diff --git a/mps/code/.p4ignore b/mps/code/.p4ignore
index 70e69009357..682821ffd84 100644
--- a/mps/code/.p4ignore
+++ b/mps/code/.p4ignore
@@ -13,6 +13,7 @@ lii6gc
 lii6ll
 w3i3mv
 w3i6mv
+xca6ll
 xci3gc
 xci3ll
 xci6gc
diff --git a/mps/code/config.h b/mps/code/config.h
index 50bd9d7ed70..aa9b70c7cce 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -576,7 +576,13 @@
    things like thread states.  These definitions fix that. */
 
 #if defined(MPS_OS_XC)
-#if defined(MPS_ARCH_I6)
+#if defined(MPS_ARCH_A6)
+
+#define THREAD_STATE_COUNT ARM_THREAD_STATE64_COUNT
+#define THREAD_STATE_FLAVOR ARM_THREAD_STATE64
+#define THREAD_STATE_S arm_thread_state64_t
+
+#elif defined(MPS_ARCH_I6)
 
 #define THREAD_STATE_COUNT x86_THREAD_STATE64_COUNT
 #define THREAD_STATE_FLAVOR x86_THREAD_STATE64
diff --git a/mps/code/eventpy.c b/mps/code/eventpy.c
index c6d9f5952ed..c8990ddc5bf 100644
--- a/mps/code/eventpy.c
+++ b/mps/code/eventpy.c
@@ -15,7 +15,7 @@
 #include "event.h"
 
 /* See <https://docs.python.org/3/library/struct.html#byte-order-size-and-alignment> */
-#if defined(MPS_ARCH_I3) || defined(MPS_ARCH_I6)
+#if defined(MPS_ARCH_A6) || defined(MPS_ARCH_I3) || defined(MPS_ARCH_I6)
 #define BYTE_ORDER "<"
 #else
 #error "Can't determine byte order for platform architecture."
diff --git a/mps/code/mps.c b/mps/code/mps.c
index 10cb47094a1..3fbfc579f7e 100644
--- a/mps/code/mps.c
+++ b/mps/code/mps.c
@@ -110,6 +110,20 @@
 #include "prmcanan.c"   /* generic architecture mutator context */
 #include "span.c"       /* generic stack probe */
 
+/* macOS on ARM64 built with Clang */
+
+#elif defined(MPS_PF_XCA6LL)
+
+#include "lockix.c"     /* Posix locks */
+#include "thxc.c"       /* macOS Mach threading */
+#include "vmix.c"       /* Posix virtual memory */
+#include "protix.c"     /* Posix protection */
+#include "protxc.c"     /* macOS Mach exception handling */
+#include "prmcanan.c"   /* generic architecture mutator context */
+#include "prmcxc.c"     /* macOS mutator context */
+#include "prmcxca6.c"   /* ARM64 for macOS mutator context */
+#include "span.c"       /* generic stack probe */
+
 /* macOS on IA-32 built with Clang or GCC */
 
 #elif defined(MPS_PF_XCI3LL) || defined(MPS_PF_XCI3GC)
diff --git a/mps/code/mpstd.h b/mps/code/mpstd.h
index d3beaed6b06..df56d29f2a6 100644
--- a/mps/code/mpstd.h
+++ b/mps/code/mpstd.h
@@ -165,6 +165,25 @@
 #define MPS_PF_ALIGN    8
 
 
+/* Apple clang version 12.0, clang -E -dM */
+
+#elif defined(__APPLE__) && defined(__arm64__) && defined(__MACH__) \
+      && defined(__clang__)
+#if defined(CONFIG_PF_STRING) && ! defined(CONFIG_PF_XCA6LL)
+#error "specified CONFIG_PF_... inconsistent with detected xca6ll"
+#endif
+#define MPS_PF_XCA6LL
+#define MPS_PF_STRING   "xca6ll"
+#define MPS_OS_XC
+#define MPS_ARCH_A6
+#define MPS_BUILD_LL
+#define MPS_T_WORD      unsigned long
+#define MPS_T_ULONGEST  unsigned long
+#define MPS_WORD_WIDTH  64
+#define MPS_WORD_SHIFT  6
+#define MPS_PF_ALIGN    8
+
+
 /* Apple clang version 3.1, clang -E -dM */
 
 #elif defined(__APPLE__) && defined(__i386__) && defined(__MACH__) \
diff --git a/mps/code/prmcxc.h b/mps/code/prmcxc.h
index 9af1e601e98..be2b6d0a974 100644
--- a/mps/code/prmcxc.h
+++ b/mps/code/prmcxc.h
@@ -12,7 +12,13 @@
 #include "mpm.h"
 
