In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture. This leads to the important fact that entire programs (which are just lists of instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer just as if they were numeric data. Since the computer's memory is able to store numbers, it can also store the instruction codes. The simplest computers are able to perform any of a handful of different instructions, the more complex computers have several hundred to choose from - each with a unique numerical code. The command to add two numbers together would have one opcode, the command to multiply them would have a different opcode and so on. In most computers, individual instructions are stored as machine code with each instruction being given a unique number (its operation code or opcode for short). However, computer hardware may fail or may itself have a fundamental problem that produces unexpected results in certain situations. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design. Sometimes otherwise benign bugs may be used for malicious intent, creating a security vulnerability. Sometimes bugs are benign and do not affect the usefulness of the program, in other cases they might cause the program to completely fail ( crash), in yet other cases there may be subtle problems. Errors in computer programs are called bugs. Large computer programs may take teams of programmers years to write and the probability of the entire program having been written completely in the manner intended is unlikely. A typical modern computer can execute billions of instructions every second and nearly never make a mistake over years of operation. In practical terms, a computer program (also called simply a program) might include anywhere from a dozen instructions to many millions of instructions for something like a word processor or a web browser.
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