FAT: clean up cluster arithmetic

a) clean up the computation of the FAT12 FAT entry offset.
b) the highest allocatable cluster is ...ff6, not ...ff0.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/core/fs/fat/fat.c b/core/fs/fat/fat.c
index d77fcb1..ce3cae6 100644 (file)
--- a/core/fs/fat/fat.c
+++ b/core/fs/fat/fat.c
@@ -54,13 +54,14 @@ static uint32_t get_next_cluster(struct fs_info *fs, uint32_t clust_num)
     
     switch(FAT_SB(fs)->fat_type) {
     case FAT12:
-       fat_sector = (clust_num + clust_num / 2) >> SECTOR_SHIFT;
+       offset = clust_num + (clust_num >> 1);
+       fat_sector = offset >> SECTOR_SHIFT;
+       offset &= (1 << SECTOR_SHIFT) - 1;
        cs = get_fat_sector(fs, fat_sector);
-       offset = (clust_num * 3 / 2) & ((1 << SECTOR_SHIFT) - 1);
-       if (offset == 0x1ff) {
+       if (offset == SECTOR_SIZE-1) {
            /* 
             * we got the end of the one fat sector, 
-            * but we don't got we have(just one byte, we need two),
+            * but we have just one byte and we need two,
             * so store the low part, then read the next fat
             * sector, read the high part, then combine it.
             */
@@ -76,7 +77,7 @@ static uint32_t get_next_cluster(struct fs_info *fs, uint32_t clust_num)
            next_cluster >>= 4;         /* cluster number is ODD */
        else
            next_cluster &= 0x0fff;     /* cluster number is EVEN */
-       if (next_cluster > 0x0ff0)
+       if (next_cluster > 0x0ff6)
            goto fail;
        break;
        
@@ -85,7 +86,7 @@ static uint32_t get_next_cluster(struct fs_info *fs, uint32_t clust_num)
        offset = clust_num & ((1 << (SECTOR_SHIFT-1)) -1);
        cs = get_fat_sector(fs, fat_sector);
        next_cluster = ((uint16_t *)cs->data)[offset];
-       if (next_cluster > 0xfff0)
+       if (next_cluster > 0xfff6)
            goto fail;
        break;
        
@@ -94,7 +95,7 @@ static uint32_t get_next_cluster(struct fs_info *fs, uint32_t clust_num)
        offset = clust_num & ((1 << (SECTOR_SHIFT-2)) -1);
        cs = get_fat_sector(fs, fat_sector);
        next_cluster = ((uint32_t *)cs->data)[offset] & 0x0fffffff;
-       if (next_cluster > 0x0ffffff0)
+       if (next_cluster > 0x0ffffff6)
            goto fail;
        break;
     }