Introduction

Real-time systems consist of different processes which are interconnected. A process diagram indicates which processes interact with each other. However, in addition to simple indicating which processes interact, it is possible for a diagram to indicate how different processes are implemented.

One way this kind of diagram can be particularly useful is that it can distinguish between events that are created by

• hardware, without software intervention
• software, as part of the normal software process
• software, as part of an interrupt service routine

This kind of diagram can also indicate how these various processes communicate with one another.

Process Diagram elements

• hardware device
  
  hardware device symbol
  
  
• software module
  
  software module symbol
  
  
• input
  
  input symbol
  
  
• output
  
  output symbol
  
  
• interrupt
  
  interrupt symbol
  
  
• interrupt service routine
  
  interrupt service routine symbol
  
  
• system generated flag
  
  system generated flag symbol
  
  
• user generated flag
  
  user generated flag symbol
  
  
• system generated register
  
  system generated register symbol
  
  
• user generated register
  
  user generated register symbol
  
  
• system generated register with flag
  
  system generated register with flag symbol
  
  
• user generated register with flag
  
  user generated register with flag symbol