Initial Pi Zero support

Both SD card and USB don't work. REPL works over UART.

Progress on #5631

lib/sdmmc/sdmmc_io.c

@@ -420,7 +420,10 @@ static sdmmc_err_t cis_tuple_func_manfid(const void* p, uint8_t* data, FILE* fp)
     int size = *(data++);
     fprintf(fp, "TUPLE: %s, size: %d\n", tuple->name, size);
     CIS_CHECK_SIZE(size, 4);
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wcast-align"
     fprintf(fp, "  MANF: %04X, CARD: %04X\n", *(uint16_t*)(data), *(uint16_t*)(data+2));
+    #pragma GCC diagnostic pop
     return SDMMC_OK;
 }
 
@@ -480,7 +483,10 @@ static sdmmc_err_t cis_tuple_func_cftable_entry(const void* p, uint8_t* data, FI
         CIS_CHECK_SIZE(size, 2);
         size-=2;
         CIS_CHECK_UNSUPPORTED(mem_space==1); //other cases not handled yet
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wcast-align"
         int len = *(uint16_t*)data;
+        #pragma GCC diagnostic pop
         fprintf(fp, "  LEN: %04X\n", len);
         data+=2;
     }

main.c

@@ -813,8 +813,8 @@ int __attribute__((used)) main(void) {
     // no SPI flash filesystem, and we might erase the existing one.
 
     // Check whether CIRCUITPY is available. Don't check if it already hasn't been found.
-    if ((safe_mode != NO_CIRCUITPY) && !filesystem_init(safe_mode == NO_SAFE_MODE, false)) {
+    if (!filesystem_init(safe_mode == NO_SAFE_MODE, false)) {
-        reset_into_safe_mode(NO_CIRCUITPY);
+        safe_mode = NO_CIRCUITPY;
     }
 
     // displays init after filesystem, since they could share the flash SPI

ports/broadcom/Makefile

@@ -31,7 +31,20 @@ include $(TOP)/supervisor/supervisor.mk
 # Include make rules and variables common across CircuitPython builds.
 include $(TOP)/py/circuitpy_defns.mk
 
-HAL_DIR=hal/$(MCU_SERIES)
+ifeq ($(CHIP_VARIANT), "bcm2711")
+CFLAGS += -mcpu=cortex-a72 -DBCM_VERSION=2711
+CROSS_COMPILE = aarch64-none-elf-
+SUFFIX = 8
+else ifeq ($(CHIP_VARIANT), "bcm2837")
+CFLAGS += -mcpu=cortex-a53 -DBCM_VERSION=2837
+CROSS_COMPILE = aarch64-none-elf-
+SUFFIX = 8
+else ifeq ($(CHIP_VARIANT), "bcm2835")
+CFLAGS += -mcpu=arm1176jzf-s -DBCM_VERSION=2835
+CROSS_COMPILE = arm-none-eabi-
+SUFFIX =
+# TODO add 32-bit support for Cortex-A7 in 2836
+endif
 
 INC += -I. \
 	   -I../.. \
@@ -69,6 +82,7 @@ SRC_C += bindings/videocore/__init__.c \
 		 peripherals/broadcom/mmu.c \
 		 peripherals/broadcom/vcmailbox.c
 
+SRC_S = peripherals/broadcom/boot$(SUFFIX).s
 
 SRC_COMMON_HAL_EXPANDED = $(addprefix shared-bindings/, $(SRC_COMMON_HAL)) \
 						  $(addprefix shared-bindings/, $(SRC_BINDINGS_ENUMS)) \
@@ -82,7 +96,6 @@ SRC_SHARED_MODULE_EXPANDED = $(addprefix shared-bindings/, $(SRC_SHARED_MODULE))
 # because a few modules have files both in common-hal/ and shared-modules/.
 # Doing a $(sort ...) removes duplicates as part of sorting.
 SRC_COMMON_HAL_SHARED_MODULE_EXPANDED = $(sort $(SRC_COMMON_HAL_EXPANDED) $(SRC_SHARED_MODULE_EXPANDED))
-SRC_S = peripherals/broadcom/boot8.s
 
