diff --git a/ports/bare-arm/Makefile b/ports/bare-arm/Makefile
index 72d5f1a277..455a9dc5ac 100644
--- a/ports/bare-arm/Makefile
+++ b/ports/bare-arm/Makefile
@@ -1,52 +1,59 @@
+# Include the core environment definitions; this will set $(TOP).
 include ../../py/mkenv.mk
 
-# qstr definitions (must come before including py.mk)
-QSTR_DEFS = qstrdefsport.h
-
-# MicroPython feature configurations
-MICROPY_ROM_TEXT_COMPRESSION ?= 1
-
-# include py core make definitions
+# Include py core make definitions.
 include $(TOP)/py/py.mk
 
+# Set makefile-level MicroPython feature configurations.
+MICROPY_ROM_TEXT_COMPRESSION ?= 1
+
+# Define toolchain and other tools.
 CROSS_COMPILE ?= arm-none-eabi-
+DFU ?= $(TOP)/tools/dfu.py
+PYDFU ?= $(TOP)/tools/pydfu.py
 
-INC += -I.
-INC += -I$(TOP)
-INC += -I$(BUILD)
+# Set CFLAGS.
+CFLAGS += -I. -I$(TOP) -I$(BUILD)
+CFLAGS += -Wall -Werror -std=c99 -nostdlib
+CFLAGS += -mthumb -mtune=cortex-m4 -mcpu=cortex-m4 -msoft-float
+CSUPEROPT = -Os # save some code space for performance-critical code
 
-CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mabi=aapcs-linux -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fsingle-precision-constant -Wdouble-promotion
-CFLAGS = $(INC) -Wall -Werror -std=c99 -nostdlib $(CFLAGS_CORTEX_M4) $(COPT)
-CSUPEROPT = -Os # save some code space
-
-#Debugging/Optimization
+# Select debugging or optimisation build.
 ifeq ($(DEBUG), 1)
-CFLAGS += -O0 -ggdb
+CFLAGS += -Og
 else
 CFLAGS += -Os -DNDEBUG
+CFLAGS += -fdata-sections -ffunction-sections
 endif
 
-LDFLAGS = -nostdlib -T stm32f405.ld -Map=$@.map --cref
-LIBS =
+# Set linker flags.
+LDFLAGS += -nostdlib -T stm32f405.ld --gc-sections
 
-SRC_C = \
-	main.c \
-#	printf.c \
-	string0.c \
-	malloc0.c \
-	gccollect.c \
+# Define the required source files.
+SRC_C += lib.c main.c system.c
 
-SRC_S = \
-#	startup_stm32f40xx.s \
-	gchelper.s \
+# Define the required object files.
+OBJ += $(PY_CORE_O)
+OBJ += $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 
-OBJ = $(PY_CORE_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o) $(SRC_S:.s=.o))
-
-all: $(BUILD)/firmware.elf
+# Define the top-level target, the main firmware.
+all: $(BUILD)/firmware.dfu
 
 $(BUILD)/firmware.elf: $(OBJ)
 	$(ECHO) "LINK $@"
-	$(Q)$(LD) $(LDFLAGS) -o $@ $^ $(LIBS)
+	$(Q)$(LD) $(LDFLAGS) -o $@ $^
 	$(Q)$(SIZE) $@
 
