Why can’t you buy external ram that plugs into a usb like you can for an ssd?
In: Engineering
This explanation might go above the ELI5 format, but I’ll try to keep it as simple as I can. This question involves how computers are built, and it’s a complicated question. I’ll put a TL:DR at the bottom with a super simple explanation though.
Your computer has 3 different types of memory.
1. Storage memory (hard drive, SSD, USB drive, etc)
2. RAM
3. Cache
Storage memory is the slowest form of memory. It is designed for long term, permanent storage of information. While storage memory has gotten faster in recent years, especially with SSDs, it’s focus is on making a good permanent copy of the data that doesn’t go away when you turn off the power.
RAM memory is the next faster memory. It is designed for temporary storage of information while a program runs. When you run a program on your computer, your computer will copy that program from your storage memory into RAM, and it will set up temporary spaces for that program to store information while it runs. To simplify an example, the location of your character in a video game might be one of the temporary storage locations. The color of his hat might be another.
Cache memory is the fastest memory in your computer. It’s typically built right into the processor and is probably only a few megabytes of storage. Your processor works by doing billions of math equations every second. It needs a place to store the numbers it is doing the math operation on, and a place to store the result of those operations. This is the cache memory. So if you wanted your processor to do 2+3 as a math operation, you would need a place to store the numbers 2, 3 and 5. This memory has to be REALLY fast, or your computer won’t be able to process billions of math operations in a second.
So what prevents us from having external memory via USB? It is how each of these memory devices are accessed. To help understand how these devices are connected, [open this image while you read the next few paragraphs.](https://en.wikipedia.org/wiki/Northbridge_(computing)#/media/File:Motherboard_diagram.svg)
In a processor cache, the memory is inside of the processor (CPU). This makes it as fast as possible.
In RAM, the memory is connected to a device called the northbridge. The northbridge is connected to the processor and is in charge of controlling the fastest devices that need the quickest access to the processor. This is typically your RAM and your graphics card.
All of the other hardware is connected to a device called the southbridge. The southbridge handles devices that are a bit slower. This includes your storage memory, your USB ports, your ethernet, your audio, and your peripherals. Even with the southbridge, you typically have controllers for each of these devices that are even slower.
Each step you get away from the processor makes the speed of the memory slower. USB is just too far away from the processor and this makes it WAY too slow to use for RAM. Your RAM needs to be quickly accessed by the processor, and it can’t do that if it’s on USB.
TL;DR/ELI5: USB is a slow connection that goes through a lot of devices to get to your processor. RAM needs to be much faster than what USB can deliver.
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