RISC-V Bare Bones: Difference between revisions

The thing we will make here is a simple serial port output/reader.

 

#include <stdint.h>

#include <stddef.h>

return;

}

 

This will print a simple "Hello world!" and a newline following, after that it will echo any input given to the serial port.

Be wary we are just setting up stack and jumping into kernel. It's the kernel's job to set up IRQs and other system stuff

 

.section .init

 

.end

 

 

Take in account that you need to implement a type of locking if you want to support SMP, and implement a trampoline or a jail for the CPUs. Be wary that we haven't enabled interrupts just yet and any return from kmain will result in invalid opcodes being executed.

We need to specify where our kernel will be loaded at. For this example we will use the start of the RAM as our load address.

 

ENTRY(start);

}

}

 

== Final ==

 

The process is pretty straightforward:

riscv64-elf-gcc -Wall -Wextra -c -mcmodel=medany kernel.c -o kernel.o -ffreestanding

riscv64-elf-as -c entry.S -o entry.o

riscv64-elf-ld -T linker.ld -lgcc -nostdlib kernel.o entry.o -o kernel.elf

 

Once this is done, the kernel can be run with the following:

qemu-system-riscv64 -machine virt -bios none -kernel kernel.elf -serial mon:stdio

 

You will promptly see a terminal with the "Hello world!" and you can "write stuff" to it. A serial terminal is very "boring", but you still have a lot at your disposal, you have a PLIC, a CLIC and a bunch of opportunities to learn PCI internals.