Embedded Software Engineer Interview Preparation Guide

Direct memory access is mainly used to overcome the disadvantages of interrupt and program controlled I/O.
DMA modules usually take the control over from the processor and perform the memory operations and this is mainly because to counteract the mismatch in the processing speeds of I/O units and the processor. This is comparatively faster.
It is an important part of any embedded systems,and the reason for their use is that they can be used for bursty data transfers instead of single byte approaches.
It has to wait for the systems resources such as the system bus in case it is already in control of it.

☛ Prototype of the function used to create a child process is pid_t fork(void);
☛ Fork is the system call that is used to create a child process. It takes no arguments and returns a value of type pid_t.
☛ If the function succeeds it returns the pid of the child process created to its parent and child receives a zero value indicating its successful creation.
☛ On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set.
☛ The child process normally performs all its operations in its parents context but each process independently of one nother and also inherits some of the important attributes from it such as UID, current directory, root directory and so on.

☛ Damage of memory devices due to transient current and static discharges.
☛ Malfunctioning of address lines due to a short in the circuit.
☛ Malfunctioning of Data lines.
☛ Some memory locations being inaccessible in storage due to garbage or errors.
☛ Improper insertion of Memory devices into the memory slots.
☛ Faulty control signals.

Functional requirements specifies the discrete and the logic to provide the services, functionality, features and the implementation that is independent from the components that are getting used in it. These are used to represent the constraints that are in the form of physical and define the probability to specify the components discretely from each other. The functional requirements are given for the hardware as well that gives more performance and measures the physical resources that are present like clock frequency, latency, etc. Functional requirements allow the system and hardware machines to transfer the functions with the non-deterministic probability.

The memory bus is used to carry the address and the data from the processor to the memory so that it can be easily accessed by the devices. These buses carry the value of the data that has to be passed for the proper functioning. The use of the technique “Time division multiplexing” is used that allow the reading and writing of the data to be done from the same bus line. This requires lots of time to be given to the bus so that it can complete the read and write operation of the data in the memory. This is very expensive process due to the data transfer technique that is used in between the processor and the memory. This also gives the concept of cache and provides algorithms to solve the problems occurring in read and writes operations.

The interrupts that comes in between the input/output operations gets detected when the input/output devices are ready. The interrupt never gets handled directly rather, it sends the interrupt signal to the thread to the input/output device that is ready to allow the thread to take necessary actions. The thread uses the signaling concept that allows the initialization to be done using the semaphore that keeps the states updated and handle the interrupt in an easy way. The input/output device getting the request and it also passes the semaphore to handle. The input/output device takes the semaphore that is ready. The thread is having the minimum latency that uses the first level interrupt handler to handle the interrupts completely. It allows the priority of the thread to be set and it doesn’t allow the context to be change as well.

To check the loop time of the program the power-save super loop is used. If the average loop time of the program is 1ms, and it requires only few instructions to be checked every second the program will save the state and build a delay that will be caused to read the input on every loop and it saves lot of energy or the power that needs to be used. The function that is required to be performed to show the functionality is:
Main_Function()
Function
{
Initialization();
Do_Forever
{
Check_Status();
Do_Calculations();
Output_Response();
Delay_For_Next_Loop();
}
}

It is the time taken to return from the interrupt service routine post handling a specific interrupt. Interrupt latency can be reduced by writing minor ISR routines.

The C's volatile keyword is a qualifier that tells the compiler not to optimize when applied to a variable. By declaring a variable volatile, we can tell the compiler that the value of the variable may change any moment from outside of the scope of the program. A variable should be declared volatile whenever its value could change unexpectedly and beyond the comprehension of the compiler.

In those cases it is required not to optimize the code, doing so may lead to erroneous result and load the variable every time it is used in the program. Volatile keyword is useful for memory-mapped peripheral registers, global variables modified by an interrupt service routine, global variables accessed by multiple tasks within a multi-threaded application.

Interrupt latency can be minimized by writing short ISR routine and by not delaying interrupts for more time.