The related ilo & napia virtual machines make use of blocks
for data storage. This implements a set of words for interacting
with the blocks from within RetroForth/Nga.

The exposed word set is compact:

  block:set-file (s-)
  block:read     (na-)
  block:write    (na-)

~~~
{{
  'Blocks var
  'BlockFile var

  :open  (n-) @BlockFile swap file:open !Blocks ;
  :close (-)  @Blocks file:close ;
  :seek  (n-) #4096 n:mul @Blocks file:seek ;
  :read  (-n) #4 [ @Blocks file:read ] times pack ;
  :store (an-a) swap store-next ;
  :write (n-) unpack 'abcd 'bcda reorder #4 [ dup n:put sp @Blocks file:write ] times ;
---reveal---
  :block:set-file (s-) s:keep !BlockFile ;
  :block:read (na-)
    file:R open swap seek #1024 [ read store ] times drop close ;
  :block:write (na-)
    file:R+ open swap seek #1024 [ fetch-next write ] times drop close ;
}}
~~~