historical: it was coined when the alternatives were tapes and drums, and it stuck as the name for read/write memory. A ROM is just as random-access as a RAM.

Random access is what the address decoder provides: it selects any word line directly, in one step.

Worked example

Example

Random versus sequential

A program reads locations 9, 2, 7 in that order.

1.
From RAM or ROM: three accesses, each taking the same access time.
2.
From a tape positioned at 0: wind past 0–8 to reach 9, rewind to 2, then wind forward to 7.
3.
The tape's time depends on the order; the memory's doesn't.

Common mistakes

Thinking ROM isn't random-access. Any ROM address can be read directly; "RAM" just means read/write memory.
Reading "random" as unpredictable. It means any address, in any order.

Practice Random access

Interactive questions with instant feedback and a worked solution for every wrong answer.

Memory lesson full course

A ROM is not random-access: its words must be read in order from address 0.

Learn it step by step

Random access is taught in Memory.

Memory →

All terms