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41 Cards in this Set

  • Front
  • Back
Phase 1: Requirements Determination
Phase 2: Requirements Structuring
Phase 3: Alternative Generation & Selection
3 Sub-phases of analysis
Graphically illustrate movement of data between external entities and the processes and data stores within a system.
Data Flow Diagrams (DFD)
1) Utilize info gathered during requirements determination
2) Model the process
3) Model the processing logic and timing of events
4) Model the structure of data
Modeling a system's process
Set of coherent, interrelated data flow diagrams
Deliverables and Outcomes
Covers the scope of the system
Context data flow diagram (DFD)
Enables analysts to understand the current system
DFDs of current physical system
Technology independent
DFDs of current logical system
Data at rest
Data Store
Causes data to move
Data comes into the system from
A source
Data leaves the system here
Destination of the data
Defines the boundary of the system
Sources and Sinks
A task or set of tasks performed on data by a person, machine, or computer so that the data are transformed, stored or distributed
Transforms input data flows into output data flows
A Process
1) Captures data from sources
2) Produces / distributes data to sinks
3) Maintains data stores
4) Creates high-level descriptions of data transformation operations
Represents data being conveyed to or from an external entity, process, or data store
Data Flow
A data flow diagram of the scope of an organizational system that shows the system's boundaries, external entities that interact with the system, and the major information flows between the entities of the system
Context Diagram
A data flow diagram that represents a system's major processes, data flows, and data stores at a high level of detail
Level-0 Diagram
DFD is organized in a framework of levels with each successive level revealing further details of the system
Hierarchical Structure
Highest level view of the system
Contains only 1 process (labeled "0")
No data stores (contained within the process)
Shows boundaries, external entities, and major info flows
Context Diagram
Represents a system's major processes, data flows, and data stores at a high level of detail
Sources and sinks same as context diagram
Each process ends in ".0"
Context level process exploded
Level-0 Diagram
Cannot have only outputs
Cannot have only inputs
Has a verb phrase label
Process DFD Rules
Data cannot be moved from one store to another
Data cannot move directly from an outside source to a data store
Data cannot move directly from a data store to a data sink
Data store has a noun phrase label
Data Store DFD Rules
Data cannot move directly from source to sink
Source/sink has a noun-phrase label
Source/sink DFD Rules
Act of going from a single system to many component processes
Repetitive procedure
Functional Decomposition
A DFD that is the result of n nested decompositions of a series of subprocesses from a process on a level-0 diagram
Level-N Diagrams
When the process (DFD) becomes so trivial that it need not be decomposed any further
Primitive Process
Primitive DFD
Has only one direction of flow between symbols
Fork means exactly the same data goes from a common location to two or more processes, data stores or sources/sinks
Data Flow DFD Rules
Represents the physical implementation of the new system
New Physical
Used as an aid for understanding the existing system.
Then used in analysis to look for improvements
Current Physical -> Current Logical -> New Logical -> New Physical -> Back to beginning
Usage of DFDs
DFD must include all components necessary for the system
Each component must be fully described in the project dictionary or CASE repository
DFD Completeness
The extent to which information contained on one level of a set of nested DFDs is also included on other levels
DFD Consistency
Time is not represented well.
Best to draw as if the system has never stared and will never stop.
DFD Timing
Analyst should expect to redraw diagram several times before reaching the closest approximation to the system being modeled
Iterative Development
The process of discovering discrepancies between two or more sets of DFDs or within a single DFD
Gap Analysis
1) When each process has been reduced to a single decision, calculation or database operation
2) When each data store represents data about a single entity
3) When the system user does not care to see any more detail
4) When every data flow does not need to be split further to show that data are handled in various ways
5) When you believe that you have shown each business form or transaction, on-line display and report as a single data flow
6) When you believe that there is a separate process for each choice on all lowest-level menu options
When to stop decomposition
Attempt to show the essence of the system without regard to the actual physical implementation. Physical aspects of the system are removed as much as possible.
Current system is reduced to data and processes that transform them. Data Stores, Data Flows and Processes are independent of media.
Current Logical
Includes Additional functions
Obsolete functions are removed
Inefficient data flows are reorganized
New Logical
Process label includes an identification of the technology (people or systems) used to process the data
Data flows and data stores are labeled with the actual name of the physical media on which data flow or in which data are stored
Current Physical
Inputs to a process are always different than outputs
Objects always have a unique name
DFD Rules
When interrupt is activated, the cpu reads a value from the low-order byte on its data bus. This value is used as the index into the interrupt service routine pointer table.
Real Mode
Instead of reading from the lowest 1k byte of memory, the interrupt descriptor table is located at another place pointed to by the IDT Register; and each entry is 8 bytes long instead of 4.
Protected Mode