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CPU's in general - what are they?

  1. Nov 23, 2011 #1
    Hello, I've done different types of programming and worked with basic electronics, but I've been curious about CPU's. I know some about how they work, but I really have no knowledge about what they are physically. I have a lot of other related questions too, so any comments are welcome.

    the basic question - what is a CPU physically? what is actually in that ~1"x1" square?

    How was windows DOS (or the first OS if not DOS) created? assuming there was no prior "structure" for commands/operations?
    It had to be 'hard wired' in the circuits, right? how do you 'hard wire' something like that?

    Any comments, advice, or related links are much appreciated!
  2. jcsd
  3. Nov 23, 2011 #2
    Inside a cpu is the physical implementation of digital logic circuits. Digital logic circuits perform boolean logic with different circuits, and also memory functions with flip-flops or some other solid-state physical method. The basic building block of logic circuits is called an inverter, which takes an input and outputs the opposite meaning of its input. So in terms of voltage, it takes a 1 and a 0 is at the output. You can wire inverters together to make nand gates and other logic. This inverter can be implemented very simply with transistors: MOSFET is the common technology now, and the topology used is CMOS inverters. You can read about those on wikipedia.

    There are different physical ways to implement these circuits, but in a cpu most of it is done with transistors which are made with semiconductor material like silicon and doped silicon, and also with metal deposited and other material layers to make the connections.

    Usually these transistors are made in 2-D layers on top of a silicon substrate, and the different layers are piled onto eachother. The patterns of the layers are made with photolitography or electron beam etching or other methods, but photolithography is the most common method where a polymer photoresist has the pattern and light causes the polymer molecules to either chain or break up depending if the photoresist will be positive or a negative (like the negative of a photograph). The actual layers are applied with deposition techniques or sputtering. All of this level is very physics/chemistry focused rather than electrical engineering. This is speaking in nanometer scales, so we are talking about atoms and molecules here.

    So to sum it up, transistors are made in layers on a silicon crystal, and they are put on it in patterns that connect the transistors as circuits. The circuits are designed to implement inverters or into more complex digital boolean logic functions. The boolean logic is what a computer's different functions are made out of.
    Last edited: Nov 23, 2011
  4. Nov 23, 2011 #3
    Here is a bad diagram but it is better than nothing, i just don't have time to make it clear and filtered with the proper information. just random thoughts for each level.

    Attached Files:

  5. Nov 23, 2011 #4


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    Staff: Mentor

    Back in the digital stone age the first machines had a set of toggle switches on their front panels that one could use to force bit patterns into the memory manually (set up an address in binary, set up a bit pattern in binary, press a "load" button, repeat). After the memory was loaded another switch forced the CPU to start running the code just entered at a memory address specified by the switches. One could also single-step instruction by instruction from the front panel.

    Usually the first program to be entered was one that would cause additional programs to be loaded automatically from some device such as a paper tape reader or card reader or magnetic tape reader. Those programs could be more complex and handle devices like disks, know something about filesystems, etc. The whole process was called 'bootstrapping', a reference to 'pulling oneself up by one's own bootstraps'. We still talk about 'booting' a computer today.

    With a primitive operating system in place, the computer itself could be used as a tool for creating more complex programs, including assemblers and compilers. These could be written to external devices for later retrieval. Thus the process builds upon itself.

    With standardization of machine architectures and instruction sets, eventually the primitive programs required to 'boot' the computer were cast into read-only memory chips. Thus the "BIOS" (Basic Input Output Subroutines) were born. The front panel switches went away.

    These days you buy a computer that has a BIOS built in, and with an operating system pre-installed on its hard disk. With suitable intervention at "boot time", the machines are still capable of loading an operating system from an alternate device such as a floppy disk or CD-ROM drive.
  6. Nov 23, 2011 #5
    thanks fellas! much appreciated
    very interesting stuff
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