INFORMATION TECHNOLGY SYSTEMS
TOPICS
Accumulator (ACC)
Arithmetic Logic Unit (ALU)
Clock
Control Unit
Generalized Computer Diagram
instruction execution cycle
Instruction Register (IR)
Main Memory
Memory Address Register (MAR)
Memory Buffer Register (MBR)
Program Counter (PC)
Register
Overview
Although Module 5 deals in translation and computer languages, many of the concepts covered in this module are easier to grasp if you have a basic understanding of the functional components of a computer and what each part does to make things work.
All general-purpose computers have five basic components: a central processing unit (CPU), main memory, input device(s), output device(s), and secondary memory - usually a hard disk unit.
The input and output devices serve to make the computer usable by humans. They essentially serve as translators which convert human generated input into a binary form that the CPU can understand and computer generated binary output into a form that humans can understand.
All the real work is done by the CPU, which uses main memory to store data and instructions until needed and results when computed. Secondary memory is used to store the main memory contents until needed again. The CPU cannot directly access secondary memory; instructions and data stored here must first be transferred into main memory, and eventually into the CPU itself before they can be accessed. Remember also that main memory is wiped clean when the power is switched off, so
secondary memory devices are essential to restore main memory when needed. Main memory has a much smaller storage capacity than secondary memory, and is much more expensive. This is why most computer systems rely heavily on secondary memory devices to function properly.
Main Memory
It is useful to examine main memory first, since the CPU has elements that look very much like those used to form memory
locations. Main memory is composed of series of cells used to store one of two electrical states (ON or OFF) which we call
binary digits (bits), arranged in groups of eight, called bytes. Modern computer systems work with binary items in groups of
one or more bytes (a computer word). Each memory location is usually comprised of a single byte and has a unique numerical
address identifying it. It is important not to confuse the address of an individual memory location with its contents, which can be
data, instructions, or results. They are NOT the same thing!!!
The bits used to form a single byte are usually numbered in descending order beginning from the left, and always ending in zero. From the discussion about converting binary numbers to decimal, you'll note that the bit position numbers also correspond to the power of two represented by that particular bit, if the content of that memory location represents a number. The rightmost bit is sometimes called the LSB for Least Significant Bit. One byte can also represent a single ASCII character.
Main memory is composed of millions of these bytes, each one with its own unique address collectively referred to as random-access memory (RAM). A figure of merit when examining computer systems is how many bytes of RAM it has. A computer with 32 million bytes of RAM is listed as a 32 Mb computer in advertisements. Secondary memory devices store binary items differently, but are also measured in terms of byte capacity. Hard drives can usually store hundreds of millions of bytes.