+$(BUILD)/firmware.bin: $(BUILD)/firmware.elf
+	$(ECHO) "Create $@"
+	$(Q)$(OBJCOPY) -O binary -j .isr_vector -j .text -j .data $^ $@
+
+$(BUILD)/firmware.dfu: $(BUILD)/firmware.bin
+	$(ECHO) "Create $@"
+	$(Q)$(PYTHON) $(DFU) -b 0x08000000:$^ $@
+
+deploy: $(BUILD)/firmware.dfu
+	$(Q)$(PYTHON) $(PYDFU) -u $^
+
+# Include remaining core make rules.
 include $(TOP)/py/mkrules.mk
diff --git a/ports/bare-arm/README.md b/ports/bare-arm/README.md
new file mode 100644
index 0000000000..496ee9c755
--- /dev/null
+++ b/ports/bare-arm/README.md
@@ -0,0 +1,21 @@
+The bare-arm port
+=================
+
+This port is intended to be the bare-minimum amount of code and configuration
+required to get MicroPython compiling and running on a bare-metal ARM-based
+target.  No external dependencies or libraries are needed for this build and
+it shows exactly what hardware and system functionality MicroPython needs to
+run.
+
+To build, simply run `make` in this directory.  The output will be
+`build/firmware.elf` (and also corresponding `.bin` and `.dfu` files).  This
+firmware can run on an STM32F405-based board (eg a PYBv1.x) and `make deploy`
+will program it to such an MCU when put in USB DFU mode.  The output is a UART
+at 115200 baud, with TX on PA0.
+
+There are some simple demonstration code strings (see `main.c`) which are
+compiled and executed when the firmware starts.  They produce output on the
+system's stdout.
+
+The size of the firmware (the machine code that is programmed to the
+microcontroller's flash/ROM) is currently around 61200 bytes.
diff --git a/ports/bare-arm/lib.c b/ports/bare-arm/lib.c
new file mode 100644
index 0000000000..ee7c1d765e
--- /dev/null
+++ b/ports/bare-arm/lib.c
@@ -0,0 +1,128 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <string.h>
+
+// These memory functions are needed when the garbage collector is disabled.
+// A full implementation should be provided, or the garbage collector enabled.
+// The functions here are very simple.
+
+extern char _heap_start;
+
+void *malloc(size_t n) {
+    static char *cur_heap = NULL;
+    if (cur_heap == NULL) {
+        cur_heap = &_heap_start;
+    }
+    void *ptr = cur_heap;
+    cur_heap += (n + 7) & ~7;
+    return ptr;
+}
+
+void *realloc(void *ptr, size_t size) {
+    void *ptr2 = malloc(size);
+    if (ptr && size) {
+        memcpy(ptr2, ptr, size); // size may be greater than ptr's region, do copy anyway
+    }
+    return ptr2;
+}
+
+void free(void *p) {
+}
+
+// These standard string functions are needed by the runtime, and can be
+// provided either by the system or lib/libc/string0.c.  The implementations
+// here are very simple.
+
+int memcmp(const void *s1, const void *s2, size_t n) {
+    const unsigned char *ss1 = s1, *ss2 = s2;
+    while (n--) {
+        int c = *ss1++ - *ss2++;
+        if (c) {
+            return c;
+        }
+    }
+    return 0;
+}
+
+void *memcpy(void *dest, const void *src, size_t n) {
+    return memmove(dest, src, n);
+}
+
+void *memmove(void *dest, const void *src, size_t n) {
+    unsigned char *d = dest;
+    const unsigned char *s = src;
+    if (s < d && d < s + n) {
+        // Need to copy backwards.
+        d += n - 1;
+        s += n - 1;
+        while (n--) {
+            *d-- = *s--;
+        }
+    } else {
+        // Can copy forwards.
+        while (n--) {
+            *d++ = *s++;
+        }
+    }
+    return dest;
+}
+
+void *memset(void *s, int c, size_t n) {
+    unsigned char *ss = s;
+    while (n--) {
+        *ss++ = c;
+    }
+    return s;
+}
+
+char *strchr(const char *s, int c) {
+    while (*s) {
+        if (*s == c) {
+            return (char *)s;
+        }
+        ++s;
+    }
+    return NULL;
+}
+
+int strcmp(const char *s1, const char *s2) {
+    while (*s1 && *s2) {
+        int c = *s1++ - *s2++;
+        if (c) {
+            return c;
+        }
+    }
+    return *s1 - *s2;
+}
+
+size_t strlen(const char *s) {
+    const char *ss = s;
+    while (*ss) {
+        ++ss;
+    }
+    return ss - s;
+}
diff --git a/ports/bare-arm/main.c b/ports/bare-arm/main.c
index df66c04eff..82c27bd097 100644
--- a/ports/bare-arm/main.c
+++ b/ports/bare-arm/main.c
@@ -1,15 +1,48 @@
-#include <stdint.h>
-#include <stdio.h>
-#include <string.h>
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014-2021 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
 
