All information in a system, including disk files, programs stored in memory, user data stored in memory, and data transferred across a network, is represented as a bunch of bits. The only thing that distinguishes different data objects is the context in which we view them.

Hardware Organization of a System

Running throughout the system is a collection of electrical conduits called buses that carry bytes of information back and forth between the components. Buses are typically designed to transfer fixed-size chunks of bytes known as words.

I/O devices are the system’s connection to the external world. Each I/O device is connected to the I/O bus by either a controller or an adapter. The purpose of a controller or an adapter is to transfer information back and forth between the I/O bus and an I/O device.

The main memory is a temporary storage device that holds both a program and the data it manipulates while the processor is executing the program. Physically, main memory consists of a collection of dynamic random access memory (DRAM) chips. Logically, memory is organized as a linear array of bytes, each with its own unique address (array index) starting at zero.

The central processing unit (CPU), or simply processor, is the engine that interprets (or executes) instructions stored in main memory. At its core is a word-size storage device (or register) called the program counter (PC). At any point in time, the PC points at some machine-language instruction in main memory.