DOWN AND UP FROM MACHINE LANGUAGE

This chapter reviews the registers in primary memory and in the central processing unit (CPU) required for executing machine-language instructions. The chapter discusses nine such instructions. The hardware activity required to fetch an instruction from memory is explored, and a detailed analysis of the execution of two of the nine instructions is presented. Machine language uses instructions that the hardware of the computer is wired up to execute directly. A test tells the computer to test whether some specified condition is met. An operation tells the computer how to change some information. The study of how the CPU executes the two kinds of instructions—test and operate—is enough to give one a general idea of all machine-language processing. The CPU has a special register called the accumulator that it uses to hold the data and the results on which it is working. Instructions and data are stored in the memory. For this reason, the CPU must also keep a register—called the instruction counter—that tells it where to go for its next instruction.