+#include <string.h>
 #include "py/compile.h"
 #include "py/runtime.h"
-#include "py/repl.h"
-#include "py/mperrno.h"
 
-void do_str(const char *src, mp_parse_input_kind_t input_kind) {
+static const char *demo_single_input =
+    "print('hello world!', list(x + 1 for x in range(10)), end='eol\\n')";
+
+static const char *demo_file_input =
+    "import micropython\n"
+    "\n"
+    "print(dir(micropython))\n"
+    "\n"
+    "for i in range(10):\n"
+    "    print('iter {:08}'.format(i))";
+
+static void do_str(const char *src, mp_parse_input_kind_t input_kind) {
     nlr_buf_t nlr;
     if (nlr_push(&nlr) == 0) {
+        // Compile, parse and execute the given string.
         mp_lexer_t *lex = mp_lexer_new_from_str_len(MP_QSTR__lt_stdin_gt_, src, strlen(src), 0);
         qstr source_name = lex->source_name;
         mp_parse_tree_t parse_tree = mp_parse(lex, input_kind);
@@ -17,121 +50,29 @@ void do_str(const char *src, mp_parse_input_kind_t input_kind) {
         mp_call_function_0(module_fun);
         nlr_pop();
     } else {
-        // uncaught exception
+        // Uncaught exception: print it out.
         mp_obj_print_exception(&mp_plat_print, (mp_obj_t)nlr.ret_val);
     }
 }
 
-int main(int argc, char **argv) {
+// Main entry point: initialise the runtime and execute demo strings.
+void bare_main(void) {
     mp_init();
-    do_str("print('hello world!', list(x+1 for x in range(10)), end='eol\\n')", MP_PARSE_SINGLE_INPUT);
-    do_str("for i in range(10):\n  print(i)", MP_PARSE_FILE_INPUT);
+    do_str(demo_single_input, MP_PARSE_SINGLE_INPUT);
+    do_str(demo_file_input, MP_PARSE_FILE_INPUT);
     mp_deinit();
-    return 0;
 }
 
-mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
-    mp_raise_OSError(MP_ENOENT);
-}
-
-mp_import_stat_t mp_import_stat(const char *path) {
-    return MP_IMPORT_STAT_NO_EXIST;
-}
-
-mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs) {
-    return mp_const_none;
-}
-MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
-
+// Called if an exception is raised outside all C exception-catching handlers.
 void nlr_jump_fail(void *val) {
-    while (1) {
-        ;
-    }
-}
-
-void NORETURN __fatal_error(const char *msg) {
-    while (1) {
-        ;
+    for (;;) {
     }
 }
 
 #ifndef NDEBUG
+// Used when debugging is enabled.
 void MP_WEAK __assert_func(const char *file, int line, const char *func, const char *expr) {
-    printf("Assertion '%s' failed, at file %s:%d\n", expr, file, line);
-    __fatal_error("Assertion failed");
+    for (;;) {
+    }
 }
 #endif
-
-/*
-int _lseek() {return 0;}
-int _read() {return 0;}
-int _write() {return 0;}
-int _close() {return 0;}
-void _exit(int x) {for(;;){}}
-int _sbrk() {return 0;}
-int _kill() {return 0;}
-int _getpid() {return 0;}
-int _fstat() {return 0;}
-int _isatty() {return 0;}
-*/
-
-void *malloc(size_t n) {
-    return NULL;
-}
-void *calloc(size_t nmemb, size_t size) {
-    return NULL;
-}
-void *realloc(void *ptr, size_t size) {
-    return NULL;
-}
-void free(void *p) {
-}
-int printf(const char *m, ...) {
-    return 0;
-}
-void *memcpy(void *dest, const void *src, size_t n) {
-    return NULL;
-}
-int memcmp(const void *s1, const void *s2, size_t n) {
-    return 0;
-}
-void *memmove(void *dest, const void *src, size_t n) {
-    return NULL;
-}
-void *memset(void *s, int c, size_t n) {
-    return NULL;
-}
-int strcmp(const char *s1, const char *s2) {
-    return 0;
-}
-int strncmp(const char *s1, const char *s2, size_t n) {
-    return 0;
-}
-size_t strlen(const char *s) {
-    return 0;
-}
-char *strcat(char *dest, const char *src) {
-    return NULL;
-}
-char *strchr(const char *dest, int c) {
-    return NULL;
-}
-#include <stdarg.h>
-int vprintf(const char *format, va_list ap) {
-    return 0;
-}
-int vsnprintf(char *str,  size_t size,  const char *format, va_list ap) {
-    return 0;
-}
-
-#undef putchar
-int putchar(int c) {
-    return 0;
-}
-int puts(const char *s) {
-    return 0;
-}
-
-void _start(void) {
-    main(0, NULL);
-}
diff --git a/ports/bare-arm/mpconfigport.h b/ports/bare-arm/mpconfigport.h
index 7fd236bfba..41b6ee71a3 100644
--- a/ports/bare-arm/mpconfigport.h
+++ b/ports/bare-arm/mpconfigport.h
@@ -1,71 +1,71 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014-2021 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
 #include <stdint.h>
 
