1.2 Hardware: Understanding Main Memory
Of all of the hardware on a computer system, main memory is perhaps the most important. In its rawest form, main memory is just a big storage area for a bunch of 0s and 1s. Each slot for a 0 or 1 is called a bit. This is where the running kernel and processes reside—they’re just big collections of bits. All input and output from peripheral devices flows through main memory, also as a bunch of bits. A CPU is just an operator on memory; it reads its instructions and data from the memory and writes data back out to the memory.
You’ll often hear the term state in reference to memory, processes, the kernel, and other parts of a computer system. Strictly speaking, a state is a particular arrangement of bits. For example, if you have four bits in your memory, 0110, 0001, and 1011 represent three different states.
When you consider that a single process can easily consist of millions of bits in memory, it’s often easier to use abstract terms when talking about states. Instead of describing a state using bits, you describe what something has done or is doing at the moment. For example, you might say, “The process is waiting for input” or, “The process is performing Stage 2 of its startup.”
Because it’s common to refer to the state in abstract terms rather than to the actual bits, the term image refers to a particular physical arrangement of bits.