How Mac OS X Works

The Purpose of Operating Systems

What's the big deal about operating systems in the first place? What do they actually do? An operating system is the level of programming that lets you do things with your computer. The operating system interacts with a computer's hardware on a basic level, transmitting your commands into language the hardware can interpret. The OS acts as a platform for all other applications on your machine. Without it, your computer would just be a paperweight.

At its heart, a computer is a number-crunching device. It takes input in the form of zeros and ones -- bits -- and channels them through various circuits and processors. The hardware behaves according to strict rules. We define these rules using things like logic gates, which take input and produce an output in a predictable way. Some simple computers have no need of an operating system because they only perform a specific task. But personal computers need to be more versatile. The operating system allows complex programs to access the capabilities of the hardware to get results. Only the hardware's physical properties and our own imaginations can limit what programs can do.


You could design an operating system by physically programming it into a computer's circuits. This would require building electrical pathways using millions of logic gates. But such an operating system would be inflexible. That's why operating systems like Mac OS X and Windows are software. Software is more malleable than hardware -- you can make changes through software patches and version updates. To do the same with hardware would mean switching out physical chips and circuit boards.

Operating systems are like the manager for a computer. It's the job of the OS to monitor what software needs and what the hardware can provide. As you run applications on your computer, the OS allocates the resources necessary to complete the task. That can include processing power, memory allocation and computer storage access, among other things. Ideally, the OS will make sure that your computer's hardware is never overtaxed.

The OS also allows programs to run on a computer. Without an OS, a programmer would have to design an application to run on the hardware directly. This isn't very efficient. An operating system acts as an application interface to the hardware. The OS does this through an application program interface (API). Program developers build applications for the API. Assuming the programmer has done a good job at building an application without any serious bugs, it should run just fine on the operating system.

One important part of the Mac computer is the firmware. Firmware is a level of programming that exists directly on top of a hardware layer. It's not part of the operating system itself. The Mac firmware is the first stored program that executes when you turn on a Mac computer. Its job is to check the computer's CPU, memory, disk drives and ports for errors. The PC equivalent to the Mac firmware is called BIOS, which stands for basic input-output systems. A second program called a bootloader loads the Mac OS X, assuming there are no errors reported by the firmware.

Next, we'll take a closer look at what makes the Mac OS X tick.