OK, since I'm bored and kind...
(This is in very brief, simple terms, so please no corrections from veterans.... It's the principle rather than the actuality that I'm gunning for here, so yes my numbers may not be that accurate)
A cpu is useless without data to work on. This data needs to come from somewhere obviously. If there is no memory, then for each request the CPU makes - even if it asked for exactly the same amount of data 2 seconds ago - the system will have to access the hard disk, physically move the heads on the disk to scan for the information, read the information, and then pass it back through the motherboard to the CPU. It can then do whatever it was going to do with that data, and send the results wherever it needs to (possibly the graphics card, or back to the hard disk), and the whole cycle repeats.
Hard disks are <i>horrendously</i> slow compared to a CPU. Don't forget a cpu can process millions of things a second, whereas a hard disk can only do hundreds or thousands. So for every time the cpu actually gets something to do, it sits there doing nothing for a thousand times as long or more.
If you now get some memory (<i>much</i> faster than a disk - no moving components for one thing) and stick it in between as a 'buffer' (or <i>cache</i> would be a correct term really), then each time the CPU requests the info, and it gets loaded from the disk, it can be stored in the memory so next time it is needed it's more readily available, and the cpu will only waste a few hundred cycles waiting for information.
The Cache on a modern CPU performs exactly the same function, but is between the CPU and the memory, and is much faster than the normal memory, so the cpu only has to wait a couple of cycles if the data resides there.
There's also clever bits in modern systems which also fetch loads of other data when some bits are requested, in anticipation of the CPU's next few requests, so the data is waiting in memory before it's asked for, thus greatly speeding things up.
So you see, <i>that's</i> why people said your computer would be slow without memory.
[/Computing 101]
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