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The first piece of genuine magic in the nature of computers is that a string of binary codes in memory tells the computer what to do, step by step The second piece of that magic is really the jewel in the crown: There are machine instructions that change the order in which machine instructions are fetched and executed In other words, once the CPU has executed a machine instruction that does something useful, the next machine instruction may tell the CPU to go back and play it again and again, and again, as many
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times as necessary The CPU can keep count of the number of times that it has executed that particular instruction or list of instructions and keep repeating them until a prearranged count has been met Or it can arrange to skip certain sequences of machine instructions entirely if they don't need to be executed at all What this means is that the list of machine instructions in memory does not necessarily begin at the top and run without deviation to the bottom The CPU can execute the first 50 or a hundred or a thousand instructions, then jump to the end of the program or jump back to the start and begin again It can skip and bounce up and down the list like a stone tossed over a calm pond It can execute a few instructions up here, then zip down somewhere else and execute a few more instructions, then zip back and pick up where it left off, all without missing a beat or even wasting too much time How is this done Recall that the CPU contains a register that always contains the address of the next instruction to be executed This register, the instruction pointer, is not essentially different from any of the other registers in the CPU Just as a machine instruction can add one to register AX, another machine instruction can add or subtract some number to or from the address stored in the instruction pointer Add 100 to the instruction pointer, and the CPU will instantly skip 100 bytes down the list of machine instructions before it continues Subtract 100 from the address stored in the instruction pointer, and the CPU will instantly jump back 100 bytes up the machine instruction list And finally, the Third Whammy: The CPU can change its course of execution based on the work it has been doing The CPU can decide whether to execute a given instruction or group of instructions, based on values stored in memory, or based on the state of special one-bit CPU registers called flags The CPU can count up how many times it needs to do something, and then do that something that number of times So, not only can you tell the CPU what to do, you can tell it where to go Better, you can sometimes let the CPU, like a faithful bloodhound, sniff out the best course forward in the interest of getting the work done the quickest possible way In 1, I spoke of a computer program being a sequence of steps and tests Most of the machine instructions understood by the CPU are steps, but others are tests The tests are always two-way tests, and in fact the choice of what to do is always the same: Jump or don't jump That's all You can test for any of numerous different conditions, but the choice is always one of jumping to another place in the program, or just keep truckin' along
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The Plan
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I can sum it all up by borrowing one of the most potent metaphors for computing ever uttered: The computer is a box that follows a plan These are the words of Ted Nelson, author of the uncanny book Computer Lib/Dream Machines, and one of those very rare people who have the infuriating habit of being right most of the time You write the plan The computer follows it by passing the instructions, byte by byte, to the CPU At the bottom of it, the process is a hellishly involved electrical chain reaction involving hundreds of thousands of switches composed of many hundreds of thousands or even millions of transistors That part of it, however, is hidden from you so that you don't have to worry about it Once you tell all those heaps of transistors what to do, they'll know how to do it This plan, this list of machine instructions in memory, is your assembly language program The whole point of this book is to teach you to correctly arrange machine instructions in memory for the use of the CPU With any luck at all, by now you'll have a reasonable conceptual understanding of both what computers do and what they are It's time to start looking more closely at the nature of the operations that machine instructions force the CPU to perform
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