[Oberon] Static variable overflow (bug?)

[Andreas Pirklbauer]

Thx. On a separate note: The fix below for ORG.Put1a only works for *non-imported*
global variables. There, the offset from the static base can now be >64KB, both
for loading the value of such variables and for determining their address.

For *imported* global variables, there still is the 64KB restriction (actually, loading
works, but not accessing the address of such variables). This could also be fixed.


> On 27 Feb 2020, at 14:35, Skulski, Wojciech <skulski at pas.rochester.edu> wrote:
> 
> Andreas:
> 
>> Below is a slightly more elaborate variant that fixes the current bug in ORG.Put1a
>> and makes MyViewers.Mod (see below) work on a Project Oberon 2013 system:
> 
> How about 
> CONST LIMIT = 10000H; ULIMIT = 0FFFFH; (* Upper limit = LIMIT -1 = 0FFFFH; *)
> 
>  PROCEDURE Put1a(op, a, b, im: LONGINT);
>      VAR r: INTEGER;
>  BEGIN 
>  (*same as Put1, but with range test  -10000H<= im < 10000H*)
>  (*same as Put1, but with range test    -LIMIT <= im < LIMIT *)
>      IF (im >= -LIMIT ) & (im <= ULIMIT) THEN Put1(op, a, b, im)
>      ELSIF op = Mov THEN
>        Put1(Mov+U, a, 0, im DIV LIMIT );
>        IF im MOD LIMIT # 0 THEN Put1(Ior, a, a, im MOD LIMIT ) END
>      ELSE r := RH;
>        IF b = RH THEN incR END ;
>        Put1(Mov+U, RH, 0, im DIV LIMIT );
>        IF im MOD LIMIT # 0 THEN Put1(Ior, RH, RH, im MOD LIMIT ) END ;
>        Put0(op, a, b, RH);
>        IF RH > r THEN DEC(RH) END
>     END
>  END Put1a;
> 
> Comments:
> 0. Using literal constants in the code is a universally condemned programming practice.
> 1. Procedure incR is to be moved up to before procedure ORG.Put0
> 2. The use of incR ensures that in fact we do have enough registers available
> 3. Register optimization: But a new register is used only when needed (when b = RH)
> 4. Instruction optimization: For op = Mov, the extra MOV instruction at the end is avoided
>