[Oberon] FJump and FixLink

[Andreas Pirklbauer]

  > The RISC5 processor has an addressbus of 24 bits («adr» in RISC5.v = 16 MB possible).
  > The PC and branch displacements are on word boundary, they have 22 bits («disp» in RISC5.v)
  >
  >  In short, I would have coded the procedures as follows:
  >
  >  PROCEDURE FixLink*(L: LONGINT);
  >    VAR L1: LONGINT;
  >  BEGIN
  >    WHILE L # 0 DO L1 := code[L] MOD 2000H; FixOne(L); L := L1 END
  >  END FixLink;
  >
  > PROCEDURE FixLinkWith(L0, dst: LONGINT); (* used for «UNTIL» *)
  >   VAR L1: LONGINT;
  >  BEGIN
  >    WHILE L0 # 0 DO
  >      L1 := code[L0] MOD 2000H; fix(L0, dst - L0 - 1); L0 := L1
  >    END
  >  END FixLinkWith;
  >

Ok. Got it. This makes more sense. But it works only for ORG.maxCode = 8000,
since 2000H = 8196. In my compilers ORG.maxCode usually is 12000 or
even 16000. So with the above approach, I’d end up having to use 4000H
= 16’384, i.e. the next higher power of 2 (for efficiency) anyway..

How about using C22 = 400000H (with FIVE zeros, not four!) = 4’194’304
= 2^22 instead of 2000H? That way, one would be...

a) correct (max displacement = 22 bits), and 

b) “safe” even for all potential values of ORG.maxCode
   up 4’194’304 = 4M words = 16MB RAM.

once and for good?

-ap