diff --git a/make-config.sh b/make-config.sh
index 3820b73c0..40fff6c79 100755
--- a/make-config.sh
+++ b/make-config.sh
@@ -629,10 +629,12 @@ case "$sbcl_os" in
         ;;
     darwin)
         printf ' :unix :bsd :darwin :mach-o' >> $ltf
+        darwin_version=`uname -r`
+        darwin_version_major=${DARWIN_VERSION_MAJOR:-${darwin_version%%.*}}
+        if (( 10 > $darwin_version_major )) || [ $sbcl_arch = "ppc" ]; then
+            printf ' :use-darwin-posix-semaphores' >> $ltf
+        fi
         if [ $sbcl_arch = "x86-64" ]; then
-            darwin_version=`uname -r`
-            darwin_version_major=${DARWIN_VERSION_MAJOR:-${darwin_version%%.*}}
-
             if (( 8 < $darwin_version_major )); then
 	        printf ' :inode64' >> $ltf
             fi
diff --git a/make-target-2-load.lisp b/make-target-2-load.lisp
index 0a10c6f67..be188f5e3 100644
--- a/make-target-2-load.lisp
+++ b/make-target-2-load.lisp
@@ -55,6 +55,8 @@
           ;; The final batch of symbols is strictly for C. The LISP_FEATURE_
           ;; prefix on the corresponding #define is unfortunate.
           :GCC-TLS :USE-SYS-MMAP
+          ;;; Enforce using of posix semaphores on Darwin instead of dispatch.
+          :USE-DARWIN-POSIX-SEMAPHORES
           ;; only for 'src/runtime/wrap.h'
           :OS-PROVIDES-BLKSIZE-T
           ;; only for src/runtime/run-program.c
diff --git a/src/runtime/darwin-os.c b/src/runtime/darwin-os.c
index f417c8dd7..804dd9b9f 100644
--- a/src/runtime/darwin-os.c
+++ b/src/runtime/darwin-os.c
@@ -55,7 +55,7 @@ void darwin_init(void)
 }
 
 
-#ifdef LISP_FEATURE_SB_THREAD
+#if defined LISP_FEATURE_SB_THREAD && defined USE_DARWIN_GCD_SEMAPHORES
 
 inline void
 os_sem_init(os_sem_t *sem, unsigned int value)
@@ -141,4 +141,53 @@ futex_wake(int *lock_word, int n)
     return 0;
 }
 #endif
+
+#elif defined LISP_FEATURE_SB_THREAD && defined CANNOT_USE_POSIX_SEM_T
+
+inline void
+os_sem_init(os_sem_t *sem, unsigned int value)
+{
+    if (KERN_SUCCESS!=semaphore_create(mach_task_self(), sem, SYNC_POLICY_FIFO, (int)value))
+        lose("os_sem_init(%p): %s", sem, strerror(errno));
+}
+
+inline void
+os_sem_wait(os_sem_t *sem)
+{
+    kern_return_t ret;
+  restart:
+    ret = semaphore_wait(*sem);
+    switch (ret) {
+    case KERN_SUCCESS:
+        return;
+        /* It is unclear just when we can get this, but a sufficiently
+         * long wait seems to do that, at least sometimes.
+         *
+         * However, a wait that long is definitely abnormal for the
+         * GC, so we complain before retrying.
+         */
+    case KERN_OPERATION_TIMED_OUT:
+        fprintf(stderr, "os_sem_wait(%p): %s", sem, strerror(errno));
+        /* This is analogous to POSIX EINTR. */
+    case KERN_ABORTED:
+        goto restart;
+    default:
+        lose("os_sem_wait(%p): %lu, %s", sem, (long unsigned)ret, strerror(errno));
+    }
+}
+
+void
+os_sem_post(os_sem_t *sem)
+{
+    if (KERN_SUCCESS!=semaphore_signal(*sem))
+        lose("os_sem_post(%p): %s", sem, strerror(errno));
+}
+
+void
+os_sem_destroy(os_sem_t *sem)
+{
+    if (-1==semaphore_destroy(mach_task_self(), *sem))
+        lose("os_sem_destroy(%p): %s", sem, strerror(errno));
+}
+
 #endif
diff --git a/src/runtime/darwin-os.h b/src/runtime/darwin-os.h
index f13669fb0..dfcc9e8ea 100644
--- a/src/runtime/darwin-os.h
+++ b/src/runtime/darwin-os.h
@@ -36,10 +36,14 @@ typedef ucontext_t os_context_t;
 
 void darwin_init(void);
 
-#ifdef LISP_FEATURE_SB_THREAD
+#if defined(LISP_FEATURE_SB_THREAD) && !defined(LISP_FEATURE_USE_DARWIN_POSIX_SEMAPHORES)
 #define USE_DARWIN_GCD_SEMAPHORES
 #include <dispatch/dispatch.h>
 typedef dispatch_semaphore_t os_sem_t;
+#elif defined LISP_FEATURE_SB_THREAD
+#define CANNOT_USE_POSIX_SEM_T
+#include <mach/semaphore.h>
+typedef semaphore_t os_sem_t;
 #endif
 
 #endif /* _DARWIN_OS_H */
diff --git a/src/runtime/os-common.c b/src/runtime/os-common.c
index 8de5e89aa..b08f1ccc1 100644
--- a/src/runtime/os-common.c
+++ b/src/runtime/os-common.c
@@ -120,7 +120,7 @@ os_get_errno(void)
     return errno;
 }
 
-#if defined LISP_FEATURE_SB_THREAD && defined LISP_FEATURE_UNIX && !defined USE_DARWIN_GCD_SEMAPHORES
+#if defined LISP_FEATURE_SB_THREAD && defined LISP_FEATURE_UNIX && !defined USE_DARWIN_GCD_SEMAPHORES && !defined CANNOT_USE_POSIX_SEM_T
 void
 os_sem_init(os_sem_t *sem, unsigned int value)
 {
diff --git a/src/runtime/os.h b/src/runtime/os.h
index 262896562..55c5296ca 100644
--- a/src/runtime/os.h
+++ b/src/runtime/os.h
@@ -237,7 +237,7 @@ extern char *os_get_runtime_executable_path();
 #define OS_VM_SIZE_FMTX PRIxPTR
 
 #if defined LISP_FEATURE_SB_THREAD && defined LISP_FEATURE_UNIX
-#  ifndef USE_DARWIN_GCD_SEMAPHORES
+#  if !defined USE_DARWIN_GCD_SEMAPHORES && !defined CANNOT_USE_POSIX_SEM_T
 #    include <semaphore.h>
      typedef sem_t os_sem_t;
 #  endif