5.2. The C Runtime Initialization, crt0.o

5.2. The C Runtime Initialization, `crt0.o`

5.2.1. Exception vector setup
5.2.2. The _start Function and Stack Initialization
5.2.3. Cache Initialization
5.2.4. Clearing BSS
5.2.5. Constructor and Destructor Handling
5.2.6. C Initialization Functions
5.2.7. Invoking the main program

The C Runtime system must carry out the following tasks.

Set up the target platform in a consistent state. For example setting up appropriate exception vectors.
Initialize the stack and frame pointers
Invoke the C constructor initialization and ensure destructors are called on exit.
Carry out any further platform specific initialization.
Call the C main function.
Exit with the return code supplied if the C main function ever terminates.

The code is invariably assembler, although it may call out to C functions, and is best illustrated by example from the OpenRISC 1000 . This is a BSP designed for use with a fast architectural simulator. It comes in two variants, one providing just standard output to the console, the other implementing a simulated UART with both standard input and standard output. The crt0.0 is common to both BSPs and found in crt0.S.