user.c

#include "user.h"

extern char __stack_top[];

int syscall(int sysno, int arg0, int arg1, int arg2) {
    register int a0 __asm__("a0") = arg0;
    register int a1 __asm__("a1") = arg1;
    register int a2 __asm__("a2") = arg2;
    register int a3 __asm__("a3") = sysno;

    /* trigger the kernel (external call instruction )*/
    __asm__ __volatile__("ecall" : "=r"(a0) : "r"(a0), "r"(a1), "r"(a2), "r"(a3) : "memory");

    return a0;
}

__attribute__((noreturn)) void exit(void) {
    syscall(SYS_EXIT, 0, 0, 0);
    for (;;)
        ;
}

void putchar(char c) { syscall(SYS_PUTCHAR, c, 0, 0); }

int getchar(void) { return syscall(SYS_GETCHAR, 0, 0, 0); }

__attribute__((section(".text.start"))) __attribute__((naked)) void start(void) {
    __asm__ __volatile__("mv sp, %[stack_top] \n"
                         "call main           \n"
                         "call exit           \n" ::[stack_top] "r"(__stack_top));
}