-// options to control how MicroPython is built
+// Options to control how MicroPython is built
 
-#define MICROPY_QSTR_BYTES_IN_HASH  (1)
-#define MICROPY_ALLOC_PATH_MAX      (512)
-#define MICROPY_EMIT_X64            (0)
-#define MICROPY_EMIT_THUMB          (0)
-#define MICROPY_EMIT_INLINE_THUMB   (0)
-#define MICROPY_COMP_MODULE_CONST   (0)
-#define MICROPY_COMP_CONST          (0)
-#define MICROPY_COMP_DOUBLE_TUPLE_ASSIGN (0)
-#define MICROPY_COMP_TRIPLE_TUPLE_ASSIGN (0)
-#define MICROPY_MEM_STATS           (0)
-#define MICROPY_DEBUG_PRINTERS      (0)
-#define MICROPY_ENABLE_GC           (0)
-#define MICROPY_HELPER_REPL         (0)
-#define MICROPY_HELPER_LEXER_UNIX   (0)
-#define MICROPY_ENABLE_SOURCE_LINE  (0)
-#define MICROPY_ENABLE_DOC_STRING   (0)
-#define MICROPY_ERROR_REPORTING     (MICROPY_ERROR_REPORTING_TERSE)
-#define MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG (0)
-#define MICROPY_PY_ASYNC_AWAIT (0)
-#define MICROPY_PY_ASSIGN_EXPR      (0)
-#define MICROPY_PY_BUILTINS_BYTEARRAY (0)
-#define MICROPY_PY_BUILTINS_DICT_FROMKEYS (0)
-#define MICROPY_PY_BUILTINS_MEMORYVIEW (0)
-#define MICROPY_PY_BUILTINS_ENUMERATE (0)
-#define MICROPY_PY_BUILTINS_FROZENSET (0)
-#define MICROPY_PY_BUILTINS_REVERSED (0)
-#define MICROPY_PY_BUILTINS_SET     (0)
-#define MICROPY_PY_BUILTINS_SLICE   (0)
-#define MICROPY_PY_BUILTINS_PROPERTY (0)
-#define MICROPY_PY_BUILTINS_STR_COUNT (0)
-#define MICROPY_PY_BUILTINS_STR_OP_MODULO (0)
-#define MICROPY_PY___FILE__         (0)
-#define MICROPY_PY_GC               (0)
-#define MICROPY_PY_ARRAY            (0)
-#define MICROPY_PY_ATTRTUPLE        (0)
-#define MICROPY_PY_COLLECTIONS      (0)
-#define MICROPY_PY_MATH             (0)
-#define MICROPY_PY_CMATH            (0)
-#define MICROPY_PY_IO               (0)
-#define MICROPY_PY_STRUCT           (0)
-#define MICROPY_PY_SYS              (0)
-#define MICROPY_CPYTHON_COMPAT      (0)
-#define MICROPY_MODULE_GETATTR      (0)
-#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_NONE)
-#define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_NONE)
-#define MICROPY_USE_INTERNAL_PRINTF (0)
+// Memory allocation policy
+#define MICROPY_QSTR_BYTES_IN_HASH              (1)
 
