YSMslides The Yourdon WardMellor Structured Method Why study YSM It has been extended for embedde systems by Ward  Mellor It is a popular and wel understood method It is applicable to both large an s
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YSMslides The Yourdon WardMellor Structured Method Why study YSM It has been extended for embedde systems by Ward Mellor It is a popular and wel understood method It is applicable to both large an s

g Select Y ourdon and EasyCase Does it have drawbacks It is most applicable to the softwar design stage its inte rfaces with th requirements and implementatio phases are rather loos brPage 2br YSMslides Basic elements of the notatio erminato Dat tra

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YSMslides The Yourdon WardMellor Structured Method Why study YSM It has been extended for embedde systems by Ward Mellor It is a popular and wel understood method It is applicable to both large an s




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Presentation on theme: "YSMslides The Yourdon WardMellor Structured Method Why study YSM It has been extended for embedde systems by Ward Mellor It is a popular and wel understood method It is applicable to both large an s"— Presentation transcript:


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YSM_slides The Yourdon (Ward-Mellor Structured Method Why study YSM It has been extended for embedde systems by Ward & Mellor It is a popular and wel understood method It is applicable to both large an small project It is well supported by CASE tool s, e.g. Select -Y ourdon and EasyCase Does it have drawbacks It is most applicable to the softwar design stage - its inte rfaces with th requirements and implementatio phases are rather loos
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YSM_slides Basic elements of the notatio erminato Dat transformatio proces Data stor Discrete data flo The represents an item

in the system environment; it acts as a data source or sink terminato Data transformatio is an input/output process (note that it can report the occurrenc of an event, i.e. initiate a control flow) A is an abstraction on a file; it act as a repository for data that is subject to storag dela . Values are modified at discrete points i time and remembered. Readout is not ordered and is non-destructive data store The is an abstraction on a transaction or other data-aggregate sent o received by the system discrete data flo
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YSM_slides Extensions of the method for real-tim For

real-time systems, it is necessary t introduce additional elements to handle: time continuous dat event data (logic signals Contro transformatio proces Continuous flo Control flow (signal Control stor Prompt : Activate Enable/ Disable Trigge Group node
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YSM_slides The controls th behaviour of data transformations by activatin or de-activating them; it is an abstraction on some portion of the system’s logic control transformatio The represents a buffe r. It i an abstraction on a stack (e.g. LIFO or FIFO) Readout is not ordered but it is destructive The control store has a

capacit control stor Prompt are initiated by control processe and activate data transformations. Ther are various types : contro proces dat proces promp Activato Activato Enable Disabl Trigge Trigger Data proces Data proces Data proces Enable Disabl
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A more complex Environmental Model. Context Diagram – Cooker Control Software * This is known as an ‘access’ flow and indicates that the system uses stored data that is shared between it and its environment. Any store shown on the context diagram must be accessed by the system and at least one terminator.
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Cooker

Control
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YSM_slides Method of us The method consists of building a mode of the system in a hierarchical manner; thi is known as . The highest level i known as the diagram. Typicall y, it consists of a single data transformation some data/control flows and terminators. levelling contex The data transformation node conceals ‘child’ data flow diagram (DFD)
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YSM_slides The control proces Control specifications are given in term of the familiar state transition diagra Transitions between states occur as a resul of input event Action routines produce output event

flows and prompts (Mealy FSM State State State Each time activated Even Actio Even Actio Even Actio Even Actio
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YSM_slides Process Specification The procedure of decomposing the syste continues until a level is found when th operations to be performed within a transformation need not be subdivided furthe At this point, description of the individua processes is done by means of a . PSPE Process specifications usually consist o , tables, mathematica formulae, graphs etc structured English Structured English has 4 basic constructs Concurrenc Sequence Decision Iteration Do function

Do function then.... Do function Do function the Do function IF conditio DO function ELS DO function WHIL conditio function @IN = list of input data item @OUT = list of output data item @PSPE FOR (all input data items calculate (output data items write (output data items @COMMEN
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YSM_slides The Data Dictionar Flows leaving and arriving at the system ar usually non-primitives, representing group of data. These groups decompose into smaller an smaller groups as they proceed dow the hierarchical levels until they reduce to primitive components. The dictionary specifies the

component and structure of each group using a special notation - the Backus-Naur for (BNF Exampl : The pilot's display of a fligh management system can be one o several types, e.g. display initialising route displa , progress display and ma at any time, carry superimpose alert/advisory messages VisualDisplay = [InitialDisplay | RouteDisplay ProgressDisplay] (Alert/AdvisoryMessage where : Alert/AdvisoryMessage [WingFallenOff | EngineOnFire | YouAreLost
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YSM_slides Data Dictionary Symbols (BNF) Symbol Meaning Description composed of The flow named on the left is composed of the

flows named on the right. together with Collects members into a group but does not imply ordering. { } iterations o The expression within the brackets may occur any number of times in a given instance of the flow. The brackets may be indexed. M{ }N indicates any number of iterations from M to N, e.g. { }2 is 0, 1 or 2 iterations 2{ } is 2 or more iterations 2{ } 2 is exactly 2 iterations [ | ] select one of A given instance of the flow will contain exactly one of the options within the brackets. ( ) optional The expression within parantheses may optionally appear in a given instance of the

flow. literal The symbols enclosed within quotes literally constitute the data flow. comment Textual information.
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YSM_slides System Decompositio use keyboar senso syste contro displa controlle mod userke data-i data-ou out mod C1 P1 P2 P3 parameter parameter data-i data-ou ou userke activat activat activat data-interna N.B. This example is not a complete specificatio
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YSM_slides on entr activate P mod mod activate P activate P deactivate P deactivate P deactivate P activate P Process specification for data transformation P1 1.2 - interpret keyboard buffer,

PSPEC02.D AT @IN = userkey @OUT = parameters @PSPEC interpret keyboard buffer SE decode userkey to decoded userke convert decoded userkey to 16 bit REAL type parameter State transition diagram fo control Process C1
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YSM_slides The Code Organisation Mode This uses the program structur chart with the following notation Constantin program module sub-system : represents som pre-defined function, perhap within the operating system o language (sub-systems cannot have children) library function : pre-define function that may be used across many module cal conditiona cal iterated cal

dat coupl contro couple A Constantine diagram shows the architectur or calling structure of modules within a project