Textbooks will have you believe this is a simple question with an equally simple answer. And it is in theory, but there's quite a bit of nuance to unpack.

Fundamentally, random access memory (RAM) is the memory your device thinks with, and read-only memory (ROM) is the memory it remembers with. RAM holds whatever the processor is working on and it's all lost when power is cut; ROM stores the permanent instructions a device needs to start up with, retaining them regardless of power state.

But hardly anything in a modern device is truly read-only anymore. That includes the bits still named after ROM, such as firmware chips in PCs and the "ROM" Xiaomi lists on spec sheets for phones like the Redmi 13.

Everything you open on a device involves RAM. When you open a browser tab, for example, it gets copied from storage into RAM, allowing the processor to reach it quickly (a matter of nanoseconds). DRAM is the most dominant type of RAM, and it stores each bit as a charge in a tiny capacitor that gets refreshed thousands of times per second. It can't be refreshed if power is cut, so if that happens, everything stored there disappears.

If you're a PC enthusiast or have built your own rig, you'll probably be familiar with the (now wildly expensive) DDR5 sticks they use. Phones and thin laptops use a different type (LPDDR) that's soldered to the board, while graphics cards cluster GDDR chips around the GPU. Higher RAM capacity means you can run more apps, open more tabs and play more powerful games before the system starts swapping to slower storage.

We saw conventional DRAM contract prices rise by a staggering 90 to 95 percent quarter-over-quarter at the beginning of the year (according to TrendForce). That's because AI data centers are absorbing virtually all the supply as companies race to get them built, like NVIDIA's plans to invest up to $100 billion in OpenAI to build 10 gigawatts of AI data centers. While that specific deal is stalled, it's one illustration of many to show what's driving prices up to highs never seen before and cutting supplies to regular consumers. 

Last year, Micron announced it would kill its Crucial consumer brand so it can focus more on high-margin AI data center demand. It ceased operations back in February. This isn't the first time a memory crunch has hit consumers: Huawei blamed slower P10 devices on a memory shortage back in 2017, after it discretely mixed faster and slower flash chips in the same phone model.

The earliest ROM chips had their data physically built into the circuitry, so any changes meant going back to the factory for new silicon. Programmable ROM (PROM) got around this in 1956. Its inventor, Wen Tsing Chow of American Bosch Arma, designed it so the US Air Force could load targeting data into missile guidance computers by blowing microscopic fuses inside the chip. But since you can only blow a fuse once, every write to PROM was permanent, including any inevitable mistakes.

Intel engineer Dov Frohman then came along in 1971 with the EPROM, an erasable chip he developed after investigating a reliability fault in another Intel product. Each EPROM carried a small quartz window, and shining UV light through it for a few minutes wiped the chip clean for reprogramming. EEPROM later dropped this by making erasure electrical. Fujio Masuoka of Toshiba then pushed the idea to its conclusion with flash memory, which was introduced as NOR flash in 1984 and followed with NAND flash in 1987. You'll now find NAND in every SSD, smartphone, memory card and USB on the market.

Even though the firmware chip on your PC's motherboard is still called the BIOS ROM, it's actually a small flash chip that gets rewritten with firmware updates. It's the same story on your phone, where the ROM Xiaomi lists for the Redmi 13 is regular UFS or eMMC flash with system partitions locked to read-only. Fun fact: You might have come across proper ROM chips in the form of old plastic game cartridges. That's why the files ripped from them for emulators are called ROMs.

Proper factory-set ROM does still exist, but you're only going to find it in the deepest depths of cheap electronics; tiny microcontrollers running code that'll never change. Everywhere else, RAM still means what it did six decades ago, while ROM has become the de facto shorthand for whatever memory you're not supposed to touch.