-// type definitions for the specific machine
+// Compiler configuration
+#define MICROPY_COMP_CONST                      (0)
+#define MICROPY_COMP_DOUBLE_TUPLE_ASSIGN        (0)
 
-#define MICROPY_MAKE_POINTER_CALLABLE(p) ((void *)((mp_uint_t)(p) | 1))
+// Python internal features
+#define MICROPY_ENABLE_EXTERNAL_IMPORT          (0)
+#define MICROPY_ERROR_REPORTING                 (MICROPY_ERROR_REPORTING_TERSE)
+#define MICROPY_CPYTHON_COMPAT                  (0)
+#define MICROPY_MODULE_GETATTR                  (0)
+#define MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG   (0)
 
-#define UINT_FMT "%lu"
-#define INT_FMT "%ld"
+// Fine control over Python builtins, classes, modules, etc
+#define MICROPY_PY_ASYNC_AWAIT                  (0)
+#define MICROPY_PY_ASSIGN_EXPR                  (0)
+#define MICROPY_PY_BUILTINS_STR_COUNT           (0)
+#define MICROPY_PY_BUILTINS_STR_OP_MODULO       (0)
+#define MICROPY_PY_BUILTINS_BYTEARRAY           (0)
+#define MICROPY_PY_BUILTINS_DICT_FROMKEYS       (0)
+#define MICROPY_PY_BUILTINS_SET                 (0)
+#define MICROPY_PY_BUILTINS_SLICE               (0)
+#define MICROPY_PY_BUILTINS_PROPERTY            (0)
+#define MICROPY_PY_BUILTINS_ENUMERATE           (0)
+#define MICROPY_PY_BUILTINS_REVERSED            (0)
+#define MICROPY_PY___FILE__                     (0)
+#define MICROPY_PY_ARRAY                        (0)
+#define MICROPY_PY_COLLECTIONS                  (0)
+#define MICROPY_PY_IO                           (0)
+#define MICROPY_PY_STRUCT                       (0)
+#define MICROPY_PY_SYS                          (0)
+
+// Type definitions for the specific machine
 
 typedef int32_t mp_int_t; // must be pointer size
 typedef uint32_t mp_uint_t; // must be pointer size
 typedef long mp_off_t;
 
-// dummy print
-#define MP_PLAT_PRINT_STRN(str, len) (void)0
-
-// extra built in names to add to the global namespace
-#define MICROPY_PORT_BUILTINS \
-    { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) },
-
-// We need to provide a declaration/definition of alloca()
+// Need to provide a declaration/definition of alloca()
 #include <alloca.h>
diff --git a/ports/bare-arm/qstrdefsport.h b/ports/bare-arm/qstrdefsport.h
deleted file mode 100644
index 00d3e2ae3c..0000000000
--- a/ports/bare-arm/qstrdefsport.h
+++ /dev/null
@@ -1,2 +0,0 @@
-// qstrs specific to this port
-// *FORMAT-OFF*
diff --git a/ports/bare-arm/stm32f405.ld b/ports/bare-arm/stm32f405.ld
index dd688a0246..33a5502d60 100644
--- a/ports/bare-arm/stm32f405.ld
+++ b/ports/bare-arm/stm32f405.ld
@@ -1,117 +1,58 @@
-/*
-    GNU linker script for STM32F405
-*/
+/* GNU linker script for STM32F405 */
 
-/* Specify the memory areas */
 MEMORY
 {
     FLASH (rx)      : ORIGIN = 0x08000000, LENGTH = 0x100000 /* entire flash, 1 MiB */
-    FLASH_ISR (rx)  : ORIGIN = 0x08000000, LENGTH = 0x004000 /* sector 0, 16 KiB */
-    FLASH_TEXT (rx) : ORIGIN = 0x08020000, LENGTH = 0x080000 /* sectors 5,6,7,8, 4*128KiB = 512 KiB (could increase it more) */
-    CCMRAM (xrw)    : ORIGIN = 0x10000000, LENGTH = 0x010000 /* 64 KiB */
     RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x020000 /* 128 KiB */
 }
 
-/* top end of the stack */
 _estack = ORIGIN(RAM) + LENGTH(RAM);
 
