1.3 System Design

1.3.1 Parts of a System InputProcessOutput Input Devices Central Processing Unit Output Devices The main parts of any computer system follow this data flow Backing Storage

The user would find it difficult to deal directly with the hardware since all operations at this level are carried out in binary machine code. Therefore, successive layers of software have developed.

1.3.2 Data in a Computer System One important process is analysis + fact- finding. This involves identifying data which needs to be held and processed. To fully describe the system it is necessary to consider what happens under many different circumstances. Often data that needs to be held and processed in a system is identified using data flow diagram.

A data flow diagram typically uses the following symbols (although there is not complete consistency in practice) A box with rounded corners represents a process. An example would be a calculation The box with an open right-hand side representing a data store. The closed rectangle is a source or sink (destination) of data. It shows the limits of our diagram, how the data gets into or out of these boxes is not a concern of this diagram.

Bicycle Returned Calculate Amount Due Renter Pays Bicycle Details Card Update Bicycle Details Card

1.3.3 Data Capture and Presentation The data flow diagram shows only data flow without reference to mechanisms of capture and display. There are a great many ways to capture data for use in a computer system.

Similarly one can classify output devices in common use. But there are so many input or output devices that not all of them will fall into a particular classification.

1.3.4 Design of Appropriate Data Structure Relevant data structure are described together with examples of when it would be appropriate to use them. In the exam, students are likely to be asked similar, but much simplified questions. The design of appropriate data structures is an important activity for the dossier and students should be able to discuss their choices. It is an excellent idea to keep a written (or web-based) log during the design stage of the dossier including sketches and notes about the problem and possible ways of representing or storing the data for a system. Discuss, including diagrams, the data structures that could be used to hold the data for the system. Remember that the ‘discussion’ keyword requires you to consider a range of possible data structures and give reasons for selecting the ones you did.

1.3.5 Hardware Components You have probably already studied a range of input, output and backing store devices.

1.3.6 User Interfaces Early operating systems operated with typed in commands (requiring command-line interpreters or CLIs) while later ones have developed graphical user interfaces (GUIs). The main features of these interfaces are: Command Line InterfacesGraphical User Interfaces Easier to implement for a programmer, requires less memory to run. Can be run on systems without graphical monitors. More complex to implement, requires more memory, a pointing device and a graphical monitor Users need to remember specific commands so new users can find them harder to use. Icons (small images) help users to remember commands, file types; commands are grouped in menus. Long term users may find it quicker to type in a command at the keyboard than to use a mouse or other pointing device New users will find it easier to use because they do not have to remember specific commands.

Graphical User Interfaces are sometimes described using the term WIMP, variously interpreted as: – Windows, Icons, Menus Pointers – Windows, Icons, Mice, Pull-down Menus

1.3.7 Systems Flowcharts Systems flowcharts are designed to link data flow and processing operations to specific pieces of hardware. They are sometime known as input- output (systems) flowcharts. They should not be confused with flowcharts used to show the structure of algorithms. As with data flow diagrams there is a wide variation in symbols used to implement systems flowcharts; below are the ones specified by IB in the Computer Science Subject Guide.

Devices and Media Action or process Input or output (word inside) On line storage Tape storage Disc storage Document

Other Symbols Annotation Lines crossing Lines joining Data flow Communications (2 ways unless indicated)

Students generally seem to have great difficulty with systems flow charts; the main problem seems to be that they think in terms of linear algorithm flow charts. Flowcharts are used to describe algorithms (although pseudocode is often preferred these days); Systems flowcharts are used to describe input-process- output in computer systems, they are the only charts to refer to hardware devices; Data flow diagrams refer to data objects and processes (people, paper files, computer files, etc); Module diagrams are used to split a large problem up into several smaller ones (stepwise refinement). This makes the problem easier to solve and divide up among a programming team

1.3.8 Constructing Systems Flowcharts Chapter 3 describes batch, online, and real- time systems; here we examine how these types of process are represented in systems flowcharts – Common batch processing tasks – Common online processing tasks – Common real-time processing tasks

Common batch processing tasks In batch processing, data is gathered first and then processed in one go. Typical operations update a master file using a sorted transaction file. Therefore in many batch processes (cheque clearing, electricity billing, payroll processing, batch update of a stock file), paper documents will be collected, validated and sorted. Items rejected by validation may be corrected and re-entered.

Common online processing tasks Recall that, in online processing, any transactions are used to update a database immediately. A typical example is supermarket stock control where barcoe scanners at a POS terminal read the barcode, look up the item details in a stock database and return the details to the POS terminal where they are printed on a receipt and shown on a display.

Common real time processing tasks Real time systems are a type of online processing system in which the processing is fast – the input data is processed quickly enough to affect the next output of the system. Usually such systems collect their data through sensors (automatic data entry). A typical example is a system which monitors a nuclear plant’s reactor core. In some reactors a set of rods are inserted into the core to damp the nuclear reaction. When this is done, a warning is sent via a communications system to the control room.