Currently working on a homework assignment for one of my Comp. Sci. classes -- the task is to write a program that can read/write FAT12 images.

So far I have read the MBR, read the FATables, and read the root directory entry (subdirectories are not so important).

I'm running into trouble trying to translate a cluster number from the FATable into an actual byte offset in the file: sometimes it works perfectly, other times I end up begining my read from halfway through where the file started.

Here's how I'm translating cluster number into byte offset:
v is a struct that contains data read from the MBR;
floppy is a pointer to the image loaded in memory.

[code]
  uint8_t ptr = 0;
  // first compute the sectors number of the cluster
  ptr += (v.hidden_sector_cnt + v.reserved_sector_cnt);
  ptr += volinfo.sectors_per_fat * volinfo.fat_cnt;
  // 32  = size of a Fat12 directory entry (bytes)
  ptr += (volinfo.root_dir_entries * 32) / volinfo.bytes_per_sector;
  // subtract two sectors for first two magic entries in the root dir
  ptr -= 2;
  // sector_number * bps
  ptr = (uint8_t*)((int)ptr * volinfo.bytes_per_sector);
  // actual offset into the file
  ptr += (int)floppy;


  // begin copying from ptr + (dirent.start_cluster * bytes_per_cluster)
[/code]

Any ideas?

Adding by hidden/reserved sector count doesn't sound right. Aren't those just mapped in the fat, a reserved sector can be anywhere on the floppy?

[quote author=Adron link=topic=11407.msg110233#msg110233 date=1114655350]
Adding by hidden/reserved sector count doesn't sound right. Aren't those just mapped in the fat, a reserved sector can be anywhere on the floppy?
[/quote]

I actually got it to work -- I'm not sure what was wrong, but now it's working 100% of the time.  I think the hidden sector is at the beginning of the image; I'm not sure about reserved, but the image we were given doesn't have any so I guess it's ok ;).