Code execution in embedded systems
I am working in embedded system domain. I would like to know how a code gets executed from a microcontroller(uC need not be subjective, in general), starting from a C file. Also i would like to know stuffs like startup code, object file, etc. I couldnt find any online documentations regarding the above stuff. If possible, please provide links which explains those things from scratch. Thanks in advance for your help
Answer
Being a microprocessor architect, I have had the opportunity to work at a very low level for software. Basically, low-level embedded is very different from general PC programming only at the hardware specific level.
Low-level embedded software can be broken down into the following:
- Reset vector - this is usually written in assembly. It is the very first thing that runs at start-up and can be considered hardware-specific code. It will usually perform simple functions like setting up the processor into a pre-defined steady state by configuring registers and such. Then it will jump to the startup code. The most basic reset vector merely jumps directly to the start-up code.
- Startup code - this is the first software-specific code that runs. Its job is basically to set up the software environment so that C code can run on top. For example, C code assumes that there is a region of memory defined as stack and heap. These are usually software constructs instead of hardware. Therefore, this piece of start-up code will define the stack pointers and heap pointers and such. This is usually grouped under the 'c-runtime'. For C++ code, constructors are also called. At the end of the routine, it will execute
main(). edit: Variables that need to be initialised and also certain parts of memory that need clearing are done here. Basically, everything that is needed to move things into a 'known state'. - Application code - this is your actual C application starting from the
main()function. As you can see, a lot of things are actually under the hood and happen even before your first main function is called. This code can usually be written as hardware-agnostic if there is a good hardware abstraction layer available. The application code will definitely make use of a lot of library functions. These libraries are usually statically linked in embedded systems. - Libraries - these are your standard C libraries that provide primitive C functions. There are also processor specific libraries that implement things like software floating-point support. There can also be hardware-specific libraries to access the I/O devices and such for stdin/stdout. A couple of common C libraries are Newlib and uClibc.
- Interrupt/Exception handler - these are routines that run at random times during normal code execution as a result of changes in hardware or processor states. These routines are also typically written in assembly as they should run with minimal software overhead in order to service the actual hardware called.
Hope this will provide a good start. Feel free to leave comments if you have other queries.