Software is a set of instructions that tells a computer how to operate.
At the lowest programming level, executable code consists of machine language instructions supported by an individual processor—typically a central processing unit (CPU) or a graphics processing unit (GPU).
For example, an instruction may change the value stored in a particular storage location in the computer—an effect that is not directly observable to the user.
An instruction may also invoke one of many input or output operations, for example, displaying some text on a computer screen; causing state changes that should be visible to the user.
The processor executes the instructions in the order they are provided, unless it is instructed to “jump” to a different instruction, or is interrupted by the operating system.
As of 2015, most personal computers, smartphone devices, and servers have processors with multiple execution units or multiple processors performing computation together, and computing has become a much more concurrent activity than in the past.
High-level programming languages are used to write the vast majority of software.
Because they are more similar to normal languages than machine languages, they are easier and more efficient for programmers to use.
A compiler, interpreter, or a mix of the two is used to transform high-level languages into machine language.
Software can also be written in a low-level assembly language, which has a close relationship to the computer’s machine language instructions and is converted into machine language by an assembler.