banks. These memory banks can hold instructions and both processed and unprocessed data. Memory access time and memory capacity are the other main factors that determine how powerful a computer is. Externally, computers can store data on magnetic disks (hard and floppy), magnetic tape, or paper tape. Disk drive units offer quicker access to the data than magnetic or paper tape units. On some systems, the disks can store more data than the internal memory of a computer. The amounts of each will depend on the design and requirements of the data system. Some systems internally store and process the data. Others depend heavily on the disks to hold and store the data, bringing the data into memory for processing, and then storing the results back onto disk. Disseminate Data After the computer has processed the data, it can send it to the I/O section or an I/O unit for immediate or  future  dissemination  to  various  equipments.  The data will exit the output section of the computer’s input/output section. It can be sent to an output device such as a printer, or to one of many storage devices such as a magnetic tape or disk unit. It can also be sent to a subsystem, such as a display system, via its associated equipment. Display  Data Computer  systems  display  two  general  types  of data—data  related  to  the  mission  of  the  system  and status information related to operation of the system and hardware  performance. The  computer  relies  on peripheral equipment, such as printers and display units, to display the processed data—the mission related output of the operational program. Your interest in output generally relates to whether the data is sent properly  by  the  computer  and  is  displaying  properly.  In other words, you want to know the computer system is functioning properly. The content of the data is usually a secondary interest to you and a primary interest to the user/operator. The other type of data/information that can be displayed relates to the operation of the system. This includes operator information, system error messages, and indications of system problems. You will be particularly   interested   in   this   information.   The maintenance panels and data terminals can display real-time data and provide you with current status of the operational  program.    For example, the maintenance panels of some computers have registers where the presence or absence of indicator lamps can indicate to the technician if the computer is communicating with a subsystem  such  as  a  display  or  communication subsystem. This is a very useful tool when you are performing  maintenance,  both  preventive  and corrective. Figure  1-2 is an example of a maintenance console panel. Notice the indicator lights for the I/O controller,  I/O  timing,  Mode,  Central  Processor Register, and soon. These will provide you with status information.  For  example,  you  can  monitor  the  I/O controller register to see if the computer is interfacing with  a  particular  subsystem  such  as  display  or communications. Look to see if the indicator of that channel is illuminated (either flashing or constantly lit). Or, you could look at the contents of a particular register in the CPU by selecting that register while installing a patch  to  a  program  using  an  inspect  and  change procedure  or  utility. TYPES OF COMPUTERS In general terms, computers can be classified into three   categories: mainframe   computers, minicomputers,  and  microcomputers.  A  computer’s power is determined by the technology it uses, NOT its physical  size. Greater speed, greater capability and capacity to store information, and greater facility to accommodate   additional   peripheral   (external) equipment  will  make  one  computer  more  powerful  than another regardless of their overall physical sizes. We do not go into detail on each of the different types of computers. Rather, we identify examples of each and point out their physical and internal differences. This will prove valuable when you are maintaining them. Let’s take a look at the types of computers you will maintain in the Navy. Later in this manual, you will study the internal workings of computers—their basic functional operation. Mainframe Computers Mainframe  computers  are  physically  the  largest computers you will maintain. Their ruggedness makes them   better   suited   than   microcomputers   and minicomputers  to  handle  the  mechanical  shock  and vibration, salt spray, temperature and humidity found aboard  Navy  vessels. The  mainframes  you  will maintain are general-purpose, digital data computers with  multiprocessing  capability. Mainframe computers are considered the heart of the afloat and ashore tactical and tactical support data systems. These mainframe computers are big, fret, multiprocessor  computers  with  correspondingly  large memories and multiple I/O channel capabilities. They process large volumes of data and require a lot of program  flexibility.  Their  operational  programs  are 1-3