diff --git a/libc/thread/sem.c b/libc/thread/sem.c
index 0789f1bd0..5d005cdb8 100644
--- a/libc/thread/sem.c
+++ b/libc/thread/sem.c
@@ -17,9 +17,8 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/bits/atomic.h"
-#include "libc/sysv/consts/futex.h"
-#include "libc/sysv/consts/nr.h"
 #include "libc/thread/sem.h"
+#include "libc/thread/wait.h"
 #include "libc/thread/yield.h"
 
 #define CTHREAD_THREAD_VAL_BITS 32
@@ -51,14 +50,9 @@ int cthread_sem_signal(cthread_sem_t* sem) {
                : "cc");
 
   if ((count >> CTHREAD_THREAD_VAL_BITS)) {
-    int flags = FUTEX_WAKE;
-
     // WARNING: an offset of 4 bytes would be required on little-endian archs
     void* wait_address = &sem->linux.count;
-    asm volatile("syscall"
-                 : /* no outputs */
-                 : "a"(__NR_futex), "D"(wait_address), "S"(flags), "d"(1)
-                 : "rcx", "r11", "cc", "memory");
+    cthread_memory_wake32(wait_address, 1);
   }
 
   return 0;
@@ -84,16 +78,9 @@ int cthread_sem_wait_futex(cthread_sem_t* sem, const struct timespec* timeout) {
       }
     }
     
-    int flags = FUTEX_WAIT;
-    register struct timespec* timeout_ asm("r10") = timeout;
-
     // WARNING: an offset of 4 bytes would be required on little-endian archs
     void* wait_address = &sem->linux.count;
-    asm volatile("syscall"
-                 : /* no outputs */
-                 : "a"(__NR_futex), "D"(wait_address), "S"(flags), "d"(count),
-                   "r"(timeout_)
-                 : "rcx", "r11", "cc", "memory");
+    cthread_memory_wait32(wait_address, count, timeout);
     count = atomic_load(&sem->linux.count);
   }
 
diff --git a/libc/thread/wait.c b/libc/thread/wait.c
new file mode 100644
index 000000000..c0ecb7427
--- /dev/null
+++ b/libc/thread/wait.c
@@ -0,0 +1,50 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/sysv/consts/futex.h"
+#include "libc/sysv/consts/nr.h"
+#include "libc/thread/wait.h"
+
+int cthread_memory_wait32(uint32_t* addr, uint32_t val,
+                          const struct timespec* timeout) {
+  if (__NR_futex != 0xfff) {
+    int flags = FUTEX_WAIT;
+    int rc;
+    register struct timespec* timeout_ asm("r10") = timeout;
+    asm volatile("syscall"
+                 : "=a"(rc)
+                 : "0"(__NR_futex), "D"(addr), "S"(flags), "d"(val),
+                   "r"(timeout_)
+                 : "rcx", "r11", "cc", "memory");
+    return rc;
+  }
+  return -1;
+}
+
+int cthread_memory_wake32(uint32_t* addr, int n) {
+  if (__NR_futex != 0xfff) {
+    int flags = FUTEX_WAKE;
+    int rc;
+    asm volatile("syscall"
+                 : "=a"(rc)
+                 : "0"(__NR_futex), "D"(addr), "S"(flags), "d"(n)
+                 : "rcx", "r11", "cc", "memory");
+    return rc;
+  }
+  return -1;
+}
diff --git a/libc/thread/wait.h b/libc/thread/wait.h
new file mode 100644
index 000000000..21deaade9
--- /dev/null
+++ b/libc/thread/wait.h
@@ -0,0 +1,16 @@
+#ifndef COSMOPOLITAN_LIBC_THREAD_WAIT_H_
+#define COSMOPOLITAN_LIBC_THREAD_WAIT_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+/**
+ * @fileoverview wait on memory
+ */
+struct timespec;
+
+int cthread_memory_wait32(uint32_t*, uint32_t, const struct timespec*);
+int cthread_memory_wake32(uint32_t*, int);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_THREAD_WAIT_H_ */