 #include <mach/mach_types.h>
+#if defined(MPS_ARCH_A6)
+#include <mach/arm/thread_status.h>
+#elif defined(MPS_ARCH_I3) || defined(MPS_ARCH_I6)
 #include <mach/i386/thread_status.h>
+#else
+#error "Unknown macOS architecture"
+#endif
 
 typedef struct MutatorContextStruct {
   Sig sig;                      /* <design/sig> */
diff --git a/mps/code/prmcxca6.c b/mps/code/prmcxca6.c
new file mode 100644
index 00000000000..ce1de415a32
--- /dev/null
+++ b/mps/code/prmcxca6.c
@@ -0,0 +1,57 @@
+/* prmcxca6.c: MUTATOR CONTEXT ARM64 (macOS)
+ *
+ * $Id$
+ * Copyright (c) 2001-2021 Ravenbrook Limited.  See end of file for license.
+ *
+ * .purpose: Implement the mutator context module. <design/prmc>.
+ *
+ *
+ * ASSUMPTIONS
+ *
+ * .sp: The stack pointer in the context is SP.
+ */
+
+#include "prmcxc.h"
+
+SRCID(prmcxca6, "$Id$");
+
+#if !defined(MPS_OS_XC) || !defined(MPS_ARCH_A6)
+#error "prmcxca6.c is specific to MPS_OS_XC and MPS_ARCH_A6"
+#endif
+
+
+Addr MutatorContextSP(MutatorContext context)
+{
+  AVERT(MutatorContext, context);
+  return (Addr)arm_thread_state64_get_sp(*(context->threadState));   /* .sp */
+}
+
+
+/* C. COPYRIGHT AND LICENSE
+ *
+ * Copyright (C) 2001-2021 Ravenbrook Limited <https://www.ravenbrook.com/>.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+ * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
diff --git a/mps/code/protxc.c b/mps/code/protxc.c
index 346fe6074f5..172908b5cd6 100644
--- a/mps/code/protxc.c
+++ b/mps/code/protxc.c
@@ -70,7 +70,6 @@
 #include <mach/thread_act.h>
 #include <mach/thread_status.h>
 #include <mach/mach_error.h>
-#include <mach/i386/thread_status.h>
 #include <mach/exc.h>
 #include <pthread.h>
 #include <stdlib.h> /* see .trans.stdlib */
diff --git a/mps/code/testlib.h b/mps/code/testlib.h
index d595cac5705..f140b0a9e1b 100644
--- a/mps/code/testlib.h
+++ b/mps/code/testlib.h
@@ -104,7 +104,7 @@
  * tests to root out any incompatible assumptions by breaking.
  */
 
-#if defined(MPS_ARCH_I6)
+#if defined(MPS_ARCH_A6) || defined(MPS_ARCH_I6)
 #define PRIwWORD "16"
 #elif defined(MPS_ARCH_I3)
 #define PRIwWORD "8"
diff --git a/mps/code/xca6ll.gmk b/mps/code/xca6ll.gmk
new file mode 100644
index 00000000000..c4834070759
--- /dev/null
+++ b/mps/code/xca6ll.gmk
@@ -0,0 +1,58 @@
+# -*- makefile -*-
+#
+# xca6ll.gmk: BUILD FOR macOS/ARM64/Clang/LLVM PLATFORM
+#
+# $Id$
+# Copyright (c) 2001-2021 Ravenbrook Limited.  See end of file for license.
+#
+# .prefer.xcode: The documented and preferred way to develop the MPS
+# for this platform is to use the Xcode project (mps.xcodeproj). This
+# makefile provides a way to compile the MPS one source file at a
+# time, rather than all at once via mps.c (which can hide errors due
+# to missing headers).
+
+PFM = xca6ll
+
+MPMPF = \
+    lockix.c \
+    prmcanan.c \
+    prmcxc.c \
+    prmcxca6.c \
+    protix.c \
+    protxc.c \
+    span.c \
+    thxc.c \
+    vmix.c
+
+include ll.gmk
+include comm.gmk
+
+
+# C. COPYRIGHT AND LICENSE
+#
+# Copyright (C) 2001-2021 Ravenbrook Limited <https://www.ravenbrook.com/>.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the
+#    distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
diff --git a/mps/configure b/mps/configure
index 912b78e46b3..cb018412424 100755
--- a/mps/configure
+++ b/mps/configure
@@ -3478,6 +3478,18 @@ $as_echo "Linux x86_64" >&6; }
       MPS_BUILD_NAME=ll
       PFMCFLAGS="$CFLAGS_LL"
     ;;
+  aarch64-*-darwin*/*)
+      { $as_echo "$as_me:${as_lineno-$LINENO}: result: Mac OS X ARM64" >&5
+$as_echo "Mac OS X ARM64" >&6; }
+      MPS_OS_NAME=xc
+      MPS_ARCH_NAME=a6
+      MPS_BUILD_NAME=ll
+      BUILD_TARGET=build-via-xcode
+      CLEAN_TARGET=clean-xcode-build
+      INSTALL_TARGET=install-xcode-build
+      TEST_TARGET=test-xcode-build
+      PFMCFLAGS="$CFLAGS_LL"
+    ;;
   i*86-*-darwin*/*)
       { $as_echo "$as_me:${as_lineno-$LINENO}: result: Mac OS X x86" >&5
 $as_echo "Mac OS X x86" >&6; }
diff --git a/mps/configure.ac b/mps/configure.ac
index c57fd0bf566..acf935b640e 100644
--- a/mps/configure.ac
+++ b/mps/configure.ac
@@ -66,6 +66,17 @@ case $host/$CLANG in
       MPS_BUILD_NAME=ll
       PFMCFLAGS="$CFLAGS_LL"
     ;;
+  aarch64-*-darwin*/*)
+      AC_MSG_RESULT([Mac OS X ARM64])
+      MPS_OS_NAME=xc
+      MPS_ARCH_NAME=a6
+      MPS_BUILD_NAME=ll
+      BUILD_TARGET=build-via-xcode
+      CLEAN_TARGET=clean-xcode-build
+      INSTALL_TARGET=install-xcode-build
+      TEST_TARGET=test-xcode-build
+      PFMCFLAGS="$CFLAGS_LL"
+    ;;
   i*86-*-darwin*/*)
       AC_MSG_RESULT([Mac OS X x86])
       MPS_OS_NAME=xc
diff --git a/mps/design/prmc.txt b/mps/design/prmc.txt
index eb427b39f49..fb141f23a8c 100644
--- a/mps/design/prmc.txt
+++ b/mps/design/prmc.txt
@@ -268,13 +268,14 @@ macOS implementation
 
