diff --git a/README.adoc b/README.adoc
index e932cc7..4067d26 100644
--- a/README.adoc
+++ b/README.adoc
@@ -1589,7 +1589,9 @@ See also: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/19
 
 The implementation is described at: https://stackoverflow.com/questions/46415059/how-to-observe-aarch64-system-registers-in-qemu/53043044#53043044
 
-=== GDB step debug multicore
+=== GDB step debug multicore userland
+
+For a more minimal baremetal multicore setup, see: <<arm-multicore>>.
 
 We can set and get which cores the Linux kernel allows a program to run on with `sched_getaffinity` and `sched_setaffinity`:
 
@@ -10510,13 +10512,13 @@ output:
 
 ==== ARM multicore
 
-TODO get working: CPU1 not waking up:
+TODO get working: CPU 1 not waking up:
 
 ....
-./run --arch aarch64 --baremetal arch/aarch64/no_bootloader/multicore
+./run --arch aarch64 --baremetal arch/aarch64/multicore --cpus 2
 ....
 
-Source: link:baremetal/arch/aarch64/no_bootloader/multicore.S[]
+Source: link:baremetal/arch/aarch64/multicore.S[]
 
 CPU 0 of this program enters a spinlock loop: it repeatedly checks if a given memory address is `1`.
 
diff --git a/baremetal/arch/aarch64/no_bootloader/multicore.S b/baremetal/arch/aarch64/multicore.S
similarity index 70%
rename from baremetal/arch/aarch64/no_bootloader/multicore.S
rename to baremetal/arch/aarch64/multicore.S
index 4494e6c..bfa2f0c 100644
--- a/baremetal/arch/aarch64/no_bootloader/multicore.S
+++ b/baremetal/arch/aarch64/multicore.S
@@ -1,7 +1,7 @@
 /* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-multicore */
 
-.global mystart
-mystart:
+.global main
+main:
     /* Reset spinlock. */
     mov x0, #0
     ldr x1, =spinlock
@@ -21,15 +21,7 @@ mystart:
     ldr x0, spinlock
     cbz x0, 1b
 
-    /* Semihost exit. */
-    mov x1, #0x26
-    movk x1, #2, lsl #16
-    str x1, [sp,#0]
-    mov x0, #0
-    str x0, [sp,#8]
-    mov x1, sp
-    mov w0, #0x18
-    hlt 0xf000
+    ret
 
