The CPU Machine Cycle

• The most important thing that a computer does is implement the CPU machine cycle. This is sometimes called the Fetch-Execute cycle because it consists of two parts--fetching and executing. The fetch part of the cycle brings the next instruction in the currently active program into the CPU. The execute part of the cycle does whatever the current instruction says to do. The machine cycle is repeated until the program ends--fetching and executing one instruction at a time. The programs do all the things we see the computer do, but the constantly running machine cycle makes the program run.
  
  

Memory Management

• In order for programs to run, they must be moved in and out of memory. Running programs must keep track of exactly where in memory the current instruction is located. Much of what the computer is doing involves memory. Moving programs in and out of memory, switching between programs that are currently running and keeping track of which instruction is next in line for the machine cycle processing. We do not see it but the movement of data in memory underlies everything that the computer does.
  
  

BIOS

• BIOS is a program that runs whenever the computer starts up. Originally it consisted of the automatic program that ran to make sure that the keyboard, mouse, screen, printer and other peripherals where still connected to the computer--hence the name. Now BIOS includes all the checks that are currently running to make sure that peripherals are connected, memory is functioning and the system is not overloaded. BIOS monitors things like how much of the time the operating system is spending moving things in and out of memory compared to how much of the time actual application programs--like Word or an Internet search engine is running.
  
  

I/O

• If the computer cannot communicate with the exterior world, it isn't worth much. In early computers, input devices read directly into memory and the input data was processed by the CPU in the same way it processed other input--output was just the reverse. Now I/O--on all but the smallest computer--involves a separate chip. The I/O chip is essentially a computer in its own right. Now when there is a document to write, the CPU just notifies the I/O chip where the document is in memory and how long it is. The CPU then continues on while the I/O chip supervises printing. This speeds things up considerably.