 _`.impl.xc`: In ``prmcix.c`` and ``prmcxc.c``, with processor-specific
 parts in ``prmci3.c`` and ``prmci6.c``, and other platform-specific
-parts in ``prmcxci3.c`` and ``prmcxci6.c``.
+parts in ``prmcxca6.c``, ``prmcxci3.c`` and ``prmcxci6.c``.
 
 _`.impl.xc.context`: The context consists of the
 ``__Request__mach_exception_raise_state_identity_t`` and
-``x86_thread_state32_t`` or ``x86_thread_state64_t`` structures. There
-doesn't seem to be any documentation for these structures, but they
-are defined in the Mach headers.
+``arm_thread_state_t``, ``x86_thread_state32_t`` or
+``x86_thread_state64_t`` structures. There doesn't seem to be any
+documentation for these structures, but they are defined in the Mach
+headers.
 
 _`.impl.xc.fault.addr`: ``__Request__mach_exception_raise_state_identity_t.code[1]`` is the
 address that the faulting instruction was trying to access.
@@ -292,9 +293,11 @@ calling |thread_get_state|_.
 .. _thread_get_state: https://www.gnu.org/software/hurd/gnumach-doc/Thread-Execution.html
 
 _`.impl.xc.context.scan`: The thread's registers are found in the
-``x86_thread_state32_t`` or ``x86_thread_state64_t`` structure.
+``arm_thread_state64_t``, ``x86_thread_state32_t`` or
+``x86_thread_state64_t`` structure.
 
-_`.impl.xc.context.sp`: The stack pointer is obtained from
+_`.impl.xc.context.sp`: The stack pointer is obtained using the
+``arm_thread_state64_get_sp()`` macro (on ARM64), or from
 ``x86_thread_state32_t.__esp`` (on IA-32) or
 ``x86_thread_state64_t.__rsp`` (on x86-64).
 