 OBJ = $(PY_O) $(SUPERVISOR_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_COMMON_HAL_SHARED_MODULE_EXPANDED:.c=.o))
@@ -96,15 +109,6 @@ OBJ += $(addprefix $(BUILD)/, $(SRC_MOD:.c=.o))
 # BCM CLFAGS
 CFLAGS += -ffreestanding -nostartfiles -DMICROPY_HW_MCU_NAME="\"$(CHIP_VARIANT)\""
 
-ifeq ($(CHIP_VARIANT), "bcm2711")
-CFLAGS += -mcpu=cortex-a72 -DBCM_VERSION=2711
-CROSS_COMPILE = aarch64-none-elf-
-else
-CFLAGS += -mcpu=cortex-a53 -DBCM_VERSION=2837
-CROSS_COMPILE = aarch64-none-elf-
-# TODO add 32-bit support for Cortex-A7 and ARM1176
-endif
-
 
 OPTIMIZATION_FLAGS ?= -O3
 CFLAGS += $(OPTIMIZATION_FLAGS)
@@ -134,32 +138,32 @@ CFLAGS += $(INC) -Wall -Werror -std=gnu11 $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) $
 
 SRC_QSTR += $(SRC_C) $(SRC_SUPERVISOR) $(SRC_COMMON_HAL_EXPANDED) $(SRC_SHARED_MODULE_EXPANDED)
 
-LDFLAGS += $(CFLAGS) -T peripherals/broadcom/link8.ld -Wl,--gc-sections -Wl,-Map=$@.map # -Wl,--cref
+LDFLAGS += $(CFLAGS) -T peripherals/broadcom/link$(SUFFIX).ld -Wl,--gc-sections -Wl,-Map=$@.map # -Wl,--cref
 
 # Use toolchain libm if we're not using our own.
 ifndef INTERNAL_LIBM
 LIBS += -lm
 endif
 
-all: $(BUILD)/firmware.kernel8.img $(BUILD)/firmware.disk.img.zip
+all: $(BUILD)/firmware.kernel$(SUFFIX).img $(BUILD)/firmware.disk.img.zip
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
-$(BUILD)/kernel8.elf: $(OBJ)
+$(BUILD)/kernel$(SUFFIX).elf: $(OBJ)
 	$(STEPECHO) "LINK $@"
 	$(Q)echo $(OBJ) > $(BUILD)/firmware.objs
 	$(Q)$(CC) -o $@ $(LDFLAGS) @$(BUILD)/firmware.objs -Wl,--start-group $(LIBS) -Wl,--end-group
 
-$(BUILD)/kernel8.img: $(BUILD)/kernel8.elf
+$(BUILD)/kernel$(SUFFIX).img: $(BUILD)/kernel$(SUFFIX).elf
 	$(STEPECHO) "Create $@"
-	$(OBJCOPY) -O binary $(BUILD)/kernel8.elf $@
+	$(OBJCOPY) -O binary $(BUILD)/kernel$(SUFFIX).elf $@
 
-$(BUILD)/firmware.kernel8.img: $(BUILD)/kernel8.img
+$(BUILD)/firmware.kernel$(SUFFIX).img: $(BUILD)/kernel$(SUFFIX).img
 	$(STEPECHO) "Create $@"
 	$(CP) $^ $@
 
-$(BUILD)/firmware.disk.img.zip: $(BUILD)/kernel8.img
+$(BUILD)/firmware.disk.img.zip: $(BUILD)/kernel$(SUFFIX).img
 	$(STEPECHO) "Create $@"
 	$(Q)dd if=/dev/zero of=$(BUILD)/circuitpython-disk.img bs=1 count=0 seek=256M
 	$(Q)parted -s $(BUILD)/circuitpython-disk.img mktable msdos
@@ -167,7 +171,7 @@ $(BUILD)/firmware.disk.img.zip: $(BUILD)/kernel8.img
 	$(Q)mkfs.fat -F 32 -n BOOT --offset=2048 $(BUILD)/circuitpython-disk.img
 
