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Block Devices

Block devices are the basic building block of storage solutions in Mbed OS.

MbedOSStorage

File systems are backed by blockdevice implementations. The BlockDevice API performs the low-level interactions with the hardware storage. To add your own block device implementation, we recommend you inherit from the BlockDevice class. For details on how to extend the BlockDevice interface, please refer to the and implementing BlockDevice section below.

Assumptions

Defined behavior

  • Erase leaves memory as undefined. It does not set memory to a predetermined value.

Undefined behavior

  • Programming without erase is undefined behavior.

Notes

blockdevicesectors

Erase, program and read block sizes may not be the same; however, they must be multiples of one another.

Implementing BlockDevice

You can find the BlockDevice class on the master branch under the features/filesystem/bd path in Mbed OS.

Public Member Functions
virtual ~BlockDevice ()
virtual int init ()=0
virtual int deinit ()=0
virtual int sync ()
virtual int read (void *buffer, bd_addr_t addr, bd_size_t size)=0
virtual int program (const void *buffer, bd_addr_t addr, bd_size_t size)=0
virtual int erase (bd_addr_t addr, bd_size_t size)
virtual int trim (bd_addr_t addr, bd_size_t size)
virtual bd_size_t get_read_size () const =0
virtual bd_size_t get_program_size () const =0
virtual bd_size_t get_erase_size () const
virtual bd_size_t get_erase_size (bd_addr_t addr) const
virtual int get_erase_value () const
virtual bd_size_t size () const =0
bool is_valid_read (bd_addr_t addr, bd_size_t size) const
bool is_valid_program (bd_addr_t addr, bd_size_t size) const
bool is_valid_erase (bd_addr_t addr, bd_size_t size) const

The primary functions to implement are:

  • int read(void *buffer, bd_addr_t addr, bd_size_t size);
  • int program(void *buffer, bd_addr_t addr, bd_size_t size);
  • int erase(bd_addr_t addr, bd_size_t size);

Testing

You can run BlockDevice tests for heap, MBR and util block devices with the following command:

mbed test -t <toolchain> -m <target> -n features-tests-filesystem-*_block_device

One way to add tests for new block devices is to copy an existing implementation, such as HeapBlocKDevice, and change the block device class to your own. You can find tests under the top level TESTS folder in the Mbed OS repository.

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