Bare metal RISC-V hello world in C

This is a copy of the riscv-probe repository stripped from everything but the 64-bit hello world program for the sifive_u QEMU machine.

The directory structure is kept. This makes this repository a good starting point to dive into riscv-probe. In particular we keep the crt.s and start.c files although they are almost empty. They are used in the original repository to initialize libfemto. Here libfemto just provides a puts function.

Having such a stripped down hello world program written in C was necessary for me to learn how to create a bare metal RISC-V assembly hello world version because I couldn't find a complete example program in assembly only that was writing to the UART.

Building

Assuming the default prefix for the GNU toolchain is correct, and the toolchain is in the $PATH, running make produce our hello program. Refer to the original repository otherwise.

$ make
AS.rv64imac build/obj/rv64imac/env/qemu-sifive_u/crt.o
CC.rv64imac build/obj/rv64imac/examples/hello/hello.o
CC.rv64imac build/obj/rv64imac/libfemto/arch/riscv/start.o
CC.rv64imac build/obj/rv64imac/libfemto/std/putchar.o
AR.rv64imac build/lib/rv64imac/libfemto.a
LD.rv64imac build/bin/rv64imac/qemu-sifive_u/hello

The result is a 64-bit RISC-V binary.

$ file build/bin/rv64imac/qemu-sifive_u/hello
build/bin/rv64imac/qemu-sifive_u/hello: ELF 64-bit LSB executable, UCB RISC-V,
version 1 (SYSV), statically linked, not stripped

Running

Run it with:

$ qemu-system-riscv64 -nographic -machine sifive_u \
  -kernel build/bin/rv64imac/qemu-sifive_u/hello 
Hello.
QEMU: Terminated

Note: the program enters an infinite loop after producing the Hello. text. Type ctrl-a x to stop QEMU.

Assembly

To dissamble the program:

$ riscv64-unknown-elf-objdump -d build/bin/rv64imac/qemu-sifive_u/hello
build/bin/rv64imac/qemu-sifive_u/hello:     file format elf64-littleriscv

Disassembly of section .text:

0000000080000000 <_start>:
    80000000:	0040006f          j	80000004 <main>

0000000080000004 <main>:
    80000004:	1141              addi	sp,sp,-16
    80000006:	e022              sd	s0,0(sp)
    80000008:	e406              sd	ra,8(sp)
    8000000a:	00000417          auipc	s0,0x0
    8000000e:	02e40413          addi	s0,s0,46 # 80000038 <putchar+0x16>
    80000012:	00044503          lbu	a0,0(s0)
    80000016:	e111              bnez	a0,8000001a <main+0x16>
    80000018:	a001              j	80000018 <main+0x14>
    8000001a:	0405              addi	s0,s0,1
    8000001c:	006000ef          jal	ra,80000022 <putchar>
    80000020:	bfcd              j	80000012 <main+0xe>

0000000080000022 <putchar>:
    80000022:	100137b7          lui	a5,0x10013
    80000026:	4398              lw	a4,0(a5)
    80000028:	02071693          slli	a3,a4,0x20
    8000002c:	fe06cde3          bltz	a3,80000026 <putchar+0x4>
    80000030:	0ff57513          andi	a0,a0,255
    80000034:	c388              sw	a0,0(a5)
    80000036:	8082              ret

I used the above listing to guide modifications to the hello-world-abs.S program provided in rv8 to finally create a pure assembly hello world.