4 Machine Language Nand2Tetris（4机器语言Nand2Tetris）.pdfVIP

4 Machine Language Make everything as simple as possible, but not simpler. —Albert Einstein (1879–1955) A computer can be described constructively, by laying out its hardware platform and explaining how it is built from low-level chips. A computer can also be described abstractly, by specifying and demonstrating its machine language capabilities. And indeed, it is convenient to get acquainted with a new computer system by first seeing some low-level programs written in its machine language. This helps us understand not only how to program the computer to do useful things, but also why its hard- ware was designed in a certain way. With that in mind, this chapter focuses on low- level programming in machine language. This sets the stage for chapter 5, where we complete the construction of a general-purpose computer designed to run machine language programs. This computer will be constructed from the chip set built in chapters 1–3. A machine language is an agreed-upon formalism, designed to code low-level programs as series of machine instructions. Using these instructions, the programmer can command the processor to perform arithmetic and logic operations, fetch and store values from and to the memory, move values from one register to another, test Boolean conditions, and so on. As opposed to high-level languages, whose basic design goals are generality and power of expression, the goal of machine language’s design is direct execution in, and total contr