D.3 The generic interface for filesystems

For any particular partition, it is presumed that only one of the normal filesystems such as FAT, FFS, or ext2fs can be used, so there is a switch table managed by the functions in disk_io.c. The notation is that you can only mount one at a time.

The block list filesystem has a special place in the system. In addition to the normal filesystem (or even without one mounted), you can access disk blocks directly (in the indicated partition) via the block list notation. Using the block list filesystem doesn't effect any other filesystem mounts.

The variables which can be read by the filesystem backend are:

current_drive

The current BIOS drive number (numbered from 0, if a floppy, and numbered from 0x80, if a hard disk).

current_partition

The current partition number.

current_slice

The current partition type.

saved_drive

The drive part of the root device.

saved_partition

The partition part of the root device.

part_start

The current partition starting address, in sectors.

part_length

The current partition length, in sectors.

print_possibilities

True when the dir function should print the possible completions of a file, and false when it should try to actually open a file of that name.

FSYS_BUF

Filesystem buffer which is 32K in size, to use in any way which the filesystem backend desires.

The variables which need to be written by a filesystem backend are:

filepos

The current position in the file, in sectors.

Caution: the value of filepos can be changed out from under the filesystem code in the current implementation. Don't depend on it being the same for later calls into the backend code!

filemax

The length of the file.

disk_read_func

The value of disk_read_hook only during reading of data for the file, not any other fs data, inodes, FAT tables, whatever, then set to NULL at all other times (it will be NULL by default). If this isn't done correctly, then the testload and install commands won't work correctly.

The functions expected to be used by the filesystem backend are:

devread

Only read sectors from within a partition. Sector 0 is the first sector in the partition.

grub_read

If the backend uses the block list code, then grub_read can be used, after setting block_file to 1.

print_a_completion

If print_possibilities is true, call print_a_completion for each possible file name. Otherwise, the file name completion won't work.

The functions expected to be defined by the filesystem backend are described at least moderately in the file filesys.h. Their usage is fairly evident from their use in the functions in disk_io.c, look for the use of the fsys_table array.

Caution: The semantics are such that then `mount'ing the filesystem, presume the filesystem buffer FSYS_BUF is corrupted, and (re-)load all important contents. When opening and reading a file, presume that the data from the `mount' is available, and doesn't get corrupted by the open/read (i.e. multiple opens and/or reads will be done with only one mount if in the same filesystem).