All blocks have a 4 byte header and a 4 byte footer

hd

ft

The header and footer have the same value.

The value is in two parts:

• the block size
  (which includes the header and footer)

• a bit indicating the block is allocated or free

The block size is in the upper 29 bits of the header/footer and the 3 lower bits of the size are implied and are 0. This means the block size is a multiple of 8.

The payload is the portion of the block between the header and footer.

The payload must begin at an 8 byte address boundary; that is, the payload address must be a multiple of 8.