Fix AArch64 write return value

arch/aarch64.c

@@ -14,15 +14,21 @@ void exit(int8_t status){
 
 intptr_t write(int32_t fd, const void* buf, intptr_t size){
   intptr_t n_written = 0;
+  //register int32_t fd_w asm("w0") = fd;
   asm volatile( 
-    "mov x0, %2\n"
-    "mov x1, %0\n"
-    "mov x2, %1\n"
-    "mov x8, #64\n"
-    "svc #0\n"
-    : //TODO: n_written
-    : "r" (buf), "r"(size), "r"(fd)
-    : "x0", "x8"           // Clobbered registers
+    // Assembly Instructions
+    "mov x0, %1\n"  // Set x0 to value of fd
+    "mov x1, %2\n"  // Set x1 to value of buf
+    "mov x2, %3\n"  // Set x2 to value of size
+    "mov x8, #64\n" // Syscall number for 'write' is 64 in AArch64
+    "svc #0\n"      // Make syscall
+    "mov %0, x0\n"  // Store return value in n_written
+    // Output operands
+    : "=r"(n_written)
+    // Input operands
+    : "r"((int64_t)fd), "r" (buf), "r"(size)
+    // Clobbered registers
+    : "x0", "x1", "x2", "x8"
   );
   return n_written;
 }

buildtest.c

@@ -3,8 +3,12 @@
 
 
 int main() {
-  uint64_t pid = fork();
+  // Test the write syscall
+  intptr_t n = write(STDIO, "Hello\n", 6);
+  if(n != 6) return n;
 
+  // Test the fork syscall
+  uint64_t pid = fork();
   // Print the pid in hex
   char msg[17] = {' '};
   msg[16] = '\n';