diff --git a/src/lang/bool.sh b/src/lang/bool.sh
index 6104cfc..ed02d08 100644
--- a/src/lang/bool.sh
+++ b/src/lang/bool.sh
@@ -9,6 +9,7 @@ readonly BOOL_MOD=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
 
 . ${BOOL_MOD}/bash.sh
 . ${BOOL_MOD}/core.sh
+. ${BOOL_MOD}/os.sh
 
 
 # ----------
@@ -255,3 +256,25 @@ function is_bash5() {
 
         [ $(bash_version_major $ctx) = "5" ]
 }
+
+function is_linux() {
+        # Return true/0 if linux.
+        local ctx; is_ctx "${1}" && ctx="${1}" && shift
+        [ $# -ne 0 ] && { ctx_wn $ctx; return $EC; }
+        shift 0 || { ctx_wn $ctx; return $EC; }
+
+        local name
+        name=$(os_name $ctx)
+        [ "${name}" = "${OS_LINUX}" ]
+}
+
+function is_mac() {
+        # Return true/0 if mac.
+        local ctx; is_ctx "${1}" && ctx="${1}" && shift
+        [ $# -ne 0 ] && { ctx_wn $ctx; return $EC; }
+        shift 0 || { ctx_wn $ctx; return $EC; }
+
+        local name
+        name=$(os_name $ctx)
+        [ "${name}" = "${OS_MAC}" ]
+}
diff --git a/src/lang/p.sh b/src/lang/p.sh
index fed7c13..7c2b782 100644
--- a/src/lang/p.sh
+++ b/src/lang/p.sh
@@ -10,6 +10,7 @@ readonly LANG_PACKAGE=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
 . ${LANG_PACKAGE}/core.sh
 . ${LANG_PACKAGE}/unsafe.sh
 . ${LANG_PACKAGE}/os.sh
+. ${LANG_PACKAGE}/runtime.sh
 . ${LANG_PACKAGE}/sys.sh
 . ${LANG_PACKAGE}/make.sh
 . ${LANG_PACKAGE}/log.sh
diff --git a/src/lang/runtime.sh b/src/lang/runtime.sh
new file mode 100644
index 0000000..6775b5a
--- /dev/null
+++ b/src/lang/runtime.sh
@@ -0,0 +1,45 @@
+#!/bin/bash
+#
+# https://github.com/EngineeringSoftware/gobash/blob/main/LICENSE
+#
+# Runtime functions.
+
+if [ -n "${LANG_RUNTIME_MOD:-}" ]; then return 0; fi
+readonly LANG_RUNTIME_MOD=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
+
+. ${LANG_RUNTIME_MOD}/core.sh
+. ${LANG_RUNTIME_MOD}/bool.sh
+
+
+# ----------
+# Functions.
+
+function runtime_num_cpu() {
+        # Return the number of logical CPUs.
+        local ctx; is_ctx "${1}" && ctx="${1}" && shift
+        [ $# -ne 0 ] && { ctx_wn $ctx; return $EC; }
+        shift 0 || { ctx_wn $ctx; return $EC; }
+
+        if is_mac; then
+                sysctl -n hw.logicalcpu_max
+        else
+                [ ! -f "/proc/cpuinfo" ] && \
+                        { ctx_w $ctx "no cpuinfo"; return $EC; }
+                cat /proc/cpuinfo | grep 'processor' | wc -l
+        fi
+}
+
+function runtime_num_physical_cpu() {
+        # Return the number of physical CPUs.
+        local ctx; is_ctx "${1}" && ctx="${1}" && shift
+        [ $# -ne 0 ] && { ctx_wn $ctx; return $EC; }
+        shift 0 || { ctx_wn $ctx; return $EC; }
+
+        if is_mac; then
+                sysctl -n hw.physicalcpu_max
+        else
+                [ ! -f "/proc/cpuinfo" ] && \
+                        { ctx_w $ctx "no cpuinfo"; return $EC; }
+                cat /proc/cpuinfo  | grep 'core id' | sort -u | wc -l
+        fi
+}
diff --git a/src/lang/runtime_test.sh b/src/lang/runtime_test.sh
new file mode 100644
index 0000000..32e1cc6
--- /dev/null
+++ b/src/lang/runtime_test.sh
@@ -0,0 +1,35 @@
+#!/bin/bash
+#
+# https://github.com/EngineeringSoftware/gobash/blob/main/LICENSE
+#
+# Unit tests for the runtime module.
+
+if [ -n "${LANG_RUNTIME_TEST_MOD:-}" ]; then return 0; fi
+readonly LANG_RUNTIME_TEST_MOD=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
+
+. ${LANG_RUNTIME_TEST_MOD}/assert.sh
+. ${LANG_RUNTIME_TEST_MOD}/os.sh
+
+
+# ----------
+# Functions.
+
+function test_runtime_num_cpu() {
+        local n
+        n=$(runtime_num_cpu) || assert_fail
+
+        ! is_int "${n}" && assert_fail
+        [ "${n}" -lt 1 ] && assert_fail
+        return 0
+}
+readonly -f test_runtime_num_cpu
+
+function test_runtime_num_physical_cpu() {
+        local n
+        n=$(runtime_num_physical_cpu) || assert_fail
+
+        ! is_int "${n}" && assert_fail
+        [ "${n}" -lt 1 ] && assert_fail
+        return 0
+}
+readonly -f test_runtime_num_physical_cpu