Advanced Embedded Systems Design

1
Advanced Embedded Systems Design

• Lecture 4 Preemptive multi-tasking inter-task
  communications
• BAE 5030 - 003
• Fall 2004
• Instructor Marvin Stone
• Biosystems and Agricultural Engineering
• Oklahoma State University

2
Goals for Class Today

• Questions over reading
• Preemptive multi-tasking
• Time triggered scheduler
• Keil Compiler
• interrupts / profiling
• Set assignment

3
inter- task communications

• Required to allow data to be passed between
  producing and consuming tasks
• Queueing is needed when inflow and outflow rates
  are not matched. (average inflow lt average
  outflow)
• Types
• message queues (ping-pong, circular)
• Pipes - FIFO buffer between tasks
• Sockets - bidirectional, sequenced flow between
  tasks
• Other
• Task synchronization and mutual exclusion
  mechanisms
• binary / mutual-exclusion semaphores
• counting semaphores where multiple of a
  resource must be metered

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Nature of inter-task communications

• Generally done with shared memory
• Tasks mutually understand a named memory as a
  communications pathway
• Preemptive multi-tasking systems
• Require inter-task coordination of access to
  shared memory
• Non-atomic access to memory corrupts
  communication
• Named shared memory is a limited shared resource
• Time triggered systems
• Time division multiplexing assures no two tasks
  run at the same time therefore do not require
  coordination to shared memory
• Assumes shared memory is completely updated
  during each tasks access

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Inter-task communications - Example

• Task 1 writes integer buffer for consumption
  during its execution
• Task 2 reads integer buffer during its execution
• If Task 1 is interrupted during the writing of
  the integer by task 2, communications will be
  corrupted. (Race condition)
• Example See 8051 MOV instruction and storage of
  int.

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Inter-task communications Binary Semaphore

• Task 1
• Tests Semaphore to assure not busy otherwise
  waits
• Sets Semaphore to Busy
• Writes integer
• Sets Semaphore to Free

• Task 2
• Tests Semaphore to assure not busy otherwise
  waits
• Sets Semaphore to busy
• Reads integer
• Sets Semaphore to Free

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Inter-task communications Binary Semaphore
Issues

• Note that in both task 1 and task 2, both test
  then set the semaphore to Busy
• Consider the potential problem that might occur
  if task 1 interrupts task 2 between the test of
  semaphore and set to busy
• Task 1 concludes semaphore is free, sets the
  semaphore to Busy, Begins writing to Integer
  Flag.
• Task 2 also sets flag to Busy and reads while
  Task 1 writes
• Corrupted communications could result. Generally
  Task 2 completes before task 1 may continue and
  the process works successfully not as intended
• An assumption in the above system is that the
  semaphore can be written in a single
  un-interruptable (atomic) process. If the
  writing of the semaphore is a multi-instruction
  process, an interrupt could occur during writing
  of the semaphore and a corrupted result would
  occur as in the original example.

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Inter-task communications Counting Semaphore

• Task 1
• Increments Semaphore to indicate an integer has
  been added to the queue
• Writes integer

• Task 2
• Tests semaphore to assure it is gt 0
• Reads integer
• Decrements Semaphore to indicate an integer has
  been removed from the queue

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Inter-task communications Counting Semaphore
Issues

• As in the original problem, Synchronized access
  to shared data must be provided.
• Here a binary semaphore could be used to control
  access to the resource and counting semaphore
• An All-in-one counting semaphore can be used
  assuming atomic access

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Strategies to test and set semaphore in an atom
Subroutine to set semaphore to Free SETB
Semaphore RTS
Subroutine to set semaphore to Busy CLRB
Semaphore RTS
Subroutine to check and wait on semaphore Set
indicates Free, Clear indicates Busy testset JBC
Semaphore, busy Jump if semaphore set and clear
semaphore JMP testset busy RTS
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Message Queues / Pipes / Sockets

• Memory buffer
• Character
• Normally organized as FIFO
• Bi-directional flow (sockets) and flow control
  handshaking between processes may be supported
• Function
• Head maintained at the read end by consumer.
• Tail maintained at write end by producer
• Tail advanced to the end of buffer and then to
  beginning
• Tail may not advance past head (Buffer full)
• Head may not be consumed beyond tail (Buffer
  empty)

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Assignment

• Code a program to receive characters from the
  serial port and to accumulate a string of them
  and print the string after receiving a carriage
  return. Use the hybrid scheduler given in Pont.
  Use a serial port ISR to receive characters and a
  time triggered task to accumulate characters and
  print after a carriage return.
• Read Pont, Chapter 17
• Tutorial 30 min
• Explain Rate Monotonic Analysis