wrote a shell but have an error will debug tomorrow

user.c

@@ -1,13 +1,32 @@
 #include "user.h"
+#include "common.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) {
     for(;;);
 }
 
 void putchar(char ch) {
-    // TODO
+    syscall(SYS_PUTCHAR, ch, 0, 0);
+}
+
+int getchar(void) {
+    return syscall(SYS_GETCHAR, 0, 0, 0);
 }
 
 __attribute__((section(".text.start")))