functional  areas  to  one  another.  This  is  how  the functional areas communicate with each other. A bus is a parallel data communication path over which information is transferred a byte or word at a time. The buses contain logic that the CPU controls. The items controlled  are  the  transfer  of  data,  instructions,  and commands  between  the  functional  areas  of  the computer:  CPU,  memory,  and  I/O  The  type  of information is generally similar on all computers; only the names or terminology of the bus types differs. The name of the bus or its operation usually implies the type of signal it carries or method of operation. The direction of signal flow for the different buses is indicated on figures in the computer’s technical manuals. The direction may be  unidirectional  or bidirectional  depending on the type of bus and type of computer. Consult the computer’s technical manual for details.  After  becoming  familiar  with  the  basic functions and operations of buses, you’ll see that regardless  of  the  names,  their  basic  concepts  are consistent throughout the computer. They provide avenues  for  information  to  be  exchanged  inside  the computer. BUS TYPES The preferred method for data/information transfer between system components is by a common data bus. Where point-to-point data transfer is required, the digital  format  is  the  preferred  method.  General Requirements for Electronic Equipment Specifications, MIL-STD-2036 series, provides a list of the industry accepted  standard  internal  data  buses.  They  include the standard and the interface as follows: IEEE   696—IEEE   Standard   696   Interface Devices, S-100 IEEE 896.l—IEEE Standard Backplane Bus Specification  for  Multiprocessor  Architecture, Future Bus I E E E    9 6 1 — S t a n d a r d    f o r    a n    8 - b i t Microcomputer Bus System, STD Bus IEEE  1014—Standard  for  a  Versatile  Backplane Bus,  VMEbus IEEE  1196—Standard  for  a  Simple  32-Bit Backplane Bus, NuBus IEEE  1296—Standard  for  a  High-Performance Synchronous 32-Bit Bus, Multibus II All computers use three types of basic buses. The name of the bus is generally determined by the type of signal it is carrying or the method of operation. We group the buses into three areas as you see them in their most common uses. They are as follows: Control  (also  called  timing  and  control  bus), address, and data (also called a memory bus) buses Instruction  (I),  Operand  (O),  Input/Output Memory (I/O MEM) or Input/Output Controller (IOC), and Computer Interconnection System (CIS) Time multiplexed bus Control  Bus The control bus is used by the CPU to direct and monitor the actions of the other functional areas of the computer. It is used to transmit a variety of individual signals  (read,  write,  interrupt,  acknowledge,  and  so forth)   necessary   to   control   and   coordinate   the operations of the computer. The individual signals transmitted over the control bus and their functions are covered in the appropriate functional area description. Address Bus The address bus consists of all the signals necessary to define any of the possible memory address locations within the computer, or for modular memories any of the possible memory address locations within a module. An address is defined as a label, symbol, or other set of characters used to designate a location or register where information is stored. Before data or instructions can be written into or read from memory by the CPU or I/O sections, an address must be transmitted to memory over the address bus. Data Bus The bidirectional data bus, sometimes called the memory bus, handles the transfer of all data and instructions  between  functional  areas  of  the  computer. The  bidirectional  data  bus  can  only  transmit  in  one direction at a time. The data bus is used to transfer instructions from memory to the CPU for execution. It carries data (operands) to and from the CPU and memory  as  required  by  instruction  translation.  The  data bus is also used to transfer data between memory and the  I/O  section  during  input/output  operations.  The information on the data bus is either written into 5-24


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