diff --git a/lib/srfi/39.module b/lib/srfi/39.module
index 11b9ed9f..e77a3c50 100644
--- a/lib/srfi/39.module
+++ b/lib/srfi/39.module
@@ -1,4 +1,21 @@
 
+;; (param) => get-cell + cdr
+;; (param value) => get-cell + [convert] + set-cdr!
+
+;; need 3 opcodes: param cell
+;;                 context params
+;;                 context-params-set!
+
+;; analyze needs to translate param calls to proper usages of the
+;; first two opcodes
+
+;; (parameterize ((param value)) body ...)
+;;  => (let* ((old-params (thread-parameters))
+;;            (new-params (cons (cons param (param-convert value)) old-params)))
+;;       (dynamic-wind (lambda () (thread-set-parameters! new-params))
+;;                     (lambda () body ...)
+;;                     (lambda () (thread-set-parameters! old-params))))
+
 (define-module (srfi 39)
   (export make-parameter parameterize)
   (import-immutable (scheme))
diff --git a/vm.c b/vm.c
index 97c162ea..4d272bbc 100644
--- a/vm.c
+++ b/vm.c
@@ -279,11 +279,12 @@ static void generate_opcode_app (sexp ctx, sexp app) {
     }
     break;
   case SEXP_OPC_PARAMETER:
-    emit_push(ctx, sexp_opcode_data(op));
 #if SEXP_USE_GREEN_THREADS
     emit(ctx, SEXP_OP_PARAMETER_REF);
     emit_word(ctx, (sexp_uint_t)op);
     sexp_push(ctx, sexp_bytecode_literals(sexp_context_bc(ctx)), op);
+#else
+    emit_push(ctx, sexp_opcode_data(op));
 #endif
     emit(ctx, ((num_args == 0) ? SEXP_OP_CDR : SEXP_OP_SET_CDR));
     break;