Current ambiguities in the Ecode machine

     While going through the Ecode generator for Amdahl it  has  become  obvious
that  varying  interpretations  are  possible for some Ecode operations and that
these interpretaions conflict.   These notes are intended to  clarify  the  more
important areas.

      1) Locations and addresses.

     Ecode operations are normally between items  in  store  (locations).    The
     locations  are  notified  by stacking them as area and offset or otherwise.
     The address of a location can be obtatined by eop(Eaddress) or by  stacking
     the address of a location(Estkaddr). An address once stacked has the status
     of a litval ie it does not have an address itself nor  is  it  possible  to
     store  into  an  address.    To  convert  an  address  into  a location use
     Erefer(0,size).

      2) Storing results.

     The operations ESTORE and EDUPSTORE are  provided  for  storing  arithmetic
     results into 1,2,4,8 & 16 byte locations. Storage is arranged so as to give
     the correct numerical result converting as necessary (eg fetch  byte  store
     into integer).  To store sets, character strings (other than 1 byte strings
     into 1 byte locations), etc MVB or specialised operations are needed. There
     is an inescapable conflict between storing a byte into a word and storing a
     one charater string into a 4 byte location.

      3) MVB

     The from and to operands for MVB are addresses;   thus to move integer i to
     integer j the operations are:-

         estkaddr i
         estkaddr j
         estklit 4
         eop(MVB)

      4) Indexing

     Indexing is an operation on an address not on a location.  If a location is
     specified  as  the  base  address of an index operation the the contents of
     that location will be taken as a base address.   Fortran is the odd man out
     here.   There is a further ambiguity between Fortran's indexing and the Imp
     users construction :- longreal(integername+4*integervalue).

      5) Procedures

     Ecode assumes nested procedure bodies are themselves nested.   Pascal  does
     not  do this and has an procedure Eproclevel to notify the generator of the
     level of any procedure body.  I propose bits 1-7 of properties be the level
     if nonzero. Zero means normal nesting.

      6) Old forms

      The operations IPUSH,IPOP,SFA,ASF are apparently no longer used.


     Pete.