Embedded C is the most popular choice of language used for developing embedded systems because of its simplicity, efficiency, less time required for development, and its portability from one system to another. As we know that the embedded systems have constraints on hardware resources such as CPU, memory sizes, etc, it becomes very important to use the resources judiciously and responsibly. To achieve this, Embedded C usually can interact with the hardware resources with necessary abstractions.
ISR expands to Interrupt Service Routines. These are the procedures stored at a particular memory location and are called when certain interrupts occur. Interrupt refers to the signal sent to the processor that indicates there is a high-priority event that requires immediate attention. The processor suspends the normal flow of the program, executes the instructions in ISR to cater for the high priority event. Post execution of the ISR, the normal flow of the program resumes. The following diagrams represent the flow of ISR.
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Void pointers are those pointers that point to a variable of any type. It is a generic pointer as it is not dependent on any of the inbuilt or user-defined data types while referencing. During dereferencing of the pointer, we require the correct data type to which the data needs to be dereferenced.
The volatile keyword is mainly used for preventing a compiler from optimizing a variable that might change its behaviour unexpectedly post the optimization. Consider a scenario where we have a variable where there is a possibility of its value getting updated by some event or a signal, then we need to tell the compiler not to optimize it and load that variable every time it is called. To inform the compiler, we use the keyword volatile at the time of variable declaration.
The Concatenation operator is indicated by the usage of ##. It is used in macros to perform concatenation of the arguments in the macro. We need to keep note that only the arguments are concatenated, not the values of those arguments.
Variables defined with static are initialized once and persists until the end of the program and are local only to the block it is defined. A static variables declaration requires definition. It can be defined in a header file. But if we do so, a private copy of the variable of the header file will be present in each source file the header is included. This is not preferred and hence it is not recommended to use static variables in a header file.
A function is called reentrant if the function can be interrupted in the middle of the execution and be safely called again (re-entered) to complete the execution. The interruption can be in the form of external events or signals or internal signals like call or jump. The reentrant function resumes at the point where the execution was left off and proceeds to completion.
Loops that involve count down to zero are better than count-up loops. This is because the compiler can optimize the comparison to zero at the time of loop termination. The processors need not have to load both the loop variable and the maximum value for comparison due to the optimization. Hence, count down to 0 loops are always better.
A null pointer is a pointer that does not point to any valid memory location. It is defined to ensure that the pointer should not be used to modify anything as it is invalid. If no address is assigned to the pointer, it is set to NULL.
Virtual memory is a means of allocating memory to the processes if there is a shortage of physical memory by using an automatic allocation of storage. The main advantage of using virtual memory is that it is possible to have larger virtual memory than physical memory. It can be implemented by using the technique of paging.
From the code given, it appears that the function intends to return the square of the values pointed by the pointer p. But, since we have the pointer point to a volatile integer, the compiler generates code as below:
Since the pointer can be changed to point to other locations, it might be possible that the values of the x and y would be different which might not even result in the square of the numbers. Hence, the correct way for achieving the square of the number is by coding as below:
Interrupt latency can be reduced by ensuring that the ISR routines are short. When a lower priority interrupt gets triggered while a higher priority interrupt is getting executed, then the lower priority interrupt would get delayed resulting in increased latency. In such cases, having smaller ISR routines for lower priority interrupts would help to reduce the delay.
Here the pointer ptr is not initialized and in the next step, we are trying to assign a valid value to it. If the ptr has a garbage location address, then that would corrupt the upcoming instructions too.
If we are trying to de-allocate this pointer and free it as well using the free function, and again if we are not assigning the pointer as NULL or any valid address, then again chances are that the pointer would still be pointing to the garbage location and accessing from that would lead to errors. These pointers are called dangling pointers.
Both declarations specify for the files to be included in the current source file. The difference is in how and where the preprocessor looks for including the files. For #include "...", the preprocessor just searches for the file in the current directory as where the source file is present and if not found, it proceeds to search in the standard directories specified by the compiler. Whereas for the #include declaration, the preprocessor looks for the files in the compiler designated directories where the standard library files usually reside.
Memory leak is a phenomenon that occurs when the developers create objects or make use of memory to help memory and then forget to free the memory before the completion of the program. This results in reduced system performance due to the reduced memory availability and if this continues, at one point, the application can crash. These are serious issues for applications involving servers, daemons, etc that should ideally never terminate.
In this example, we have created pointer p inside the function and we have not freed the pointer before the completion of the function. This causes pointer p to remain in the memory. Imagine 100s of pointers like these. The memory will be occupied unnecessarily and hence resulting in a memory leak.
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What really helped me were a compilation of my university scrapbooks and online blogs to get my technical basics right, so this post is about list of questions which might help individuals whether freshers or little seasoned Engineers who are preparing for their Interviews.
I hope these embedded system interview questions will be helpful to you. If you have any other important questions related to the embedded systems and concepts or have better answers, then please write in the comment box. I will make sure to add edits.
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With these ideas in mind, I have attempted to construct a test that is heavily slanted towards the requirements of embedded systems. This is a lousy test to give to someone seeking a job writing compilers! The questions are almost all drawn from situations I have encountered over the years. Some of them are very tough; however, they should all be informative.
This test may be given to a wide range of candidates. Most entry-level applicants will do poorly on this test, while seasoned veterans should do very well. Points are not assigned to each question, as this tends to arbitrarily weight certain questions. However, if you choose to adapt this test for your own uses, feel free to assign scores.
(c) An understanding that the pre-processor will evaluate constant expressions for you. Thus, it is clearer, and penalty free to spell out how you are calculating the number of seconds in a year, rather than actually doing the calculation yourself.
(a) Basic knowledge of the #define directive as used in macros. This is important, because until the inline operator becomes part of standard C, macros are the only portable way of generating inline code. Inline code is often necessary in embedded systems in order to achieve the required performance level.
(b) Knowledge of the ternary conditional operator. This exists in C because it allows the compiler to potentially produce more optimal code than an if-then-else sequence. Given that performance is normally an issue in embedded systems, knowledge and use of this construct is important.
(b) A variable declared static within a module [1], (but outside the body of a function) is accessible by all functions within that module. It is not accessible by functions within any other module. That is, it is a localized global. 152ee80cbc
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