.NH 2
External representation of the intermediate code
.PP
The syntax of the intermediate code was given
in the previous section.
In this section we will make some remarks about
the representation of the code in sequential files.
.sp
We use sequential files in order to avoid
the bookkeeping of complex file indices.
As a consequence of this decision
we can't store all components
of the intermediate code
in one file.
If a phase wishes to change some attribute
of a procedure,
or wants to add or delete entire procedures
(inline substitution may do the latter),
the procedure table will only be fully updated
after the entire EM text has been scanned.
Yet, the next phase undoubtedly wants
to read the procedure table before it
starts working on the EM text.
Hence there is an ordering problem, which
can be solved easily by putting the
procedure table in a separate file.
Similarly, the data block table is kept
in a file of its own.
.PP
The control flow graphs (CFGs) could be mixed
with the EM text.
Rather, we have chosen to put them
in a separate file too.
The control flow graph file should be regarded as a
file that imposes some structure on the EM-text file,
just as an overhead sheet containing a picture
of a Flow Chart may be put on an overhead sheet
containing statements.
The loop tables are also put in the CFG file.
A loop imposes an extra structure on the
CFGs and hence on the EM text.
So there are four files:
.IP -
the EM-text file
.IP -
the procedure table file
.IP -
the object table file
.IP -
the CFG and loop tables file
.LP
Every table is preceded by its length, in order to
tell where it ends.
The CFG file also contains the number of instructions of
every basic block,
indicating which part of the EM text belongs
to that block.
.DS
.UL syntax
  intermediate_code:
                object_table_file
                proctable_file
                em_text_file
                cfg_file ;
  object_table_file:
                LENGTH          -- number of objects
                object_table ;
  proctable_file:
                LENGTH          -- number of procedures
                procedure_table ;
  em_text_file:
                em_text ;
  cfg_file:
                {per_proc} ;    -- one for every procedure
  per_proc:
                BLENGTH         -- number of basic blocks
                LLENGTH         -- number of loops
                control_flow_graph
                looptable ;
.DE