diff --git a/mps/manual/build.txt b/mps/manual/build.txt
index 0e4828abbf5..8a5e55c1795 100644
--- a/mps/manual/build.txt
+++ b/mps/manual/build.txt
@@ -144,6 +144,7 @@ Platform     OS          Architecture    Compiler      Makefile
 ``lii6ll``   Linux       x86-64          Clang         ``lii6ll.gmk``
 ``w3i3mv``   Windows     IA-32           Microsoft C   ``w3i3mv.nmk``
 ``w3i6mv``   Windows     x86-64          Microsoft C   ``w3i6mv.nmk``
+``xca6ll``   macOS       ARM64           Clang         ``mps.xcodeproj``
 ``xci3ll``   macOS       IA-32           Clang         ``mps.xcodeproj``
 ``xci6ll``   macOS       x86-64          Clang         ``mps.xcodeproj``
 ==========   =========   =============   ============  =================
diff --git a/mps/manual/source/code-index.rst b/mps/manual/source/code-index.rst
index 0f172a8dd5c..f30ac3a1e59 100644
--- a/mps/manual/source/code-index.rst
+++ b/mps/manual/source/code-index.rst
@@ -183,6 +183,7 @@ prmcw3i3.c    Mutator context implementation for Windows, IA-32.
 prmcw3i6.c    Mutator context implementation for Windows, x86-64.
 prmcxc.c      Mutator context implementation for macOS.
 prmcxc.h      Mutator context interface for macOS.
+prmcxca6.c    Mutator context implementation for macOS, ARM64.
 prmcxci3.c    Mutator context implementation for macOS, IA-32.
 prmcxci6.c    Mutator context implementation for macOS, x86-64.
 prot.h        Protection interface. See design.mps.prot_.
@@ -393,6 +394,7 @@ w3i3mv.nmk     NMAKE file for platform W3I3MV.
 w3i3pc.nmk     NMAKE file for platform W3I3PC.
 w3i6mv.nmk     NMAKE file for platform W3I6MV.
 w3i6pc.nmk     NMAKE file for platform W3I6PC.
+xca6ll.gmk     GNU makefile for platform XCA6LL.
 xci3gc.gmk     GNU makefile for platform XCI3GC.
 xci3ll.gmk     GNU makefile for platform XCI3LL.
 xci6gc.gmk     GNU makefile for platform XCI6GC.
diff --git a/mps/manual/source/release.rst b/mps/manual/source/release.rst
index b8fb7239fa9..70d5b7ccded 100644
--- a/mps/manual/source/release.rst
+++ b/mps/manual/source/release.rst
@@ -12,6 +12,8 @@ Release 1.118.0
 New features
 ............
 
+#. New supported platform ``xca6ll`` (macOS, ARM64, Clang/LLVM).
+
 #. The MPS no longer supports building for the xci3ll platform (macOS,
    IA-32, Clang/LLVM) using Xcode. This is because Xcode 10.0 no
    longer supports this platform. The platform is still supported via
diff --git a/mps/manual/source/topic/platform.rst b/mps/manual/source/topic/platform.rst
index 0538ec52f16..d72f3fb7a28 100644
--- a/mps/manual/source/topic/platform.rst
+++ b/mps/manual/source/topic/platform.rst
@@ -28,6 +28,7 @@ The second pair of characters names the processor architecture:
 ======  ======================  ======================
 ``AR``  Processor architecture  Constant
 ======  ======================  ======================
+``a6``  ARM64                   :c:macro:`MPS_ARCH_A6`
 ``i3``  Intel/AMD IA-32         :c:macro:`MPS_ARCH_I3`
 ``i6``  Intel/AMD x86-64        :c:macro:`MPS_ARCH_I6`
 ======  ======================  ======================
@@ -66,6 +67,13 @@ Platform interface
     #include "mpstd.h"
 
 
+.. c:macro:: MPS_ARCH_A6
+
+    A :term:`C` preprocessor macro that indicates, if defined, that
+    the target processor architecture of the compilation is a member
+    of the ARM64 family of 64-bit processors.
+
+
 .. c:macro:: MPS_ARCH_I3
 
     A :term:`C` preprocessor macro that indicates, if defined, that
@@ -206,6 +214,13 @@ Platform interface
     compiler.
 
 
+.. c:macro:: MPS_PF_XCA6LL
+
+    A :term:`C` preprocessor macro that indicates, if defined, that
+    the :term:`platform` consists of the macOS operating system, the
+    ARM64 processor architecture, and the Clang/LLVM compiler.
+
+
 .. c:macro:: MPS_PF_XCI3GC
 
     A :term:`C` preprocessor macro that indicates, if defined, that
@@ -378,6 +393,7 @@ Platform    Status
 ``w3i6mv``  Supported
 ``w3i6pc``  *Not supported*
 ``w3ppmv``  *Not supported*
+``xca6ll``  Supported
 ``xci3gc``  *Not supported*
 ``xci3ll``  Supported
 ``xci6gc``  *Not supported*
diff --git a/mps/manual/source/topic/porting.rst b/mps/manual/source/topic/porting.rst
index ea497dd9776..30e039d9627 100644
--- a/mps/manual/source/topic/porting.rst
+++ b/mps/manual/source/topic/porting.rst
@@ -76,8 +76,8 @@ usable.
    stack` can be scanned.
 
    See :ref:`design-prmc` for the design, and ``prmc.h`` for the
-   interface. There are implementations on Unix, Windows, and macOS for
-   IA-32 and x86-64.
+   interface. There are implementations on Unix and Windows for
+   IA-32 and x86-64, and on macOS for IA-32, x86-64, and ARM64.
 
    There is a generic implementation in ``prmcan.c``, which can't
    provide these features, and so only supports a single thread.