spikeroot wrote on Monday, July 24, 2017:
Hi.
I’m currently using the ELM-FAT library on my embedded system to create two partitions on an SD card, and format them both as FAT32. The problem is that when I then pop that SD card into my PC, I can only access the first partition from Windows. This is a known limitation of that system.
If I do this using the FreeRTOS +FAT system instead, will I be able to access all partitions from Windows?
Thanks.
-Spike
rtel wrote on Monday, July 24, 2017:
If I do this using the FreeRTOS +FAT system instead, will I be able to
access all partitions from Windows?
Provided they are formatted in accordance with the Windows requirements,
then yes.
spikeroot wrote on Monday, July 24, 2017:
Thanks for your response. I’m confused though: If I use your FF_Partition and FF_Format functions, why wouldn’t they be in accordance with the Windows requirements? Is it just a matter of using the right parameters?
rtel wrote on Monday, July 24, 2017:
Is it just a matter of using the right
parameters?
Yes. The official FAT spec has requirements for the minimum size, etc.
If you create a disk that does not fall into that spec then +FAT might
be happy to use it, but Windows won’t.
heinbali01 wrote on Tuesday, July 25, 2017:
Hi Michael, in the FreeRTOS+FAT demo projects, in ff_sddisk.c, you will see that :
BaseType_t xPartitionNumber = 0;
Just because 99% percent of the memory cards have a single partition.
But there may be good reasons to use 2 partitions and +FAT will indeed recognise them: both primary as well as extended/logical partitions.
If you are using SD-cards, be aware that only the first partition will have it’s FAT table in a FAT-optimised area ( the second 4MB erase block ). So you might notice that subsequent partitions are slower, and maybe even less reliable.
Within ff_ioman.c you see how partitions are being discovered, with the function FF_PartitionSearch():
xPartitionCount = FF_PartitionSearch( pxIOManager, &partsFound );
if( FF_isERR( xPartitionCount ) )
{
xError = xPartitionCount;
break;
}
heinbali01 wrote on Tuesday, July 25, 2017:
Forgot to tell how the two ( or more ) partitions will be represented in the file system.
Normally there is an SD-card with a single partition, which will be found in the root ("/").
As an example, the following files are located in the first partition mounted on "/":
/readme.txt
/etc/config.ini
/firmware/bootloader.hex
The modules ff_sys.c and ff_stdio.c allow you to combine several volumes into a single file system. For instance, if you create a RAM disk, you can mount its root on e.g. "/ram" :
/ram
/ram/files/picture1.jpg
/ram/tmp/_WM68C7.tmp
When the first partition of the SD-card is mounted as "/", you can mount the second partition as follows:
FF_FS_Add( "/part2", pxDisk_2 );
The user interface of FreeRTOS+TCP has two layers:
Lower level functions that work on a single volume. FF_Open(), FF_Read(), FF_Write(). These functions will not recognise your "/part2".
STDIO-compatible functions that do recognise mounted volumes, such as "/ram" and "/part2", These functions have a prefix "ff_", such as ff_fopen(), ff_fread()/ff_fwrite(), and ff_fclose().
It sounds all more complicated than it actually is. It works very much like mounting in Linux, except that the mount-points are always located in the root directory.
There have recently been some essential changes to a few +FAT source files, I’m attaching these changes here below in ff_sys.zip.
spikeroot wrote on Thursday, August 03, 2017:
So, in order to use this system to format an SD card with two partitions, do I need to create two FF_Disk_t objects with two different calls to FF_SDDiskInit?
heinbali01 wrote on Friday, August 04, 2017:
That is correct, you will need everything double. Two partitions are being treated as different disks.
And it is only in ff_sys.c and ff_stdio.c where the two drives come together in a single “file system”
But the first thing to do is call FF_PartitionSearch(), as described here above.
heinbali01 wrote on Saturday, August 05, 2017:
Hi Machael, your original question in the email was:
Why would the two drives come together in a single file system?
Shouldn’t each partition have its own file system?
I will answer it for the other people following this thread:
you are right, each partition is a file system on its own. But ff_sys.c and ff_stdio.c will map one of the partitions onto a symbolic directory.
All files under e.g."/part2" will refer to the second partition. Other files and directories within the root directory are all located on the first partition.
Better questions to follow…
I’m all curious 
richard_damon wrote on Saturday, August 05, 2017:
Another way to look at it is that there are several ways to express a multi-volume/partition file system. In windows we are used to ‘Drive Letters’ where the main hard drive is C:, and other drivers are given a distinct letter. This has a fundamental limit of 26 volumes/partitions.
A second method, also used in windows is the \server\share\ prefix.
A third method, used on *nix like systems is that / is the root of all systems and some directory (like /part2) is mapped to point to the other volumes/partitions.