RISC-V C Baremetal Example

This project demonstrates how to use C on baremetal RISC-V, emulated via QEMU. This example is actually short (unlike everything else on the internet apparently).

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 and frame 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 (otherwise, the execution tends to just up and die, I don't know why).

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 system's Device Tree. On QEMU, the Device Tree can be obtained by adding dumpdtb=[Device Tree Blob target file here] to QEMU's machine arguments:

qemu-system-riscv64 -M virt,dumpdtb=virt.dtb -cpu rv64

The resulting blob then needs to be converted to a human readable format via Device Tree Compiler:

dtc -I dtb -O dts virt.dtb -o virt.dts

After that, you can look through virt.dts to find the RAM's properties. Example:

	memory@80000000 {
		device_type = "memory";
		reg = <0x00 0x80000000 0x00 0x8000000>;
	};

This tells us that RAM space begins at address 0x80000000, and is 0x8000000 bytes long.

Makefile just build everything.

Usage

Firstly, install everything necessary, most notably qemu-system-riscv64 and a RISC-V cross compiler.

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

Lastly, run the example by invoking qemu-system-riscv64 -M virt -cpu rv64 -bios riscv.bin. If everything is working correctly, the register a0 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.