-/* RAM extents for the garbage collector */
-_ram_end = ORIGIN(RAM) + LENGTH(RAM);
-_heap_end = 0x2001c000; /* tunable */
-
-/* define output sections */
 SECTIONS
 {
-    /* The startup code goes first into FLASH */
     .isr_vector :
     {
         . = ALIGN(4);
-        KEEP(*(.isr_vector)) /* Startup code */
-
+        KEEP(*(.isr_vector))
         . = ALIGN(4);
-    } >FLASH_ISR
+    } >FLASH
 
-    /* The program code and other data goes into FLASH */
     .text :
     {
         . = ALIGN(4);
-        *(.text)           /* .text sections (code) */
-        *(.text*)          /* .text* sections (code) */
-        *(.rodata)         /* .rodata sections (constants, strings, etc.) */
-        *(.rodata*)        /* .rodata* sections (constants, strings, etc.) */
-    /*  *(.glue_7)   */    /* glue arm to thumb code */
-    /*  *(.glue_7t)  */    /* glue thumb to arm code */
-
+        *(.text)
+        *(.text*)
+        *(.rodata)
+        *(.rodata*)
         . = ALIGN(4);
-        _etext = .;        /* define a global symbol at end of code */
-        _sidata = _etext;  /* This is used by the startup in order to initialize the .data secion */
-    } >FLASH_TEXT
-
-    /*
-    .ARM.extab :
-    {
-        *(.ARM.extab* .gnu.linkonce.armextab.*)
+        _etext = .;
+        _sidata = _etext;
     } >FLASH
 
-    .ARM :
-    {
-        __exidx_start = .;
-        *(.ARM.exidx*)
-        __exidx_end = .;
-    } >FLASH
-    */
-
-    /* This is the initialized data section
-    The program executes knowing that the data is in the RAM
-    but the loader puts the initial values in the FLASH (inidata).
-    It is one task of the startup to copy the initial values from FLASH to RAM. */
     .data : AT ( _sidata )
     {
         . = ALIGN(4);
-        _sdata = .;        /* create a global symbol at data start; used by startup code in order to initialise the .data section in RAM */
-        _ram_start = .;    /* create a global symbol at ram start for garbage collector */
-        *(.data)           /* .data sections */
-        *(.data*)          /* .data* sections */
-
+        _sdata = .;
+        *(.data)
+        *(.data*)
         . = ALIGN(4);
-        _edata = .;        /* define a global symbol at data end; used by startup code in order to initialise the .data section in RAM */
+        _edata = .;
     } >RAM
 
-    /* Uninitialized data section */
     .bss :
     {
         . = ALIGN(4);
-        _sbss = .;         /* define a global symbol at bss start; used by startup code */
+        _sbss = .;
         *(.bss)
         *(.bss*)
         *(COMMON)
-
         . = ALIGN(4);
-        _ebss = .;         /* define a global symbol at bss end; used by startup code */
+        _ebss = .;
     } >RAM
 
