Apache NuttX App for Ox64 BL808 SBC, compiled with TCC Compiler. See https://github.com/lupyuen/tcc-riscv32-wasm

nuttx-tcc-ox64.c

int main(int argc, char *argv[]) {

  // Make NuttX System Call to write(fd, buf, buflen)
  const unsigned int nbr = 61; // SYS_write
  const void *parm1 = 1;       // File Descriptor (stdout)
  const void *parm2 = "Hello, World!!\n"; // Buffer
  const void *parm3 = 15; // Buffer Length

  // Load the Parameters into Registers A0 to A3
  // TODO: This doesn't work, so we load again below
  register long r0 asm("a0") = (long)(nbr);
  register long r1 asm("a1") = (long)(parm1);
  register long r2 asm("a2") = (long)(parm2);
  register long r3 asm("a3") = (long)(parm3);

  // Execute ECALL for System Call to NuttX Kernel
  // Load the Parameters into Registers A0 to A3
  asm volatile (

    // Load 61 to Register A0 (SYS_write)
    "addi a0, zero, 61 \n"

    // Load 1 to Register A1 (File Descriptor)
    "addi a1, zero, 1 \n"

    // Load 0x80101000 to Register A2 (Buffer)
    "lui   a2, 0x80 \n"
    "addiw a2, a2, 257 \n"
    "slli  a2, a2, 0xc \n"

    // Load 15 to Register A3 (Buffer Length)
    "addi a3, zero, 15 \n"

    // ECALL for System Call to NuttX Kernel
    "ecall \n"

    // NuttX needs NOP after ECALL
    ".word 0x0001 \n"

    // Input+Output Registers: None
    // Input-Only Registers: A0 to A3
    // Clobbers the Memory
    :
    : "r"(r0), "r"(r1), "r"(r2), "r"(r3)
    : "memory"
  );

  // Loop Forever
  for(;;) {}
  return 0;
}