( u -- a-addr )

a-addr is the address of the first character of the block buffer assigned to mass-storage block u. An ambiguous condition exists if u is not an available block number.

If block u is already in a block buffer, a-addr is the address of that block buffer.

If block u is not already in memory and there is an unassigned block buffer, transfer block u from mass storage to an unassigned block buffer. a-addr is the address of that block buffer.

If block u is not already in memory and there are no unassigned block buffers, unassign a block buffer. If the block in that buffer has been UPDATEd, transfer the block to mass storage and transfer block u from mass storage into that buffer. a-addr is the address of that block buffer.

At the conclusion of the operation, the block buffer pointed to by a-addr is the current block buffer and is assigned to u.