 	$(Q)mcopy -i $(BUILD)/circuitpython-disk.img@@1M config.txt firmware/bootcode.bin firmware/fixup* firmware/start* firmware/*.dtb ::
-	$(Q)mcopy -i $(BUILD)/circuitpython-disk.img@@1M $(BUILD)/kernel8.img ::
+	$(Q)mcopy -i $(BUILD)/circuitpython-disk.img@@1M $(BUILD)/kernel$(SUFFIX).img ::
 	$(Q)zip $@ $(BUILD)/circuitpython-disk.img
 	$(Q)rm $(BUILD)/circuitpython-disk.img
 

ports/broadcom/boards/raspberrypi_zero_w/board.c

@@ -0,0 +1,56 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Scott Shawcroft for Adafruit Industries
+ *
+ * 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 "supervisor/board.h"
+#include "mpconfigboard.h"
+
+#include "bindings/videocore/Framebuffer.h"
+#include "shared-module/displayio/__init__.h"
+#include "shared-bindings/framebufferio/FramebufferDisplay.h"
+
+void board_init(void) {
+    videocore_framebuffer_obj_t *fb = &allocate_display_bus()->videocore;
+    fb->base.type = &videocore_framebuffer_type;
+    common_hal_videocore_framebuffer_construct(fb, 640, 480);
+
+    framebufferio_framebufferdisplay_obj_t *display = &displays[0].framebuffer_display;
+    display->base.type = &framebufferio_framebufferdisplay_type;
+    common_hal_framebufferio_framebufferdisplay_construct(
+        display,
+        MP_OBJ_FROM_PTR(fb),
+        0,
+        true);
+}
+
+bool board_requests_safe_mode(void) {
+    return false;
+}
+
+void reset_board(void) {
+}
+
+void board_deinit(void) {
+}

ports/broadcom/boards/raspberrypi_zero_w/mpconfigboard.h

@@ -0,0 +1,6 @@
+#define MICROPY_HW_BOARD_NAME "Raspberry Pi Zero"
+
+#define DEFAULT_I2C_BUS_SCL         (&pin_GPIO3)
+#define DEFAULT_I2C_BUS_SDA         (&pin_GPIO2)
+
+#define MICROPY_HW_LED_STATUS       (&pin_GPIO47)

ports/broadcom/boards/raspberrypi_zero_w/mpconfigboard.mk

@@ -0,0 +1,6 @@
+USB_VID = 0x2E8A
+USB_PID = 0xf015
+USB_PRODUCT = "Raspberry Pi Zero"
+USB_MANUFACTURER = "Raspberry Pi"
+
+CHIP_VARIANT = "bcm2835"

