Why can’t you buy external ram that plugs into a usb like you can for an ssd?
In: Engineering
RAM is very high speed memory that sits very close to the CPU. Think of the CPU as a worker making custom widgets on an assembly line where they need to be constantly supplied with parts. Because they are custom ordered they don’t know which parts they will need for the next widget until it arrives in front of them so the parts must be located very close to them. The time it takes for the part to show up at their station on the assembly line is called latency. RAM is like a coworker in the factory bringing parts to the CPU. If the latency is too high the CPU stalls and sits idle waiting to begin working again until the RAM can provide them with the next thing they need. Although RAM still causes the CPU to stall while it is fetching the next part, it can get it quite quickly because it is located close to the CPU.
Now imagine that we tried to save some money buy not having RAM in the factory and instead just ordered everything online (e.g. from Amazon.) Yes, you can gets things in a day or two but if the CPU can normally make 10 items per day but now has to wait a day for the parts to arrive it will be bottlenecked by the wait time and slow down tremendously.
That’s basically what’s happening inside your PC all the time so there are many layers of storage that are designed to keep a CPU working all the time. The closer the memory the faster it can respond but the more it will cost so you try to find a balance.
The very fast cache memory is right on the chip with the CPU but you only get a few megabytes then next we have RAM where you can have gigabytes followed by solid state drive (SSD) / hard disk drive (HDD) or network storage where you can have terabytes.
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.
You can, kind of.
[https://en.wikipedia.org/wiki/ReadyBoost](https://en.wikipedia.org/wiki/ReadyBoost)
But it’s massively limited in the data throughput. DDR4 RAM has a throughput of up to 25 GB/s. USB 3.0 comes in at a paltry 625MB/s, less than 1/40th of the speed. USB can’t even compare with the first version of DDR RAM from 20 years ago, which was still 1600MB/s.
It’s analogous to attempting to route a river through your kitchen faucet. There just isn’t enough space.
Ram is what the cpu speak with directly, usb is too slow and the cpu can’t access it directly so it’s not possible
Interestingly enough you could buy ram expansion packs on some computers back in the 1980s, as computers often hard connectors directly connected to the cpu for game cartridges
Everything in a computer needs to talk to the CPU to do anything. RAM sits like a fire truck next door to your CPU. It can fight a fire at your home at a moment’s notice. SSDs are sitting across the country in NYC while the CPU is in LA. It has to travel across the country to fight the fire. The USB interface is in a similar situation, but is technically even further in London. For stuff you do on the SSD, this distance and travel time isn’t as big of an issue. But for RAM, it is.
In more technical terms, almost everything your CPU does needs to work with data. RAM and your SSD store said data. RAM will give your CPU the data it needs on the order of tens of CPU clock cycles. An SSD can in thousands of CPU clock cycles. RAM also sits right next to your CPU in your computer, which means it has fewer chips and wires to go through before it gets to your CPU. SSDs and USB sit much further away, needing to go through several chips and wires to get to your CPU.
No port on your computer is fast enough to make that work well.
The point of RAM is that it is extremely fast even though it is low density, it is orders of magnitude faster than an SSD and several orders of magnitude faster than a hard drive.
RAM can be accessed in tens of nanoseconds, SSDs take up to 100 us (~1000x longer), and HDDs take 10 ms (100x longer than SSDs)
Then you’ve got to deal with bandwidth. A DDR4-3200 RAM stick can transfer 25.6 GB/s, while the maximum bandwidth for DisplayPort using all 4 lanes is just 9.6 GB/s. That’s the fastest external port you have on a computer and it has only half the bandwidth of your CPU’s connection to a single RAM stick, plus a lot of overhead in the process so it’d raise your access times to the tens of microseconds range making it a very expensive and low density SSD.
RAM only works because it is right next to the CPU with a couple hundred little signal wires running right to it, you simply cannot get the performance required over a longer range.
RAM has to be very fast to access. This is why it is mounted directly on the motherboard. Any of the external ports are inherently slower because they are dynamic.
The whole purpose of ram is to have a verry fast storage to temporarily store data that’s currently needed. Usb does not have anywhere near the transfer speeds needed to serve as ram
You can always just hook up a SSD and tell the OS to place a page file on it.
Problem is that even the fastest internal SSDs are FAR slower than RAM. The OS knows this and strategically handles moving data to/from a page file in what it thinks will be the most optimal way.
USB just adds even more steps between the drive and RAM, so I don’t think Windows even allows this anymore.
No, USB isn’t fast enough to make viable RAM access technology.
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