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fortuna-fat

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This is a very small very small (< 8 kB) and memory conscious (< 40 bytes + a shared 512 byte buffer) C11 code for accessing FAT images. Compilable to both AVR and x64, for use in Fortuna computers and emulator.

To use in your project, just add the files in the src/ directory to the project source.

It is implemented in a series of layers:

  • Layer 0: raw access to 512-bytes sectors in the device;
  • Layer 1: FAT aware access to sectors. Permit creating/navigating/resizing files. Does not understand the concept of directory listings, treats everything as a file. Keeps FSINFO updated.
  • Layer 2: Understand directory listings. Can update file date and size. Can create/delete and list directories. (NOT IMPLEMENTED YET)

Each layer builds on top of the previous one.

Layers can be added or removed from the implementation by setting the define LAYER_IMPLEMENTED. For example, to enable only layers 0 and 1, use:

#define LAYER_IMPLEMENTED 1

Layer 0 (raw access)

Registers

Name Description Size RW
F_RSLT Result of the last operation (see below). 8-bit Read-only
F_RAWSEC Number of the 512-byte sector in the device. 64-bit Read/write

Supported operations

Operation Description
F_READ_RAW Reads a 512-byte sector from the device to the buffer.
F_WRITE_RAW Writes the buffer into a 512-byte sector in the device.

Possible responses (F_RSLT)

Code Response
F_OK Operation successful.
F_IO_ERROR I/O error in the device.
F_INVALID_OP A non-existing operation was requested.

Layer 1 (FAT, FSINFO and data clusters)

All requests from layer 0 are also available in layer 1.

Layer 1 mostly allows navigation and modification of the FAT. Creating a new file will update the FAT and FSINFO, but will not create a new file in the directory. This needs to be done in a superior layer.

Registers

When refering to the registers, all clusters/sectors are counted from beginning of partition, except for F_ABS.

These registers can be used to pass parameters and receive values from requests:

Name Description Size RW
F_CLSTR Number of the data cluster to which the operation refers to. 32-bit Read/write
F_SCTR Number of the sector inside the cluster. 16-bit Read/write
F_PARM Additional parameter used in some operations. 32-bit Read/write
F_ROOT Number of the root directory sector (relative to start of data sectors). 16-bit Read-only
F_SPC Sectors per cluster. This is important while reading/writing to files. Since most commands are based on clusters, and not sectors, it is necessary to know how many sectors are in a cluster. 8-bit Read-only

These registers are usually not required, but can provide additional information to the user:

Name Description Size RW
F_ABS Partition sector start (counting from beginning of disk) 32-bit Read-only
F_FATST FAT sector start 16-bit Read-only
F_FATSZ FAT size 32-bit Read-only
F_DATA Data starting sector 32-bit Read-only
F_NFATS Number of FATs 3 bits (0 ~ 7) Read-only
F_TYPE Filesystem type 2 bits (0: FAT16, 1: FAT32) Read-only

Supported operations

Operation Description Additional information
F_INIT Reads a partition boot sector and initialize the partition. When changing partitions, new F_INIT must be issued. F_PARM: partition number
F_BOOT Load boot sector into buffer.
F_FREE Returns number of free clusters into first 4 bytes of buffer.
F_FSI_CALC Recalculate FSINFO values (free clusters and next free cluster).
F_SEEK Advance F_CLSTR a number of clusters. F_PARM: number of clusters to move forward. Use 0xFFFFFFFF to go to EOF.
F_APPEND Advance F_CLSTR until the cluster that marks the end of file, and append a new cluster. If F_CLSTR == 0, it'll create a new file.
F_TRUNCATE Limit the file size to the one specified in F_CLSTR, clearing all following FAT entries.
F_REMOVE Remove a file from FAT. F_CLSTR should be pointing to the initial file cluster.
F_READ_DATA Read a sector in a data cluster, based on F_CLSTR and F_SCTR.
F_WRITE_DATA Write a sector in a data cluster, based on F_CLSTR and F_SCTR.

Possible responses (F_RSLT)

Code Response
F_NO_PARTITION The partition does not exist.
F_UNSUPPORTED_FS The file system is not FAT16 or FAT32.
F_BPS_NOT_512 Bytes per sector is not 512.
F_DEVICE_FULL There's no space left in the device.
F_SEEK_PAST_EOF A seek command moved the pointer past EOF.
F_INVALID_FAT_CLUSTER A command was issued on a invalid FAT cluster.
F_NOT_IMPLEMENTED A request is being made to a command that was not implemented yet.

Layer 2 (files and directories)

Layer 2 is not yet implemented, but will have the following functionalities:

  • Navigate between directories
  • Create/delete files
  • Read/write files
  • Create/delete directories
  • Stat files and directories
  • Move and rename files and directories