1. Introduction to Embedded Systems
Rabie A. Ramadan 7

2. Real Time Synchronization
Required when tasks are not independent and need to share information and synchronize
If two tasks want to access the same data, semaphores are used to ensure non-simultaneous access.
Potential Issues
Priority Inversion – A situation in which a higher priority job is blocked by lower priority jobs for an indefinite period of time
Chain Blocking – A situation in which more than two resources are available and a high priority task is blocked off from both because of two or more lower priority tasks holding locks on one or both of the resources.

3. Priority Inversions
Low priority task holds resource that prevents execution of higher priority tasks.
- Task L acquires lock
- Task H needs resource, but is blocked by Task L, so Task L is allowed to execute
- Task M preempts Task L, because it is higher priority
- Thus Task M delays Task L, which delays Task H

4. Priority Inheritance Protocol
PIP eliminates priority inversion problem
The algorithm will increase the priority of a task to the maximum priority of any task waiting for any resource the task has a resource lock on
- i.e. if a lower priority task L has a lock on a resource required by a higher priority task H, then the priority of the lower task is increased to the priority of the higher task
- Once the lock is released the task resumes back its original priority.

5. Priority Ceiling Protocol
Each resource is assigned a priority ceiling, which is a priority equal to the highest priority of any task which may lock the resource.
PCP eliminates chain blocking which considers the use of more than one resource or semaphore
A task can acquire a lock on resource S only if no other task holds a lock on resource R. Thus higher priority tasks will not be blocked through both S and R
If a high priority task is blocked through a resource, then the task holding that resource gets the priority of the high priority task. Once the resource is released, the priority is reset back to its original

6. Priority inversion - A scheduling scenario where a high priority task is indirectly preempted by a medium priority task because it is blocked waiting for a shared resource.
Priority inheritance - A scheduling policy that attempts to solve the problem of priority inversion as follows:
whenever a high priority task has to wait for some resource, shared with an executing low priority task, the low priority task is temporarily assigned the priority of the highest waiting priority task for the duration of its own use of the shared resource. This strategy prevents medium priority tasks from preempting it. When the shared resource is released the low priority task continues to execute at its original priority level.

7. Priority ceiling inheritance - An enhancement of the priority inheritance policy.
A priority ceiling is defined for each resource. The scheduling policy is the same as for priority inheritance except that tasks can only acquire a resource if their static priority is higher than the ceiling of any currently locked resource (excluding that it has already blocked itself).

8. Timers and Interrupts

9. The tight-coupling between the computer and external world distinguishes an embedded system from a regular computer system.
synchronization between the executing software and its external environment is critical for the success of an embedded system. 

10. Definition of ‘Interrupt’
Event that disrupts the normal execution of a program and causes the execution of special instructions

11. Interrupts
Interrupt
Program
time t
UBC 104 Embedded Systems

12. Interrupt Service Routine
Program
Program
Interrupt Service Routine
time t
UBC 104 Embedded Systems

13. Interrupt Service Routine
Program
Program
Save Context
Restore Context
Interrupt Service Routine
mul R1, 9
mov R1, cent
eg push R1
eg pop R1
time t
UBC 104 Embedded Systems

14. Hardware event is called a trigger.
The hardware event can either be a busy to ready transition in an external I/O device (like the UART input/output) or an internal event (like bus fault, memory fault, or a periodic timer).
The execution of the interrupt service routine is called a background thread.
killed when the interrupt service routine returns from interrupt 
A new thread is created for each interrupt request. 

15. Threads share access to I/O devices, system resources, and global variables, while processes have separate global variables and system resources. Processes do not share I/O devices.

16. Exception and Interrupt Handling in ARM as a Case Study

17. Interrupts vs. polling
polling mechanism wastes the CPU time in looping forever checking some flags to know that the event occurred.
we have nowadays systems with more than one interrupt source.
We may need priorities to be assigned to different Events
Interrupt is the best way to handle events

18. ARM modes of operation Internally has 7 different modes of operation
User mode: used for normal program execution state
Fast Interrupt Request (FIQ) mode: This mode is used for interrupts requiring fast response and low latency.
like for example data transfer with DMA
Interrupt Request (IRQ) mode: used for general interrupt services
Abort mode: selected when data or instruction fetch is aborted
System mode: Operating system privilege mode for users
Undefined mode: When undefined instruction is fetched
Supervisor mode: used when operating system support is needed where it works as protected mode

19. Protected Vs. Original Mode
Protected mode differed from the original mode:
Later dubbed “real mode”,
Areas of memory could be physically isolated by the processor itself to prevent illegal writes to other programs running in memory at the same time.
Prior to protected mode, multiple programs could be running in memory at the same time, but any program could access any area of memory and, therefore, if malicious or errant, for example, could take down the entire system.
Isolates that possibility by allowing the operating system (OS) to dictate where each program should run.

20. ARM Modes of Operation

21. ARM Register set
Register structure in ARM depends on the mode of operation.
There are 16 (32-bit) registers named from R0 to R15 in ARM mode (usr).
Registers R0 to R12 are general purpose registers,
R13 is stack pointer (SP),
R14 is subroutine link register
R15 is program counter (PC).
R16 is the current program status register (CPSR) and it plays a main role in the process of switching between modes – eg. the current processor mode

22. Banked Registers
Some registers are available with the same name but as another physical register in memory which is called (banked)
Decreases the effort needed when context switching is required
The new mode has its own physical register space and no need to store the old mode’s register values

23. Register Organization in ARM

24. ARM Exceptions
Exception : any condition that needs to halt normal execution of the instructions.
State of resetting ARM core,
Failure of fetching instructions or memory access
There is always software associated with each exception,
Exception handler.
Each of the ARM exceptions causes the ARM core to enter a certain mode automatically.
Ex.

25. Vector Table
It is a table of instructions that the ARM core branches to when an exception is raised.
These instructions are places in a specific part in memory and its address is related to the exception type

26. Exception priorities Since exceptions can occur simultaneously
We may have more than one exception raised at the same time,
the processor has to have a priority for each exception so it can decide which of the currently raised exceptions is more important

27. Exception priorities
various exceptions that occur on the ARM and their associated priorities.
Both are caused by an instruction entering the execution stage of the ARM instruction
pipeline

28. Link Register Offset
This register is used to return the PC to the appropriate place in the interrupted task
not always the old PC value.
It is modified depending on the type of exception.
In the case of IRQ exception, the link register is pointing initially to the last executed instruction
In data abort exception , the exception is handled and the PC should point to the same instruction again to retry accessing the same memory location again.
Data Exception : processor can fetch and store data or they can refuse to do either.

29. Entering and exiting an exception handler
Save the address of the next instruction in the appropriate Link Register (LR).
Copy Current Program Status Register (CPSR) to the Saved Program Status Register (SPSR) of new mode.
Change the mode by modifying bits in CPSR.
Fetch next instruction from the vector table.

30. Leaving exception handler
Move the Link Register LR (minus an offset) to the PC.
Copy SPSR back to CPSR, this will automatically changes the mode back to the previous one.
Clear the interrupt disable flags (if they were set).

31. Interrupts

32. Interrupts

33. Interrupts

34. Interrupt handling schemes

35. Interrupt handling schemes

36. Interrupt handling schemes

37. Interrupt handling schemes

38. In class work