FAT-Calculating free disk space

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Hello everybody,

I want to calculate the free space of a SD-Card. The SD-Card is either formatted to FAT16 or
to FAT32.
The calculation is performed by a M16C uC which is connected via SPI to the card.

I have a little clue how to calculate but I don't know if my idea is high in performance.

My idea:
Reading following from parameters "Boot sector"
- Total sector (0x13/0x20)
- Sector per cluster (0x0d)
- Bytes per sector (0x0b)

Then calculating
Total sector / Sectors per cluster=> Total clusters

Then I look up the FAT and calculate how much clusters of total cluster are free

For example

free_cluster=0;
for(int i=0;i<total mode="hold" />{
  if(FAT[i]==0)//Then cluster is free
    free_cluster++;
}

Finally, I would calculate the size like this:

Size_of_cluster=Bytes per sector*Sector per cluster;
free_space=free_cluster * Size_of_cluster;

What do you think about my solution?
Is there any other way how to solve the problem?
How does Windows calculate this if I insert a SD-Card?
How would you solve it?

Thank you for helping

Posted 21-Jan-11 11:51am

Mxrxix

Updated 21-Jan-11 14:06pm

Henry Minute

v2

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Nish Nishant · Answer 1 · 2011-01-21T12:09:00

Solution 1

You can just use GetDiskFreeSpaceEx (you need to know the drive name that Windows has mapped the SD card to).

Example:
-----------------

C++

int _tmain()
{
    ULARGE_INTEGER freeBytesAvailable;
    ULARGE_INTEGER totalNumberOfBytes;
    ULARGE_INTEGER totalNumberOfFreeBytes;
    GetDiskFreeSpaceEx(L"J:\\", &freeBytesAvailable, &totalNumberOfBytes, &totalNumberOfFreeBytes);
}

Posted 21-Jan-11 12:09pm

Nish Nishant

Updated 21-Jan-11 12:10pm

v2

Comments

Sergey Alexandrovich Kryukov 21-Jan-11 18:13pm

As simple as that - my 5.

Nish Nishant 21-Jan-11 18:17pm

A hypothetical 5 apparently, but thanks nonetheless ;-)

Sergey Alexandrovich Kryukov 21-Jan-11 19:35pm

I agree if you say it's not that easy. It depends what you want want, because the disk is fragmented, etc. But that's would be beyond the question. Now I see, OP makes your answer totally irrelevant. But that's the (big) fault of OP, not yours (in fact, very nasty of OP to formulate a question like that, waste our time for nothing).
So, this is not a valid reason to re-vote, its the OP deserved big down-vote. :-)

Nish Nishant 21-Jan-11 21:33pm

Kryukov, I was kidding there. I said that because you commented with "5" and yet there were no votes. It was just an attempt at humor on my part :-)

Sergey Alexandrovich Kryukov 21-Jan-11 22:05pm

I kind of noticed your smile :-)
It's just so happened that when answering to your comment I spotted another "joke" -- by OP, and then...

Sergey Alexandrovich Kryukov 21-Jan-11 19:36pm

Mxrxix commented:

"No I cant use a function like "GetDiskFreeSpaceEx".

The calculation is made by a microcontroller (M16C) which is connected via SPI to the SD-Card.
Only the uC can access the card."

Nish Nishant 21-Jan-11 21:32pm

To the OP: okay, you had not mentioned that in your question originally.

Sergey Alexandrovich Kryukov 21-Jan-11 22:12pm

OP did not mention it as I just spotted, I apologized for my over-reaction.
But I cannot blame you, how could you understand that?
Cheers,
--SA

Nish Nishant 22-Jan-11 10:47am

No problem, no worries :-)

Sergey Alexandrovich Kryukov 21-Jan-11 22:11pm

Oh, Mxrxix, I'm sorry.

I did not realize "The calculation is performed by a M16C uC" might mean different platform, because I was only looking to Nishant's Answer. Also, it's hard to understand what it was, because one could assume this is controller or something working on Windows. You see, this is too exotic thing; most people (including myself) not familiar with.
At least I understand you did not silently confused Nishant...

Nish Nishant 22-Jan-11 10:49am

Yeah, while I think he did not make it very obvious, on re-reading his question it was mentioned. It just did not stand out enough to be obvious.

User 7429338 · Answer 2 · 2011-01-21T23:44:00

Your method to calculate the free space seems fine.
I do spot one error though in the implementation:

The first two entries of the FAT don't map to clusters but hold other information. This is either 2x16 or 2x32 bits depending on if you're using FAT16 or FAT32. Although this data won't be 0 anyway, it shouldn't be used in the calculation.