This document provides comprehensive guidance for board profile developers on how to add ROMFS (Read-Only Memory File System) support to MicroPython boards.

ROMFS allows embedding a read-only filesystem directly in flash memory that can be accessed at runtime through the /rom mount point. This feature enables:

• Fast access to Python modules and data files (faster than FAT/LittleFS)
• Deployment of applications and libraries without modifying firmware
• Runtime updates of ROM content via vfs.rom_ioctl() and mpremote

Add these defines to your board's mpconfigboard.h:

#define MICROPY_VFS_ROM (1)

This automatically enables MICROPY_VFS_ROM_IOCTL unless explicitly disabled.

Note: When both MICROPY_VFS_ROM and MICROPY_VFS_ROM_IOCTL are enabled, ROMFS will be automatically mounted at /rom during mp_init(). Both /rom and /rom/lib are added to sys.path.

Each port must implement mp_vfs_rom_ioctl() in a vfs_rom_ioctl.c file. This function handles:

• MP_VFS_ROM_IOCTL_GET_NUMBER_OF_SEGMENTS - Query number of ROM partitions
• MP_VFS_ROM_IOCTL_GET_SEGMENT - Get partition info (address, size)
• MP_VFS_ROM_IOCTL_WRITE_PREPARE - Prepare partition for writing (erase)
• MP_VFS_ROM_IOCTL_WRITE - Write data to partition
• MP_VFS_ROM_IOCTL_WRITE_COMPLETE - Finalize write operation

Configuration Options:

// For internal flash storage
#define MICROPY_HW_ROMFS_ENABLE_INTERNAL_FLASH (1)

// For external QSPI flash storage  
#define MICROPY_HW_ROMFS_ENABLE_EXTERNAL_QSPI (1)
#define MICROPY_HW_ROMFS_QSPI_SPIFLASH_OBJ (&spi_bdev_obj)

// Enable partitions (up to 2 supported)
#define MICROPY_HW_ROMFS_ENABLE_PART0 (1)
#define MICROPY_HW_ROMFS_ENABLE_PART1 (1)  // Optional second partition

Linker Script Requirements:

Define partition symbols in your .ld file:

MEMORY
{
    /* Other memory regions... */
    FLASH_ROMFS (rx): ORIGIN = 0x90000000, LENGTH = 1024K
}

/* ROMFS location symbols */
_micropy_hw_romfs_part0_start = ORIGIN(FLASH_ROMFS);
_micropy_hw_romfs_part0_size = LENGTH(FLASH_ROMFS);

/* Optional second partition */
_micropy_hw_romfs_part1_start = ORIGIN(FLASH_ROMFS2);
_micropy_hw_romfs_part1_size = LENGTH(FLASH_ROMFS2);

Example Boards:

• PYBD_SF2/SF6: Uses external QSPI flash, 1MB partition
• ARDUINO_GIGA: External QSPI flash support
• ARDUINO_NICLA_VISION: External QSPI flash support
• ARDUINO_PORTENTA_H7: External QSPI flash support

Configuration:

Add to mpconfigboard.h:

#define MICROPY_HW_ROMFS_BYTES (128 * 1024)  // Size in bytes

Memory Layout:

• ROMFS partition placed at end of firmware allocation
• Lives between firmware and read/write filesystem
• Reduces available firmware space by MICROPY_HW_ROMFS_BYTES

Automatic Enablement:

#define MICROPY_VFS_ROM (MICROPY_HW_ROMFS_BYTES > 0)

Configuration:

Add to mpconfigboard.h:

#define MICROPY_VFS_ROM (1)

Partition Table:

Create or modify partition CSV file with romfs entry:

# Name,   Type, SubType, Offset,  Size, Flags
nvs,      data, nvs,     0x9000,  0x6000,
phy_init, data, phy,     0xf000,  0x1000,
factory,  app,  factory, 0x10000, 0x1D0000,
romfs,    data, 0x8f,    0x1E0000, 0x20000,    # 128KB ROM partition
vfs,      data, fat,     0x200000, 0x200000,

