RAM and ROM explained

Last Updated on February 5, 2022 by Dave Farquhar

It used to be that chapter one of any book about computers explained RAM and ROM. Based on my conversations with computer professionals younger than me, it doesn’t sound like they do that anymore. So here’s RAM and ROM explained, by a guy who learned the difference in the early 1980s.

First up: ROM

RAM and ROM explained: a ROM chip
The large chip in this Nintendo 64 cartridge is a ROM chip.

ROM stands for Read Only Memory. Outwardly, ROM chips look a lot like RAM chips, just a bit bigger. But they act very differently inside.

Under normal circumstances, the computer can’t change it. It can just read it and run code from it. Every PC has some ROM in it that tells it how to load an operating system from disk and do other fundamental setup. The BIOS or UEFI in your computer is an example of ROM.

In the old days, ROM really was unchangeable. My printer had a bug in its ROM chip that caused it to malfunction occasionally. It would slam the print head up against the edge of the platten and grind its gears. I called the printer manufacturer and asked if they had a new version of my printer’s ROM. They said they did. I pried the chip out of the printer with a screwdriver, mailed it to them, and they sent me a new chip to replace it. Scout’s honor–that’s how we used to have to do things. This was in about 1988.

Enter firmware

Today, you can download what we call a firmware update. Firmware normally acts like ROM, but you can kick a device into a special mode that lets it reprogram the firmware. Your phone does this routinely. When we talk about loading a custom ROM into an Android phone, it’s just loading a new image into the phone’s firmware. If you ever update the BIOS or UEFI on your computer, you’re doing something similar.

ROM is slower than RAM, but its contents survive even with the power off. If you have an old Atari 2600 cartridge from 1977 that nobody has played since the early 1980s, no worries. It has a ROM chip in it, and it’s a very good bet that it still works perfectly. Game cartridges up to the Nintendo 64 days had ROM chips inside.

Second up: RAM

RAM and ROM explained: RAM chips
RAM chips frequently come on plug-in modules like these.

RAM is faster than ROM, and it’s changeable. The acronym RAM stands for random access memory, because they needed something pronounceable. But RAM is a misnomer. You can access ROM randomly too. What really sets RAM apart is that you can read and write to it. As you use your computer, your computer is constantly changing data inside its RAM, to store whatever it is you’re working on, and possibly even to load different program code to help the computer do whatever you need next.

The disadvantage with RAM is that it quickly loses its contents after you turn the computer off. That’s why you have to save your work before you turn off or reboot your computer.

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