diff --git a/spike/backend/driver.ml b/spike/backend/driver.ml
index 355a748..99deda9 100644
--- a/spike/backend/driver.ml
+++ b/spike/backend/driver.ml
@@ -103,6 +103,49 @@ let run src =
      close_out oc
    with Not_found -> ());
 
+  (* ── The SysV boundary ──────────────────────────────────────────────
+     Three synthetic functions, built as Tast by hand rather than written in
+     Flan, because the surface language has no way to spell a call to an
+     arbitrary C symbol with eight arguments. [Tast.Rt] is the node a runtime
+     call already uses and the one a [declare-c] shim lands on, so this is the
+     real path with a made-up callee. *)
+  let loc = Flan.Loc.unknown in
+  let i64 = Flan.Types.Int Flan.Types.I64 in
+  let ex e = { Flan.Tast.e; ty = i64; loc } in
+  let lit n = ex (Flan.Tast.Int (Int64.of_int n, Flan.Types.I64)) in
+  let probe name params body =
+    { Flan.Tast.name; params; slots = Array.make (List.length params) i64;
+      snames = Array.make (List.length params) None; ret = i64;
+      body = [ body ]; fdefers = []; fparent = None; floc = loc }
+  in
+  let arg0 = ex (Flan.Tast.Local 0) in
+  let probes = [
+    (* Eight integers: six in registers and two on the stack, which is the case
+       a register-only convention gets silently wrong. *)
+    probe "abi-8" [ i64 ]
+      (ex (Flan.Tast.Prim (Flan.Tast.Rt "spike_probe8",
+         [ arg0; lit 2; lit 3; lit 4; lit 5; lit 6; lit 7; lit 8 ])));
+    (* rsp % 16 == 0 at the call. The callee does an aligned 16-byte spill and
+       answers -1 if it was entered misaligned. *)
+    probe "abi-align" [ i64 ]
+      (ex (Flan.Tast.Prim (Flan.Tast.Rt "spike_probe_align", [ arg0 ])));
+    (* The same call, but underneath a binary operator -- so it is evaluated
+       with the left operand spilled on the stack. This is the one that matters:
+       alignment at a call site is not a property of the prologue, it is a
+       property of how much the expression evaluator has pushed. *)
+    probe "abi-align-nested" [ i64 ]
+      (ex (Flan.Tast.Prim (Flan.Tast.Add,
+         [ lit 0;
+           ex (Flan.Tast.Prim (Flan.Tast.Rt "spike_probe_align", [ arg0 ])) ])));
+  ] in
+  List.iter
+    (fun (fd : Flan.Tast.fn) ->
+       let code = X86.fn ~resolve fd in
+       let p = jit_alloc page in
+       jit_write p code; jit_protect p page;
+       Hashtbl.replace addrs fd.Flan.Tast.name p)
+    probes;
+
   print_endline "results:";
   let at n = Hashtbl.find addrs n in
   check "spike-add 3 4" (call2 (at "spike-add") 3L 4L) 7L;
@@ -112,6 +155,9 @@ let run src =
   check "spike-if 1 2" (call2 (at "spike-if") 1L 2L) 1L;
   check "spike-if 9 2" (call2 (at "spike-if") 9L 2L) 7L;
   check "spike-calls 5" (call1 (at "spike-calls") 5L) 656L;
+  check "abi-8 1" (call1 (at "abi-8") 1L) 87654321L;
+  check "abi-align 10" (call1 (at "abi-align") 10L) 13L;
+  check "abi-align-nested 10" (call1 (at "abi-align-nested") 10L) 13L;
 
   Printf.printf "\n%d checks, %d failures\n" !checks !failures;
   exit (if !failures = 0 then 0 else 1)