/*
* walk the fat one level ( n is a current cluster number ).
* return the new cluster number or -1 if no more.
*/
static long
fatwalk(Dos *dos, int n)
{
ulong k, sect;
Clustbuf *p;
int o;
chat("fatwalk %d\n", n);
if(n < 2 || n >= dos->fatclusters)
return -1;
switch(dos->fatbits){
case 12:
k = (3*n)/2; break;
case 16:
k = 2*n; break;
case 32:
k = 4*n; break;
default:
return -1;
}
if(k >= dos->fatsize*dos->sectsize)
panic("getfat");
if (dos->sectsize == 0 || dos->clustsize == 0)
panic("fatwalk: zero sector or cluster size");
sect = (k/(dos->sectsize*dos->clustsize))*dos->clustsize + dos->fataddr;
o = k%(dos->sectsize*dos->clustsize);
p = getclust(dos, sect);
k = p->iobuf[o++];
if(o >= dos->sectsize*dos->clustsize){
p = getclust(dos, sect+dos->clustsize);
o = 0;
}
k |= p->iobuf[o++]<<8;
if(dos->fatbits == 12){
if(n&1)
k >>= 4;
else
k &= 0xfff;
if(k >= 0xff8)
k = -1;
}
Iobuf*
getbuf(Xdata *dev, ulong addr, ulong tag)
{
int off;
Ioclust *c;
off = addr%BUFPERCLUST;
c = getclust(dev, addr - off, tag);
if(c->nbuf < off){
c->busy--;
error("I/O read error");
}
return &c->buf[off];
}
