New as function

[CBFalconer]

Frank Heckenbach wrote:
> CBFalconer wrote:
> 
... snip ...
> 
> > Proper use of the standard mechanism allows almost anything to be
> > attached to the file system with clear semantics.  This includes
> > such things as complete screens, LANs, pipes, etc.  No need for
> > so-called CRT units, although they may be convenient.
> 
> I don't agree here. The Pascal file system only allows for
> sequential read or write access (ISO 7185), or record-based random
> access (ISO 10206). A CRT unit does much more than this.
> 
> To start with, coordinates are two-dimensional. Of course, you could
> compute a sequential offset yourself, but this doesn't exactly sound
> like a high-level language to me.

Just as an example, one of my systems in bygone years did screen
updates.  The file was defined as a FILE OF RECORD .... where the
record included a 24x80 array of char and a "cursorlocation" x y
pair.  The main program could examine and alter these fields in f^
as it desired, and then execute "put(f)", which did the actual
transmission.

There was then no difference in the code running an external
terminal on a serial line, and that running a memory mapped
display.  There was a great difference in the drivers, which were
mapped into the actual file system via a set of tables for such
things as open, close, put, get, status.  The actual operation
could be asynchronous, and handled by interrupts in a serial drive,
none of which complication was needed in a local memory driver. 
That meant a difference in throughput to the actual program, but
the apparent speed was virtually unaffected.

If significant variations in the end device existed, they could be
reported by invariant fields in f^, such as MAXX and MAXY.  After a
put to the serial device completion could be detected by examining
a 'busy' field.  Etc.  The details are unimportant, the point is
that all this can be mapped into the standard Pascal file system.

-- 
Chuck F (cbfalconer at yahoo.com) (cbfalconer at worldnet.att.net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>  USE worldnet address!