direct memory access (DMA) gives the computer an advantage-speed. It allows information to be moved quickly in and out of memory without the intervention of the CPU. DMA is given control and takes over from the  CPU  as  director  of  electronic  traffic  on  the computer’s network of communication buses. It allows blocks of information to be transferred directly in and out of memory and from and to an external device without   any   CPU   intervention. Information   is transferred at a speed compatible with the speed of the external device. Therefore, the use of DMA would be advantageous  when  using  a  high-speed  external  device, such as a magnetic disk. The DMA acts the same as an I/O processor; it is just another method to control the flow of information. A DMA controller is usually placed between the external device and the computer’s bus. The controller uses  circuits  consistent  with  the  computer’s  other  major functional areas. The controller consists of several functional parts. Two counter registers are used. One generates the next main memory addresses from which information is read or in which it is stored. This counter register is incremented by successive information transfers. The second counter keeps track of the number of information words that are remaining to be transferred. A data register serves as a buffer between main memory and the external device. And of course, the control circuits, will control DMA operations. Other registers are provided for more complex external devices. In its most usual form, a DMA assumes command of  the  computer’s  bus  when  the  DMA  controller receives an interrupt signal from an external device. It then gives the CPU a hold/suspend operations message. The  CPU  will  respond  with  a  hold-acknowledge  signal. It turns over control of the bus and then, in effect, takes a short break. Meanwhile, the DMA controller moves information  between  main  memory  and  the  I/O  external devices and independently carries out the I/O transfers. The DMA controller will inform the CPU when it is finished  with  an  interrupt.  During  DMA  operations,  the CPU performs other tasks. If the CPU and the DMA controller try to access main memory simultaneously, the DMA has priority. TOPIC 4—INPUT/OUTPUT INTERFACING Input/output (I/O) interfacing is affected by many factors. Among them are the method of connection, serial  or  parallel  interfacing,  and  the  type  of  equipment the   computer   is   interfacing   with.   Input/output operations allow the computer to communicate with an assortment of external devices. Most computers use an I/O processor of some sort, so we concentrate our discussion  in  that  area.  The  external  devices  are connected  to  the  I/O  processor  via  I/O channels  or ports. An I/O channel or port is nothing more than the wiring necessary to interconnect the computer’s I/O processor with one or more external devices. The type of interfacing used will dictate the wiring of each channel or port. Computers may have a small number of   channels   or   ports   with   multiple   equipments connected to each channel, or they may, particularly in larger computers, have a number of I/O channels with limited numbers or types of external equipments on each  channel  or  port. METHODS OF CONNECTIONS There is a great deal of variety not only in the types of  external  devices  but  also  in  the  methods  of connecting  them  to  a  computer. One  thing  that computer  external  devices  have  in  common  is  that  they communicate  wit-h  the  computer  indiscrete  binary  data. The function of the external equipment may be to convert that data to other forms, but when a data exchange is done over I/O channels, the data exchange is in some form of binary data. We now look at two methods of connecting the external equipments where more than one external device is involved: daisy chaining  and  independent  request  control. Daisy Chaining When more than one peripheral device is connected to a single port/channel, a technique called daisy chaining  is  used.  When  daisy  chained,  the  peripheral devices  receive  or  transmit  information  over  a  common path. A separate set of addressing or control lines is used to identify (address) specific devices and to control the transmission or reception of information. When the CPU dictates the use of the computer’s bus, there is no difficulty in deciding which external device will have access to the computer’s bus. But in more complex situations, such as DMA transfers, simultaneous requests for the computer’s bus may be made by two or more external devices. Then a 7-17