AVR C Baremetal Example

This project demonstrates how to use C on baremetal AVR, emulated via QEMU. This example is very short, for a quick start.

Overview

• cstart.c: The C code to be executed.
• start.s: Short loader assembly code, which initializes the CPU, and then invokes cstart() from cstart.c.
• link.ld: Linker script, linking everything together, as well as defining the RAM and the entry point.
• Makefile: The Makefile that compiles and links everything.

Firstly, the CPU needs to be initialized (most notably the stack pointer). This is done using assembly (start.s). After that, C can be called.

Lastly, if the C code returns, it is important that execution ends in a loop which does nothing.

Link.ld Contains the linker script needed to load _start from start.s at the correct address (the beginning of RAM space), so that it actually executes. It also contains a pointer to the end of RAM space, which is where the stack pointer is initialized to.

Note: RAM Space may differ system to system. You may need to modify its definition in link.ld with data obtained from your board's datasheet. Since Arduino's aren't defined via Device Trees, we can't use QEMU's dumpdtb (like we could with my other examples).

Makefile just build everything.

Usage

Firstly, install everything necessary, most notably qemu-system-avr and an AVR cross compiler.

Then, you can compile everything by running make, which will produce avr.bin.

Lastly, run the example by invoking qemu-system-avr -M uno -bios avr.bin. If everything is working correctly, the registers r24 and r25 should contain 0x489. You can check by running info registers in the QEMU monitor console, or by adding -d cpu to the QEMU invocation command.