Booting A Cortex M with NO IDE : like a caveman : Part 3 Startup File

 

How about those AI generated images, that supposed to be a C start file haha!!

Purpose of the Startup File:

The startup file is a crucial part of an embedded system's initialization process. Its primary purpose is to:

1. Initialize the Hardware: Set up the initial state of the microcontroller, including configuring the stack pointer, initializing static data, and setting up the vector table.

2. Prepare the Runtime Environment: Zero-initialize the .bss section (uninitialized data) and copy the initialized data from Flash to RAM (.data section).

3. Jump to the Main Program: After setting up the environment, the startup file typically transfers control to the main() function, where the user’s application code begins execution.

Why It’s Usually Written in Assembly:

1. Low-Level Control: Assembly language provides direct access to the CPU registers and hardware features, allowing precise control over the initialization process that might be challenging or inefficient to achieve in C.

2. Minimal Overhead: Startup code needs to run before any other code, including the C runtime library. Writing it in assembly ensures there’s minimal overhead and no dependency on a higher-level runtime, which might not be initialized yet.

3. CPU-Specific Initialization: Certain tasks, like setting the stack pointer or configuring the CPU’s vector table, require instructions that are specific to the processor and are best implemented in assembly. However as you saw in the vector table we can just place the stack pointer at the right location without any need for code per say.
   

For typical applications with standard initialization needs, like our use case here, writing the startup file in C is perfectly acceptable. It simplifies development and leverages the C language's capabilities to prepare the system for running the main application. Assembly is more critical when you need fine-grained control over the hardware or when optimizing for the smallest possible code size and maximum performance.

 

myStartUp.c

The first step is to include the necessary standard integer header and reference all the symbols defined in our linker script:

Keep in mind that these symbols represent address locations, not the values stored at those addresses.

Next, we need to define the reset handler function, as it is the first function executed upon reset. Since the reset handler will eventually call the main function, we should also declare main as an external variable to inform the compiler that it exists elsewhere. Additionally, we'll create a default interrupt handler. All unimplemented interrupt handlers will be mapped to this default handler. By marking this mapping as weak, we ensure that any actual interrupt handlers we implement can override the default one. For now we will make the function prototypes.

Now lets make the vector table. You can find the layout of the vector table for your device in its reference manual, below is a snippet of mine.

As you can see in the table an per my explanation before, 0x00000000 is reserved this is where the initial stack pointer will go and right after that comes the Reset_Handler, and they are all 1 word apart (4 bytes). 

Lets first add the prototypes for the exception and interrupt handlers and define them as weak with the default being Default_Handler

Below is my vector table itself.

The next and final two pieces we need are the Reset_Handler itself and the Default_Handler

And that is all you need as far as the startup file goes. The C code pretty much speaks for itself so im not going to do into detail about it. 

The next step is to use our toolchain to compile, link and flash to the device. See you in the next post!

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