ports/broadcom/boards/raspberrypi_zero_w/pins.c

@@ -0,0 +1,62 @@
+#include "shared-bindings/board/__init__.h"
+
+#include "shared-module/displayio/__init__.h"
+
+STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
+    CIRCUITPYTHON_BOARD_DICT_STANDARD_ITEMS
+    // These match the names used in Blinka
+    { MP_ROM_QSTR(MP_QSTR_D0), MP_ROM_PTR(&pin_GPIO0) },
+    { MP_ROM_QSTR(MP_QSTR_D1), MP_ROM_PTR(&pin_GPIO1) },
+
+    { MP_ROM_QSTR(MP_QSTR_D2), MP_ROM_PTR(&pin_GPIO2) },
+    { MP_ROM_QSTR(MP_QSTR_SDA), MP_ROM_PTR(&pin_GPIO2) },
+    { MP_ROM_QSTR(MP_QSTR_D3), MP_ROM_PTR(&pin_GPIO3) },
+    { MP_ROM_QSTR(MP_QSTR_SCL), MP_ROM_PTR(&pin_GPIO3) },
+
+    { MP_ROM_QSTR(MP_QSTR_D4), MP_ROM_PTR(&pin_GPIO4) },
+    { MP_ROM_QSTR(MP_QSTR_D5), MP_ROM_PTR(&pin_GPIO5) },
+    { MP_ROM_QSTR(MP_QSTR_D6), MP_ROM_PTR(&pin_GPIO6) },
+
+    { MP_ROM_QSTR(MP_QSTR_D7), MP_ROM_PTR(&pin_GPIO7) },
+    { MP_ROM_QSTR(MP_QSTR_CE1), MP_ROM_PTR(&pin_GPIO7) },
+    { MP_ROM_QSTR(MP_QSTR_D8), MP_ROM_PTR(&pin_GPIO8) },
+    { MP_ROM_QSTR(MP_QSTR_CE0), MP_ROM_PTR(&pin_GPIO8) },
+    { MP_ROM_QSTR(MP_QSTR_D9), MP_ROM_PTR(&pin_GPIO9) },
+    { MP_ROM_QSTR(MP_QSTR_MISO), MP_ROM_PTR(&pin_GPIO9) },
+    { MP_ROM_QSTR(MP_QSTR_D10), MP_ROM_PTR(&pin_GPIO10) },
+    { MP_ROM_QSTR(MP_QSTR_MOSI), MP_ROM_PTR(&pin_GPIO10) },
+    { MP_ROM_QSTR(MP_QSTR_D11), MP_ROM_PTR(&pin_GPIO11) },
+    { MP_ROM_QSTR(MP_QSTR_SCLK), MP_ROM_PTR(&pin_GPIO11) },
+    { MP_ROM_QSTR(MP_QSTR_SCK), MP_ROM_PTR(&pin_GPIO11) },
+
+    { MP_ROM_QSTR(MP_QSTR_D12), MP_ROM_PTR(&pin_GPIO12) },
+    { MP_ROM_QSTR(MP_QSTR_D13), MP_ROM_PTR(&pin_GPIO13) },
+
+    { MP_ROM_QSTR(MP_QSTR_D14), MP_ROM_PTR(&pin_GPIO14) },
+    { MP_ROM_QSTR(MP_QSTR_TXD), MP_ROM_PTR(&pin_GPIO14) },
+    { MP_ROM_QSTR(MP_QSTR_D15), MP_ROM_PTR(&pin_GPIO15) },
+    { MP_ROM_QSTR(MP_QSTR_RXD), MP_ROM_PTR(&pin_GPIO15) },
+    { MP_ROM_QSTR(MP_QSTR_TX), MP_ROM_PTR(&pin_GPIO14) },
+    { MP_ROM_QSTR(MP_QSTR_RX), MP_ROM_PTR(&pin_GPIO15) },
+
+    { MP_ROM_QSTR(MP_QSTR_D16), MP_ROM_PTR(&pin_GPIO16) },
+    { MP_ROM_QSTR(MP_QSTR_D17), MP_ROM_PTR(&pin_GPIO17) },
+    { MP_ROM_QSTR(MP_QSTR_D18), MP_ROM_PTR(&pin_GPIO18) },
+    { MP_ROM_QSTR(MP_QSTR_D19), MP_ROM_PTR(&pin_GPIO19) },
+    { MP_ROM_QSTR(MP_QSTR_MISO_1), MP_ROM_PTR(&pin_GPIO19) },
+    { MP_ROM_QSTR(MP_QSTR_D20), MP_ROM_PTR(&pin_GPIO20) },
+    { MP_ROM_QSTR(MP_QSTR_MOSI_1), MP_ROM_PTR(&pin_GPIO20) },
+    { MP_ROM_QSTR(MP_QSTR_D21), MP_ROM_PTR(&pin_GPIO21) },
+    { MP_ROM_QSTR(MP_QSTR_SCLK_1), MP_ROM_PTR(&pin_GPIO21) },
+    { MP_ROM_QSTR(MP_QSTR_SCK_1), MP_ROM_PTR(&pin_GPIO21) },
+    { MP_ROM_QSTR(MP_QSTR_D22), MP_ROM_PTR(&pin_GPIO22) },
+    { MP_ROM_QSTR(MP_QSTR_D23), MP_ROM_PTR(&pin_GPIO23) },
+    { MP_ROM_QSTR(MP_QSTR_D24), MP_ROM_PTR(&pin_GPIO24) },
+    { MP_ROM_QSTR(MP_QSTR_D25), MP_ROM_PTR(&pin_GPIO25) },
+    { MP_ROM_QSTR(MP_QSTR_D26), MP_ROM_PTR(&pin_GPIO26) },
+    { MP_ROM_QSTR(MP_QSTR_D27), MP_ROM_PTR(&pin_GPIO27) },
+
+    { MP_ROM_QSTR(MP_QSTR_I2C), MP_ROM_PTR(&board_i2c_obj) },
+    { MP_ROM_QSTR(MP_QSTR_DISPLAY), MP_ROM_PTR(&displays[0].display)},
+};
+MP_DEFINE_CONST_DICT(board_module_globals, board_global_dict_table);

