...

src/runtime/.gitignore

@@ -1 +1,2 @@
 *.o
+out

src/runtime/makefile

@@ -1,5 +1,15 @@
-x86_64:
-	gcc -nostdlib -ffreestanding -c platform/x86_64.s runtime.c
+CC = gcc
+CFLAGS = -nostdlib -ffreestanding -Wall -Werror -Wextra
+
+x64: runtime.o x64.o
+	mkdir -p out
+	ar rcs out/libruntime.a runtime.o x64.o
+
+runtime.o: runtime/runtime.c
+	$(CC) $(CFLAGS) -c runtime/runtime.c -o runtime.o
+
+x64.o: runtime.o platform/x64.s
+	$(CC) $(CFLAGS) -c platform/x64.s -o x64.o
 
 clean:
-	rm runtime.o
+	rm -r out runtime.o x64.o

src/runtime/platform/x64.s

@@ -4,9 +4,12 @@
 .globl nub_write_string
 # void nub_write_string(const char* str)
 nub_write_string:
-    mov rsi, rdi
-    mov rdi, 1
+    push rdi
+    call nub_cstring_length
+    mov rdx, rax
+    pop rsi
     mov rax, 1
+    mov rdi, 1
     syscall
 
 .text

src/runtime/runtime/runtime.c

@@ -45,11 +45,11 @@ void nub_panic_oom()
     nub_exit(NUB_PANIC_ERROR_CODE);
 }
 
-extern int main();
+extern uint64_t main();
 
 __attribute__((noreturn)) void _start()
 {
-    int exit_code = main();
+    uint64_t exit_code = main();
     nub_exit(exit_code);
     __builtin_unreachable();
 }