BLK_PER_READ
static int prvdevoff = -dbtob(BLK_PER_READ);
pos = prvdevoff + dbtob(BLK_PER_READ);
for (; pos < devoff; pos += dbtob(BLK_PER_READ))
dbtob(BLK_PER_READ), pos);
boot_input(diskbuf, dbtob(BLK_PER_READ), devoff);
readlen = dbtob(BLK_PER_READ) - off;
blkpos = (blkpos & ~MASK_BLK_PER_READ) + BLK_PER_READ;
#define MASK_BLK_PER_READ (BLK_PER_READ - 1)