Bad disk-block numbers in assignment 2.

  The pa2-diskio.dsk batch disk is only 20 disk-blocks long, but the program it
  contains reads and writes disk block 511.  Shouldn't those reads and writes
  fail with a bad block-number error?

No, and the reason depends the difference between a disk block not presently
existing and a disk block not ever possibly existing.

A disk block with a negative number or a number at least the maximum number of
blocks on the disk (Hardware.Disk.blockCount) can't ever exist.  Attempts to
access such a block should cause a bad block-number error.

A disk block with a number at least zero but less than the maximum number of
blocks on the disk could exist, even if it isn't actually on the disk at the
time the block is accessed.  In particular, a disk block with a number at least
the number of blocks on the disk and less than the maximum possible number of
blocks on the disk could exist, even if it doesn't actually appear on the disk.
Accessing such a phantom disk block shouldn't cause a bad block-number error.

There is no difficulty if the first access to a phantom disk block is a write,
and a read is only slightly trickier: if the first access to a phantom disk
block is a read, the disk block read contains undefined values, although the
simulator returns a block of zeros.