Board Configuration:

Specify partition file in board makefile:

PART_SRC = partitions-4MiB-romfs.csv

Configuration:

Add to board variant makefile:

CFLAGS += -DMICROPY_VFS_ROM=1

Linker Script:

Define FLASH_ROMFS memory region and symbols:

MEMORY
{
    /* Other regions... */
    irom0_0_seg :  org = 0x40209000, len = 1M - 36K - 320K  # Reduced firmware space
    FLASH_ROMFS :  org = 0x402b0000, len = 320K             # ROM partition
}

/* ROMFS symbols */
_micropy_hw_romfs_part0_start = ORIGIN(FLASH_ROMFS);
_micropy_hw_romfs_part0_size = LENGTH(FLASH_ROMFS);

Configuration:

Add to mpconfigport.h:

#define MICROPY_VFS_ROM (1)

Hardware Support:

• OSPI flash memory (external)
• MRAM (Magnetoresistive RAM) memory
• Up to 2 partitions supported
• Similar partition configuration to STM32

Configuration Options:

// Enable partitions
#define MICROPY_HW_ROMFS_ENABLE_PART0 (1)
#define MICROPY_HW_ROMFS_ENABLE_PART1 (1)  // Optional second partition

Linker Script:

Similar to STM32, define partition symbols:

/* ROMFS location symbols */
_micropy_hw_romfs_part0_start = ORIGIN(FLASH_ROMFS);
_micropy_hw_romfs_part0_size = LENGTH(FLASH_ROMFS);

Key Features:

• Automatic detection of OSPI vs MRAM based on memory addresses
• Support for both XIP (execute-in-place) and memory-mapped access
• Hardware-specific address validation for memory safety

1. vfs_rom_ioctl.c - Port-specific ROM I/O control implementation
2. Board configuration - Add ROMFS defines to mpconfigboard.h
3. Memory layout - Update linker script or partition table
4. Build integration - Add vfs_rom_ioctl.c to port Makefile

1. Partition Management:

   • Support 1-2 partitions per port capabilities
   • Expose partition info via mp_obj_array_t structures
   • Handle partition bounds checking

2. Flash Operations:

   • Implement erase before write
   • Support incremental writes with offset
   • Handle flash-specific alignment requirements
   • Provide proper error handling

3. Memory Safety:

   • Validate write boundaries
   • Check partition sizes and alignment
   • Prevent corruption of adjacent areas

4. Performance Considerations:

   • ROMFS stat operations are significantly faster than FAT/LittleFS
   • Automatically added to sys.path as /rom and /rom/lib
   • Minimal import overhead compared to frozen modules

import vfs

# Query available ROM segments
num_segments = vfs.rom_ioctl(1)
print(f"Available ROM segments: {num_segments}")

# Get segment info
if num_segments > 0:
    segment_info = vfs.rom_ioctl(2, 0)  # Get first segment
    print(f"Segment 0: {segment_info}")

# Create and deploy ROMFS via mpremote
mpremote romfs build lib/ --output romfs.img
mpremote romfs deploy romfs.img

import os
print("ROM filesystem contents:")
for item in os.listdir('/rom'):
    print(f"  {item}")

1. Insufficient Flash Space: Ensure adequate flash allocation for ROM partition
2. Alignment Issues: Respect flash sector/page alignment requirements
3. Linker Symbol Errors: Properly define partition start/size symbols
4. Missing Dependencies: Include required flash driver headers
5. Partition Overlap: Avoid conflicts with firmware or filesystem areas

1. Size Planning: Reserve 64KB-1MB for typical applications
2. Partition Layout: Place ROM between firmware and R/W filesystem
3. Error Handling: Implement comprehensive error checking in rom_ioctl
4. Documentation: Document partition layout in board documentation
5. Testing: Validate on actual hardware with various file sizes

This guide provides the foundation for implementing ROMFS support on any MicroPython port. Refer to existing implementations for port-specific patterns and adapt as needed for your target hardware.