FAT12

[Guest]

How do you read a specific cluster's data from the FAT. The problem is that each cluster's data is 12-bits (1.5 bytes) I've heard you can't have an array of that i.e The bellow won't work:-

typedef struct
{
int data : 12;
}fatdata_t
fatdata_t TheFat[1000];
readfloppy(whateversector,TheFat);
kprintf("%d\n",TheFat[clusterno]);

So what can I do to read the data?

Thanks in advanced

Pete

[Tim]

This:

Code: Select all

uint32_t Fat::GetNextCluster(uint32_t cluster)
{
    uint32_t FATOffset;
    uint16_t w;
    uint8_t* b;

    if (cluster >=
        (uint32_t) (m_sectors / m_bpb.sectors_per_cluster))
    {
        wprintf(L"Cluster 0x%x (%d) beyond total clusters 0x%x (%d)\n",
            cluster, cluster,
            m_sectors / m_bpb.sectors_per_cluster,
            m_sectors / m_bpb.sectors_per_cluster);
        assert(false);
    }

    switch (m_fat_bits)
    {
    case 12:
        FATOffset = cluster + cluster / 2;
        break;
    case 16:
        FATOffset = cluster * 2;
        break;
    default:
        assert(m_fat_bits == 12 || m_fat_bits == 16);
        return (uint32_t) -1;
    }

    b = m_fat + FATOffset;
    /* xxx -- won't work for FAT32 */
    w = *(uint16_t*) b;

    if (m_fat_bits == 12)
    {
        if (cluster & 1)    /* cluster is odd */
            w >>= 4;
        else                /* cluster is even */
            w &= 0xfff;
        
        if (w >= 0xff0)
            w |= 0xf000;
    }
    
    return w;
}

(m_fat is a uint8_t* pointing to a copy of the entire FAT read from disk)

[Guest]

It still won't work the code is at:-

http://xinit.port5.com/fat12.c
http://xinit.port5.com/fat12.h

Please can someone look at what might be the bug. The code works fine for files <513 bytes but when getting the next cluster number it gets 1070 instead of 3 no matter what code I use. This makes me think that it might be an error reading the FAT in, although my Floppy Driver works for everything else.

Thanks in advance

Pete