.  Memory  bus l I/O bus The I/O bus has historically been the slowest of all buses, and the main focus when computer design engineers try to improve bus speeds. Processor  Bus The  processor  bus  is  communications  path  between the  CPU  and  the  main  bus.  It  is  also  used  for communications between the CPU and the processor support  chipset.  The  processor  support  chipset  includes chips such as an external memory cache and the bus controller chip found on some microcomputers. The size of the processor bus matches the size of the data words used by CPU. For example, the 80486DX chip uses 32-bit words; therefore the processor bus has 32 data lines, 32 address lines, and the control lines. The Pentium processors have 64-bit words and use 32-bit addresses. Processor buses can have a maximum data transfer rate of the motherboard clock. Memory Bus The memory bus transfers data between the RAM and the CPU. This bus can be the processor bus or will be implemented by a dedicated chipset that controls the memory   bus. In   most   computers   that   have   a motherboard clock that is faster then 16MHz, a special memory controller chipset will control the memory bus. Address Bus The address bus transfers the next memory or I/O address to be used in the next data transfer. The address bus in 486 and Pentium systems is 32 bits wide. I/O Buses To  thoroughly  understand  the  I/O  buses  used  in modern  microcomputer  systems,  an  understanding  of the development and evolution of bus systems is required. The microcomputer’s architecture is directly related to the type of buses in the computer. Originally, microcomputers used a bus system called the S-100 bus. Using this system, any board could be plugged into any open slot. The S-100 bus has 62 lines, each connect to each of the 62-pin connectors. This system dedicated eight lines for the eight data bits used in the Intel 8088 microprocessor. Twenty lines are used for memory addressing. The same 20 lines are also used to address I/O  devices.  A  control  line  determines  whether  the  data 5-27 on these 20 lines will be a memory address or an I/O address. There are also several control lines and power distribution  lines. The S-100 bus also provided four lines to designate channels for Direct Memory Accessing (DMA). A DMA channel allows a device, such as the hard drive, to transfer data directly into RAM, vice transferring data to the CPU and then having the CPU transfer it to the RAM. The DMA channel number identifies which device is requesting and transferring data on the data bus. Buses also need to be clocked to properly transfer data. The early microcomputer buses were designed to run at the speed of the microprocessor that was installed on  the  board.  The  4.7  MHZ  8088  microprocessor  clock was  also  used  to  clock  the  bus.  The  7.16  MHZ microprocessor clocked the bus at the same rate. The ISA standard set the bus clock speed at 8 MHZ. To maintain  compatibility  with  the  older  controller  boards, this speed is still common in many computers today. This speed is fine when getting input from a mouse or a keyboard, even for most disk drives. The biggest problem with bus speeds has occurred because of the increase in video resolution, the development of video capture boards and some network interfaces. INDUSTRY  STANDARD  ARCHITECTURE (ISA).—  As  the  microcomputer  evolved,  the  eight  data lines and 20 address lines became insufficient to handle the increased data capacity of the 16-bit processor. This led  to  the  development  of  the  Industry  Standard Architecture (ISA). To be compatible with the boards used in eight-bit computers, an additional 36-wire connector was added to the circuit boards and the bus. This added eight more data lines, four more address lines, four more DMA channels, and five more IRQ channels. LOCAL  BUSES.—  A local bus is a bus that is a dedicated path between the processor and a specific board. There are several local buses built into various types  of  computers  to  increase  the  speed  of  data transfers. Local buses for expanded memory and video boards  are  the  most  common. Some  high-end computers also provide a local bus for the hard drive. The VESA Local Bus is one of the more popular buses and was developed to increase the speed of data transfer  between  memory  and  the  video  processing board  (video  graphics  adapter).  VESA  stands  for  Video Electronics Standards Association. The VESA Local Bus is a direct bus that connects the video processor