ports/broadcom/common-hal/neopixel_write/__init__.c

@@ -48,9 +48,9 @@ void common_hal_neopixel_write(const digitalio_digitalinout_obj_t *digitalinout,
     while (port_get_raw_ticks(NULL) < next_start_raw_ticks) {
     }
 
-    BP_Function_Enum alt_function;
+    BP_Function_Enum alt_function = GPIO_FUNCTION_OUTPUT;
     uint8_t index;
-    uint8_t channel;
+    uint8_t channel = 0;
     bool found = false;
     for (size_t i = 0; i < NUM_ALT_FUNC; i++) {
         const pin_function_t *f = &digitalinout->pin->functions[i];

ports/broadcom/common-hal/videocore/Framebuffer.c

@@ -16,7 +16,13 @@ void common_hal_videocore_framebuffer_construct(videocore_framebuffer_obj_t *sel
     // TODO: Make the FB twice as tall if double buffering.
     uint32_t physical_height = height;
     uint32_t pitch = 0;
-    self->framebuffer = vcmailbox_get_framebuffer(&virtual_width, &virtual_height, &physical_width, &physical_height, &pitch);
+    uint32_t bits_per_pixel = 0;
+    self->framebuffer = vcmailbox_get_framebuffer(&virtual_width,
+        &virtual_height,
+        &physical_width,
+        &physical_height,
+        &pitch,
+        &bits_per_pixel);
     if (self->framebuffer == NULL) {
         if (gc_alloc_possible()) {
             mp_raise_ValueError(translate("no fb"));

ports/broadcom/mpconfigport.h

@@ -36,7 +36,11 @@
 #define MICROPY_PY_SYS_PLATFORM                     "BROADCOM"
 #define MICROPY_PY_BUILTINS_NOTIMPLEMENTED          (1)
 #define MICROPY_PY_FUNCTION_ATTRS                   (1)
+#if BCM_VERSION == 2837 || BCM_VERSION == 2711
 #define MICROPY_OBJ_REPR            (MICROPY_OBJ_REPR_A)
+#elif BCM_VERSION == 2835
+#define MICROPY_OBJ_REPR            (MICROPY_OBJ_REPR_C)
+#endif
 #define CIRCUITPY_DEFAULT_STACK_SIZE            (0x10000)
 #define CIRCUITPY_DISPLAY_AREA_BUFFER_SIZE (1920)
 #define CIRCUITPY_PROCESSOR_COUNT (4)

