diff --git a/CMakeLists.txt b/CMakeLists.txt
index ca6fdf7..10fe745 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -169,6 +169,7 @@ if (NOT FZL_SKIP_VM)
 		src/vm/reader/vm_rd_sections_sz.c
 		src/vm/fzl_vm.c
 		src/vm/fzl_vm_env.c
+		src/vm/fzl_vm_extern.c
 		src/vm/fzl_vm_reader.c
 		src/vm/fzl_vm_debug.c
 		src/vm/main.c)
diff --git a/src/vm/fzl_vm.c b/src/vm/fzl_vm.c
index 6bc7184..fb41615 100644
--- a/src/vm/fzl_vm.c
+++ b/src/vm/fzl_vm.c
@@ -8,6 +8,7 @@
 #include <string.h>
 #include <vm/fzl_vm.h>
 #include <vm/fzl_vm_env.h>
+#include <vm/fzl_vm_extern.h>
 #include <vm/fzl_vm_object.h>
 #include <vm/fzl_vm_reader.h>
 #include <vm/internals/opcode.h>
diff --git a/src/vm/fzl_vm_extern.c b/src/vm/fzl_vm_extern.c
index 7cefb9b..118cb04 100644
--- a/src/vm/fzl_vm_extern.c
+++ b/src/vm/fzl_vm_extern.c
@@ -87,10 +87,37 @@ bool prepare_sym(fzl_vm_reader_t *reader, void *sym, fzl_extern_fn_t *fn,
              "specified");
     return false;
   }
+  memset(out, 0, sizeof(fzl_vm_sym_t));
 
-  out->func_ptr = sym;
-  // TODO: fill the FFI CIF struct values
-  return false;
+  out->func_ptr  = sym;
+  out->arg_types = malloc(sizeof(ffi_type *) * fn->args_count);
+  if (!out->arg_types)
+  {
+    snprintf(ex_err, sizeof(ex_err), "out of memory");
+    return false;
+  }
+
+  memset(out->arg_types, 0, sizeof(ffi_type *) * fn->args_count);
+  for (size_t i = 0; i < fn->args_count; i++)
+  {
+    fzl_extern_arg_t *arg = &fn->args[i];
+    if (arg->is_out)
+      out->arg_types[i] = &ffi_type_pointer;
+    else
+      out->arg_types[i] = resolve_type(reader, &arg->type);
+  }
+
+  ffi_type *ret =
+      fn->has_return ? resolve_type(reader, &fn->ret_type) : &ffi_type_void;
+  ffi_status res = ffi_prep_cif(&out->cif, FFI_DEFAULT_ABI, fn->args_count, ret,
+                                out->arg_types);
+  if (res != FFI_OK)
+  {
+    snprintf(ex_err, sizeof(ex_err), "failed to prepare FFI");
+    return false;
+  }
+
+  return true;
 }
 
 fzl_vm_extern_t *fzl_vm_init_externs(fzl_vm_reader_t *reader)
diff --git a/src/vm/handlers/group_fn.c b/src/vm/handlers/group_fn.c
index 9d1c35e..42e1bcd 100644
--- a/src/vm/handlers/group_fn.c
+++ b/src/vm/handlers/group_fn.c
@@ -1,5 +1,8 @@
+#include <common/fzl_value.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <vm/fzl_vm.h>
+#include <vm/fzl_vm_extern.h>
 #include <vm/internals/opcode.h>
 
 // Call function (store return address and jump into function init)
@@ -28,7 +31,31 @@ int op_ret(fzl_vm_t *vm)
 // Call externally defined function
 int op_call_extern(fzl_vm_t *vm)
 {
-  (void)vm;
-  // TODO: implement
+  size_t fn_n = vm->reader->ops[++vm->ip];
+  if (fn_n >= vm->reader->extern_fns_count)
+  {
+    snprintf(last_err, sizeof(last_err),
+             "runtime error - unknown external function");
+    print_err_data(vm);
+    return -1;
+  }
+
+  fzl_vm_sym_t *fn = &vm->prep_fns->syms[fn_n];
+  if (!fn)
+  {
+    snprintf(last_err, sizeof(last_err),
+             "runtime error - failed to load extern function");
+    print_err_data(vm);
+    return -1;
+  }
+
+  if (vm->dtop < fn->cif.nargs)
+  {
+    snprintf(last_err, sizeof(last_err),
+             "runtime error - expected %u valued on dstack", fn->cif.nargs);
+    print_err_data(vm);
+    return -1;
+  }
+
   return 0;
 }
diff --git a/src/vm/main.c b/src/vm/main.c
index 5c420ee..420f574 100644
--- a/src/vm/main.c
+++ b/src/vm/main.c
@@ -145,6 +145,7 @@ int main(int argc, char **argv)
   if (!vm->prep_fns)
   {
     printf("%sError%s: %s\n", FZL_CERROR, FZL_CRST, fzl_vm_externs_geterror());
+    fzl_vm_free_externs(vm->prep_fns);
     return -1;
   }
 
@@ -163,6 +164,7 @@ int main(int argc, char **argv)
         printf("%sError%s: %s\n", FZL_CERROR, FZL_CRST, err);
 
       free(args);
+      fzl_vm_free_externs(vm->prep_fns);
       fzl_vm_freereader(reader);
       fzl_freevm(vm);
       return -1;
@@ -170,6 +172,7 @@ int main(int argc, char **argv)
   }
 
   free(args);
+  fzl_vm_free_externs(vm->prep_fns);
   fzl_vm_freereader(reader);
   fzl_freevm(vm);