Obviously, four (or even eight) registers aren't even close to the theoretically infinite storage space I've been talking about throughout this article. In order to make a viable computer that does useful work, you need to be able to store much larger data sets. This is where main memory, or RAM, comes in. Main memory stores the data set on which the computer operates, and only a small portion of that data set at a time is moved to the registers for easy access from the ALU. To return to our file clerk metaphor, we can think of main memory as the filing cabinet and the registers as the desktop where the file clerk (the ALU) temporarily places the papers on which it's currently working. So the computer must first retrieve a file from the cabinet (memory) and place it on the desktop (the registers) before it can manipulate it.
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