New guy here. :D I've been reading up on the AVR-GCC compiler toolchain and trying to get a complete understanding of the process. I understand most of it from a top-level except for the "Start-Up Code":
http://www.mygooglest.com/fni/el...
I've read references to how the start-up code is specific to a particular device, but what is it and where does it come from? What does it do?
It depends on what you are talking about.
There is "start up code" that is generated by the compiler that includes things like initialized variables.
There is also start-up code that YOU write. It does things like set up timers, configure I/O ports, set UART baud rate, and such.
By the way, in my opinion, that page you referenced does NOT give very good information. You want to use EMACS? We have a semi-great IDE called AVR Studio 6 that provides an editor, automatically generated make files, compilation with gcc, linking, code downloading to the target, simulation, and debugging in one complete package.
Jim
Koshchi The startup code referred to on that page comes from avr-libc. As Jim said, this is code that sets up global variables, ISR vectors and the stack so that it is all ready for main().
JohanEkdahl 
The startup code is doing several things. For example:
- It sets up a default interrupt handler so that if an interrupt occurs for which you have not created an ISR then the default one will be activated.
- It does what every C tool chain must do: It sets uninitialized statically allocated variables to zero, and sets initialized statically allocated variables to their respective init values.
- It initializes the stack pointer.
- It initializes the heap.
(and perhaps more that I've forgotten.)
- Finally it calls your main() function.
Just asa there is startup code, there is code that will be executed should your main() ever return. From memory, it
- Disables interrupts
- Goes into an endless loop
Thanks everyone! Here's the page I was reading originally (I couldn't find it earlier):
https://www.avrfreaks.net/wiki/in...
The diagram shows two elements, Lib and startup code as input to the linker. That's what threw me. It makes sense for them to be the same file.
By the way, in my opinion, that page you referenced does NOT give very good information. You want to use EMACS? We have a semi-great IDE called AVR Studio 6 that it provides an editor, automatically generated make files, compilation with gcc, linking, code downloading to the target, simulation, and debugging in one complete package.Jim
Thanks Jim! I downloaded Studio 6 and started messing around with it. I'm designing into an ATmega2561 on a custom board. Also I'm very new with C and C++ so I'm still reading up on how to read and write registers and do bit-manipulation. I'm coming from a strong background in FPGA/digital design and trying my hand at embedded software. This is exciting! :D
clawson 
The diagram shows two elements, Lib and startup code as input to the linker. That's what threw me. It makes sense for them to be the same file.
If you want to learn more search here for threads that include "CRT", "_do_copy_data" and "_do_clear_bss" which are two of the main components of the CRT. Maybe also search for "gcrt1.S" which is the single source file in AVR-LibC that builds about 100+ different crtXXX.o files in the run time library.
