Real Time Systems

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Real Time Systems

• Embedded System
• Issues - Part I
• Low Level Programming

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Review
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Outline

• Device (Hardware) Input / Output Mechanisms
• Status Driven Device Control
• Interrupt Driven Device Control
• An Example the Bytronic IBM Programmable
  Peripheral Interface Card

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How does an RTS interact with the real world?

• Recall that one the main characteristics of a
  real-time system is to interact with its
  environment. In order for it to do so, software
  must interface to hardware via special purpose
  input / output devices.
• Interfaces to these devices and techniques to
  control them vary widely because
• of different computer architectures
• of different control interfaces
• of different device requirements

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Device Input / Output Mechanisms

• Separate bus architecture
• access to a device is typically through a set of
  registers
• logically separate bus for memory and device I/O
• two sets of assembly instructions
• example Intel x86 architectures

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Device Input / Output Mechanisms

• Memory-mapped architecture
• devices still typically accessed through
  registers
• addresses divided among memory and devices
• a single set of assembly instructions
• example Motorola 68000 architecture

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Status Driven Device Control

• In this mechanism, a controlling program
  explicitly checks (polls) the status of a device
  through its register(s)
• typical hardware instructions include
• test operations which allow the program to
  determine the status of a device
• control operations which allow the program to
  send device-dependent action commands to a device
• input/output operations which allow the program
  to transfer data to/from a device

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Interrupt Driven Device Control

• In this mechanism, the device itself generates an
  interrupt which must then be handled by the
  controlling program
• allows I/O to be performed asynchronously, thus
  avoiding the busy waiting or constant polling of
  pure status controlled systems
• however this requires support for
• context switching
• interrupt device handler identification
• interrupt control

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An Example the Bytronic IBM PPI Card

• See handouts for example showing how to control a
  robotic arm
• the arm consists of two pulse-width modulated
  stepper motors which control the azimuth and
  elevation movement of the arm
• the position of the motors is fed back via two
  potentiometers

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References

• 1 Burns, A. and Wellings, A., Real-Time
  Systems and Programming Languages, Chapter 15,
  Addison Wesley, 1997.
• 2 Bytronic handbook, The Bytronic IBM Internal
  and External Interfaces MPIBM1 and MPIBM2,
  Version No 2.2.

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Questions