-    /* this is to define the start of the heap, and make sure we have a minimum size */
     .heap :
     {
-        . = ALIGN(4);
-        _heap_start = .;    /* define a global symbol at heap start */
+        . = ALIGN(8);
+        _heap_start = .;
     } >RAM
-
-    /* this just checks there is enough RAM for the stack */
-    .stack :
-    {
-        . = ALIGN(4);
-    } >RAM
-
-    /* Remove information from the standard libraries */
-    /*
-    /DISCARD/ :
-    {
-        libc.a ( * )
-        libm.a ( * )
-        libgcc.a ( * )
-    }
-    */
-
-    .ARM.attributes 0 : { *(.ARM.attributes) }
 }
diff --git a/ports/bare-arm/system.c b/ports/bare-arm/system.c
new file mode 100644
index 0000000000..f66dd2d80a
--- /dev/null
+++ b/ports/bare-arm/system.c
@@ -0,0 +1,142 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <stdint.h>
+#include <string.h>
+
+#define RCC     ((periph_rcc_t *)0x40023800)
+#define GPIOA   ((periph_gpio_t *)0x40020000)
+#define UART4   ((periph_uart_t *)0x40004C00)
+
+typedef struct {
+    volatile uint32_t CR;
+    volatile uint32_t PLLCFGR;
+    volatile uint32_t CFGR;
+    volatile uint32_t CIR;
+    uint32_t _1[8];
+    volatile uint32_t AHB1ENR;
+    volatile uint32_t AHB2ENR;
+    volatile uint32_t AHB3ENR;
+    uint32_t _2;
+    volatile uint32_t APB1ENR;
+    volatile uint32_t APB2ENR;
+} periph_rcc_t;
+
+typedef struct {
+    volatile uint32_t MODER;
+    volatile uint32_t OTYPER;
+    volatile uint32_t OSPEEDR;
+    volatile uint32_t PUPDR;
+    volatile uint32_t IDR;
+    volatile uint32_t ODR;
+    volatile uint16_t BSRRL;
+    volatile uint16_t BSRRH;
+    volatile uint32_t LCKR;
+    volatile uint32_t AFR[2];
+} periph_gpio_t;
+
+typedef struct {
+    volatile uint32_t SR;
+    volatile uint32_t DR;
+    volatile uint32_t BRR;
+    volatile uint32_t CR1;
+} periph_uart_t;
+
+extern uint32_t _estack, _sidata, _sdata, _edata, _sbss, _ebss;
+
+void Reset_Handler(void) __attribute__((naked));
+void bare_main(void);
+
+static void stm32_init(void);
+static void gpio_init_alt(periph_gpio_t *gpio, int pin, int alt);
+
+// Very simple ARM vector table.
+const uint32_t isr_vector[] __attribute__((section(".isr_vector"))) = {
+    (uint32_t)&_estack,
+    (uint32_t)&Reset_Handler,
+};
+
+// The CPU runs this function after a reset.
+void Reset_Handler(void) {
+    // Set stack pointer.
+    __asm volatile ("ldr sp, =_estack");
+
+    // Copy .data section from flash to RAM.
+    memcpy(&_sdata, &_sidata, (char *)&_edata - (char *)&_sdata);
+
+    // Zero out .bss section.
+    memset(&_sbss, 0, (char *)&_ebss - (char *)&_sbss);
+
+    // SCB->CCR: enable 8-byte stack alignment for IRQ handlers, in accord with EABI.
+    *((volatile uint32_t *)0xe000ed14) |= 1 << 9;
+
+    // Initialise the cpu and peripherals.
+    stm32_init();
+
+    // Now that there is a basic system up and running, call the main application code.
+    bare_main();
+
+    // This function must not return.
+    for (;;) {
+    }
+}
+
+// Set up the STM32 MCU.
+static void stm32_init(void) {
+    // Note: default clock is internal 16MHz.
+    RCC->AHB1ENR |= 1 << 0; // GPIOAEN
+    RCC->APB1ENR |= 1 << 19; // UART4EN
+    gpio_init_alt(GPIOA, 0, 8);
+    UART4->BRR = (8 << 4) | 11; // 16MHz/(16*8.6875) = 115107 baud
+    UART4->CR1 = 0x00002008; // USART enable, tx enable, rx enable
+}
+
+// Configure a GPIO pin in alternate-function mode.
+static void gpio_init_alt(periph_gpio_t *gpio, int pin, int alt) {
+    gpio->MODER = (gpio->MODER & ~(3 << (2 * pin))) | (2 << (2 * pin));
+    // OTYPER is left as default push-pull
+    // OSPEEDR is left as default low speed
+    // PUPDR is left as default no-pull
+    gpio->AFR[pin >> 3] = (gpio->AFR[pin >> 3] & ~(15 << (4 * (pin & 7)))) | (alt << (4 * (pin & 7)));
+}
+
+// Write a character out to the UART.
+static inline void uart_write_char(int c) {
+    // Wait for TXE, then write the character.
+    while ((UART4->SR & (1 << 7)) == 0) {
+    }
+    UART4->DR = c;
+}
+
+// Send string of given length to stdout, converting \n to \r\n.
+void mp_hal_stdout_tx_strn_cooked(const char *str, size_t len) {
+    while (len--) {
+        if (*str == '\n') {
+            uart_write_char('\r');
+        }
+        uart_write_char(*str++);
+    }
+}