Basic Computer Structure and the 6812 (Microcontrollers)

Computers, and microcomputers in particular, are among the most useful tools that humans have developed. They are not the news media’s mysterious half-human forces implied by “The computer will decide . . .” or “It was a computer error!” No, computers are actually like levers; as a lever amplifies what the human arm can do, so the computer amplifies what the human brain can do. Good commands are amplified, and the computer is a great tool, but bad commands are likewise amplified, and good commands incorrectly programmed are also amplified. “To err is human, but to really foul things up, you need a computer.” You have to study and exercise this tool to make it useful; that is the purpose of this topic. The computer also has to be used with insight and consideration for its effects on society, but that will not be studied in this topic.
We shall study the computer as an engineer studies any tool—we begin by finding out just how it ticks. We make our discussion concrete using the well-designed Motorola 6812 microcomputer, as a means of teaching the operations of computers in general. In this topic we introduce basic computer structure. We discuss memory, how memory words are read to tell the microcomputer what to do, and how these words are written and read to save the microcomputer’s data. Finally, we describe a small but useful subset of 6812 instructions to show how a computer reads and carries out an instruction and a program, to introduce the idea of programming.
After reading this topic, you should be able to approach a typical instruction, to be introduced in the next two topics, with an understanding about what the mnemonic, the machine code, and a sequence of memory reads and writes may mean for that instruction. This topic then provides background for the discussion of instructions that we will present in the next two topics.

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