ports/broadcom/mphalport.c

@@ -41,16 +41,16 @@ mp_uint_t cpu_get_regs_and_sp(mp_uint_t *regs) {
     __asm__ ("mov %[out], x28" : [out] "=r" (regs[9]));
     #else
     __asm__ ("mov %[out], sp" : [out] "=r" (sp));
-    __asm__ ("mov %[out], x19" : [out] "=r" (regs[0]));
+    __asm__ ("mov %[out], r4" : [out] "=r" (regs[0]));
-    __asm__ ("mov %[out], x20" : [out] "=r" (regs[1]));
+    __asm__ ("mov %[out], r5" : [out] "=r" (regs[1]));
-    __asm__ ("mov %[out], x21" : [out] "=r" (regs[2]));
+    __asm__ ("mov %[out], r6" : [out] "=r" (regs[2]));
-    __asm__ ("mov %[out], x22" : [out] "=r" (regs[3]));
+    __asm__ ("mov %[out], r7" : [out] "=r" (regs[3]));
-    __asm__ ("mov %[out], x23" : [out] "=r" (regs[4]));
+    __asm__ ("mov %[out], r8" : [out] "=r" (regs[4]));
-    __asm__ ("mov %[out], x24" : [out] "=r" (regs[5]));
+    __asm__ ("mov %[out], r9" : [out] "=r" (regs[5]));
-    __asm__ ("mov %[out], x25" : [out] "=r" (regs[6]));
+    __asm__ ("mov %[out], r10" : [out] "=r" (regs[6]));
-    __asm__ ("mov %[out], x26" : [out] "=r" (regs[7]));
+    __asm__ ("mov %[out], r11" : [out] "=r" (regs[7]));
-    __asm__ ("mov %[out], x27" : [out] "=r" (regs[8]));
+    __asm__ ("mov %[out], r12" : [out] "=r" (regs[8]));
-    __asm__ ("mov %[out], x28" : [out] "=r" (regs[9]));
+    __asm__ ("mov %[out], r13" : [out] "=r" (regs[9]));
     #endif
 
     return sp;

ports/broadcom/peripherals

@@ -1 +1 @@
-Subproject commit 2c10889a4b2d78987bc4e0783db2e7584aa4d572
+Subproject commit 4e1632ab1f15996a9bbe2aa74f92bf138dff691f

ports/broadcom/supervisor/port.c

@@ -116,42 +116,57 @@ void reset_to_bootloader(void) {
 }
 
 void reset_cpu(void) {
-
+    // Don't actually reset because we can't store the safe mode info.
+    // PM->WDOG = 1 << PM_WDOG_TIME_Pos | PM_WDOG_PASSWD_PASSWD << PM_WDOG_PASSWD_Pos;
+    // PM->RSTC = PM_RSTC_WRCFG_FULL_RESET << PM_RSTC_WRCFG_Pos | PM_RSTC_PASSWD_PASSWD << PM_RSTC_PASSWD_Pos;
     while (true) {
     }
 }
 
 bool port_has_fixed_stack(void) {
+    #ifdef __aarch64__
     return true;
-}
+    #else
-
+    return false;
-uint32_t *port_stack_get_limit(void) {
+    #endif
-    return (uint32_t *)0x4;
-}
-
-uint32_t *port_stack_get_top(void) {
-    return (uint32_t *)0x80000;
 }
 
 // From the linker script
 extern uint32_t __bss_end;
+extern uint32_t _ld_ram_end;
+uint32_t *port_stack_get_limit(void) {
+    #ifdef __aarch64__
+    return (uint32_t *)0x4;
+    #else
+    return &__bss_end;
+    #endif
+}
+
+uint32_t *port_stack_get_top(void) {
+    #ifdef __aarch64__
+    return (uint32_t *)0x80000;
+    #else
+    return &_ld_ram_end;
+    #endif
+}
+
 uint32_t *port_heap_get_bottom(void) {
     return &__bss_end;
 }
 
 uint32_t *port_heap_get_top(void) {
     // 32M heap
-    return (uint32_t *)(((uint64_t)&__bss_end) + 32 * 1024 * 1024);
+    return (uint32_t *)(((size_t)&__bss_end) + 32 * 1024 * 1024);
 }
 
 void port_set_saved_word(uint32_t value) {
     // NOTE: This doesn't survive pressing the reset button (aka toggling RUN).
-    // watchdog_hw->scratch[0] = value;
+    __bss_end = value;
+    data_clean_and_invalidate(&__bss_end, sizeof(uint32_t));
 }
 
 uint32_t port_get_saved_word(void) {
-    // return watchdog_hw->scratch[0];
+    return __bss_end;
-    return 0;
 }
 
 uint64_t port_get_raw_ticks(uint8_t *subticks) {