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Well, then I have to agree the program is nothing more than a sequence of plain characters, stored in a text file. This abstraction is a complete mystery for the computer, which understands only instructions written in machine language. Thus, if we want to execute this program, the first thing we must do is parse the string of characters of which the high-level code is made, uncover its semantics—figure out what the program seeks to do—and then generate low-level code that reexpresses this semantics using the machine language of the target computer. The result of this elaborate translation process, known as compilation, will be an executable sequence of machine language instructions.
Of course, machine language is also an abstraction—an agreed upon set of binary codes. To make this abstraction concrete, it must be realized by some hardware architecture. And this architecture, in turn, is implemented by a certain set of chips—registers, memory units, adders, and so on. Now, every one of these hardware devices is constructed from lower-level, elementary logic gates. And these gates, in turn, can be built from primitive gates like Nand and Nor. These primitive gates are very low in the hierarchy, but they, too, are made of several switching devices, typically implemented by transistors. And each transistor is made of—Well, we won’t go further than that, because that’s where computer science ends and physics starts.
Excerpt from https://www.nand2tetris.org/book |