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The C Run-Time Environment (crt0)

To link and run C or C++ programs, you need to define a small module (usually written in assembler as crt0.s) that makes sure the hardware is initialized for C conventions before calling main.

There are some examples of crt0.s code (along with examples of system call stub code) available in the source code for GNUPro Toolkit.

Look in the following path.

installdir/progressive-97r1a/src/newlib/libc/sys

installdir refers to your installation directory, by default /usr/cygnus.

For example, look in .../sys/h8300hms for Hitachi H8/300 bare boards, or in .../sys/sparclite for the Fujitsu SPARClite board.

More examples are in the following directory.

installdir/progressive-97r1a/src/newlib/stub

To write your own crt0.s, you need the following information about your target.

- A memory map. What memory is available, and where?
- Which way does the stack grow?
- What output format do you use?

At a minimum, your crt0.s must do the following processes.

- Define the symbol, start (_start in assembler code). Execution begins at this symbol.
- Set up the stack pointer, sp. It is largely up to you to choose where to store your stack within the constraints of your targets memory map. Perhaps the simplest choice is to choose a fixed-size area somewhere in the uninitialized data section (often called bss). Remember that whether you choose the low address or the high address in this area depends on the direction your stack grows.
- Initialize all memory in the uninitialized-data (bss) section to zero.
  The easiest way to do this is with the help of a linker script (see Command Language in Using LD in GNUPro Utilities). Use a linker script to define symbols such as bss_start and bss_end to record the boundaries of this section; then you can use a for loop to initialize all memory between them in crt0.s.
- Call main. Nothing else will!

A more complete crt0.s might also do the following processes.

- Define an _exit subroutine. This is the C name; in your assembler code. Use the label, _ _exit, with two leading underbars. Its precise behavior depends on the details of your system, and on your choice. Possibilities include trapping back to the boot monitor, if there is one; or to the loader, if there is no monitor; or even back to the symbol start.
- If your target has no monitor to mediate communications with the debugger, you must set up the hardware exception handler in crt0.s. See The GDB remote serial protocol in Debugging with GDB in GNUPro Debugging Tools for details on how to use the GDB generic remote-target facilities for this purpose.
- Perform other hardware-dependent initialization; for example, initializing an MMU or an auxiliary floating-point chip.
- Define low-level input and output subroutines. For example, crt0.s is a convenient place to define the minimal assembly-level routines described in System Calls in GNUPro C Library in GNUPro Libraries.