 spinlock:
     .skip 8
diff --git a/build-baremetal b/build-baremetal
index b8e1294..16594bb 100755
--- a/build-baremetal
+++ b/build-baremetal
@@ -44,29 +44,32 @@ class BaremetalComponent(common.Component):
             uart_address = 0x09000000
         os.makedirs(build_dir, exist_ok=True)
         os.makedirs(common.baremetal_build_lib_dir, exist_ok=True)
-        common.run_cmd(
-            [gcc,  common.Newline] +
-            cflags +
-            [
-                '-c', common.Newline,
-                '-o', bootloader_obj, common.Newline,
-                os.path.join(common.baremetal_src_lib_dir, '{}{}'.format(args.arch, common.asm_ext)), common.Newline,
-            ]
-        )
-        for src, obj in [
-            (common_src, common_obj),
-            (syscalls_src, syscalls_obj),
-        ]:
+        src = os.path.join(common.baremetal_src_lib_dir, '{}{}'.format(args.arch, common.asm_ext))
+        if common.need_rebuild([src], bootloader_obj):
             common.run_cmd(
                 [gcc,  common.Newline] +
                 cflags +
                 [
                     '-c', common.Newline,
-                    '-D', 'UART0_ADDR={:#x}'.format(uart_address), common.Newline,
-                    '-o', obj, common.Newline,
+                    '-o', bootloader_obj, common.Newline,
                     src, common.Newline,
                 ]
             )
+        for src, obj in [
+            (common_src, common_obj),
+            (syscalls_src, syscalls_obj),
+        ]:
+            if common.need_rebuild([src], obj):
+                common.run_cmd(
+                    [gcc,  common.Newline] +
+                    cflags +
+                    [
+                        '-c', common.Newline,
+                        '-D', 'UART0_ADDR={:#x}'.format(uart_address), common.Newline,
+                        '-o', obj, common.Newline,
+                        src, common.Newline,
+                    ]
+                )
         self._build_dir(
             '',
             gcc=gcc,
@@ -137,38 +140,39 @@ Build the baremetal examples with crosstool-NG.
         in_dir = os.path.join(common.baremetal_src_dir, subpath)
         out_dir = os.path.join(common.baremetal_build_dir, subpath)
         os.makedirs(out_dir, exist_ok=True)
+        common_objs = common_objs.copy()
         if bootloader:
-            bootloader_cmd = [bootloader_obj, common.Newline]
-        else:
-            bootloader_cmd = []
+            common_objs.append(bootloader_obj)
         for in_basename in os.listdir(in_dir):
             in_path = os.path.join(in_dir, in_basename)
             if os.path.isfile(in_path) and os.path.splitext(in_basename)[1] in (common.c_ext, common.asm_ext):
                 in_name = os.path.splitext(in_basename)[0]
                 main_obj = os.path.join(common.baremetal_build_dir, subpath, '{}{}'.format(in_name, common.obj_ext))
-                common.run_cmd(
-                    [gcc,  common.Newline] +
-                    cflags +
-                    [
-                        '-c', common.Newline,
-                        '-o', main_obj, common.Newline,
-                        os.path.join(common.baremetal_src_dir, in_path), common.Newline,
-                    ]
-                )
-                common.run_cmd(
-                    [gcc,  common.Newline] +
-                    cflags +
-                    [
-                        '-Wl,--section-start=.text={:#x}'.format(entry_address), common.Newline,
-                        '-o', os.path.join(common.baremetal_build_dir, subpath, in_name + common.baremetal_build_ext), common.Newline,
-                        '-T', os.path.join(common.baremetal_src_dir, 'link.ld'), common.Newline,
-                    ] +
-                    bootloader_cmd +
-                    common.add_newlines(common_objs) +
-                    [
-                        main_obj, common.Newline,
-                    ]
-                )
+                src = os.path.join(common.baremetal_src_dir, in_path)
+                if common.need_rebuild([src], main_obj):
+                    common.run_cmd(
+                        [gcc,  common.Newline] +
+                        cflags +
+                        [
+                            '-c', common.Newline,
+                            '-o', main_obj, common.Newline,
+                            src, common.Newline,
+                        ]
+                    )
+                objs = common_objs + [main_obj]
+                out = os.path.join(common.baremetal_build_dir, subpath, in_name + common.baremetal_build_ext)
+                link_script = os.path.join(common.baremetal_src_dir, 'link.ld')
+                if common.need_rebuild(objs + [link_script], out):
+                    common.run_cmd(
+                        [gcc,  common.Newline] +
+                        cflags +
+                        [
+                            '-Wl,--section-start=.text={:#x}'.format(entry_address), common.Newline,
+                            '-o', out, common.Newline,
+                            '-T', link_script, common.Newline,
+                        ] +
+                        common.add_newlines(objs)
+                    )
 
 if __name__ == '__main__':
     BaremetalComponent().build()
diff --git a/common.py b/common.py
index a591b89..5cf552a 100644
--- a/common.py
+++ b/common.py
@@ -193,6 +193,39 @@ def write_string_to_file(path, string, mode='w'):
         with open(path, 'a') as f:
             f.write(string)
 
