When a word modifies the buffer, it sets the buffer as dirty
by calling BLK!!. BLK@ checks, before it reads its buffer,
whether the current buffer is dirty and implicitly calls BLK!
when it is.

The index of the block currently in memory is kept in BLK>.

Many blocks contain code. That code can be interpreted through
LOAD. Programs stored in blocks frequently have "loader blocks"
that take care of loading all blocks relevant to the program.