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;

	__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 ch)
{
	syscall(SYS_PUTCHAR, ch, 0, 0);
}

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

int read(const char *filename, char *buf, int len)
{
	return syscall(SYS_READ, (int)filename, (int)buf, len);
}

int write(const char *filename, const char *buf, int len)
{
	return syscall(SYS_WRITE, (int)filename, (int)buf, len);
}

__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)		
	);
}