+def cmd_to_string(cmd, cwd=None, extra_env=None, extra_paths=None):
+    '''
+    Format a command given as a list of strings so that it can
+    be viewed nicely and executed by bash directly and print it to stdout.
+    '''
+    last_newline = ' \\\n'
+    newline_separator = last_newline + '  '
+    out = []
+    if extra_env is None:
+        extra_env = {}
+    if cwd is not None:
+        out.append('cd {} &&'.format(shlex.quote(cwd)))
+    if extra_paths is not None:
+        out.append('PATH="{}:${{PATH}}"'.format(':'.join(extra_paths)))
+    for key in extra_env:
+        out.append('{}={}'.format(shlex.quote(key), shlex.quote(extra_env[key])))
+    cmd_quote = []
+    newline_count = 0
+    for arg in cmd:
+        if arg == common.Newline:
+            cmd_quote.append(arg)
+            newline_count += 1
+        else:
+            cmd_quote.append(shlex.quote(arg))
+    if newline_count > 0:
+        cmd_quote = [' '.join(list(y)) for x, y in itertools.groupby(cmd_quote, lambda z: z == common.Newline) if not x]
+    out.extend(cmd_quote)
+    if newline_count == 1 and cmd[-1] == common.Newline:
+        ending = ''
+    else:
+        ending = last_newline + ';'
+    return newline_separator.join(out) + ending
+
 def copy_dir_if_update_non_recursive(srcdir, destdir, filter_ext=None):
     # TODO print rsync equivalent.
     os.makedirs(destdir, exist_ok=True)
@@ -207,6 +240,11 @@ def copy_dir_if_update_non_recursive(srcdir, destdir, filter_ext=None):
                     update=1,
                 )
 
+def cp(src, dest):
+    print_cmd(['cp', src, dest])
+    if not common.dry_run:
+        shutil.copy2(src, dest)
+
 def gem_list_checkpoint_dirs():
     '''
     List checkpoint directory, oldest first.
@@ -475,38 +513,13 @@ def make_run_dirs():
     os.makedirs(common.p9_dir, exist_ok=True)
     os.makedirs(common.qemu_run_dir, exist_ok=True)
 
-def cmd_to_string(cmd, cwd=None, extra_env=None, extra_paths=None):
-    '''
-    Format a command given as a list of strings so that it can
-    be viewed nicely and executed by bash directly and print it to stdout.
-    '''
-    last_newline = ' \\\n'
-    newline_separator = last_newline + '  '
-    out = []
-    if extra_env is None:
-        extra_env = {}
-    if cwd is not None:
-        out.append('cd {} &&'.format(shlex.quote(cwd)))
-    if extra_paths is not None:
-        out.append('PATH="{}:${{PATH}}"'.format(':'.join(extra_paths)))
-    for key in extra_env:
-        out.append('{}={}'.format(shlex.quote(key), shlex.quote(extra_env[key])))
-    cmd_quote = []
-    newline_count = 0
-    for arg in cmd:
-        if arg == common.Newline:
-            cmd_quote.append(arg)
-            newline_count += 1
-        else:
-            cmd_quote.append(shlex.quote(arg))
-    if newline_count > 0:
-        cmd_quote = [' '.join(list(y)) for x, y in itertools.groupby(cmd_quote, lambda z: z == common.Newline) if not x]
-    out.extend(cmd_quote)
-    if newline_count == 1 and cmd[-1] == common.Newline:
-        ending = ''
-    else:
-        ending = last_newline + ';'
-    return newline_separator.join(out) + ending
+def need_rebuild(srcs, dst):
+    if not os.path.exists(dst):
+        return True
+    for src in srcs:
+        if os.path.getmtime(src) > os.path.getmtime(dst):
+            return True
+    return False
 
 def print_cmd(cmd, cwd=None, cmd_file=None, extra_env=None, extra_paths=None):
     '''
@@ -576,11 +589,6 @@ def resolve_args(defaults, args, extra_args):
     argcopy.__dict__ = dict(list(defaults.items()) + list(argcopy.__dict__.items()) + list(extra_args.items()))
     return argcopy
 
-def cp(src, dest):
-    print_cmd(['cp', src, dest])
-    if not common.dry_run:
-        shutil.copy2(src, dest)
-
 def rmrf(path):
     print_cmd(['rm', '-r', '-f', path])
     if not common.dry_run and os.path.exists(path):