S7: Pointer offset as variabel

[PeterW]

L D [AR2,P#0.0], did you use LAR2 P#DIX 0.0 just to set the upper area bits? Isn't there way around having to do this by forcing the L D [AR2,#P0.0] instruction to use the DI register by saying L DIW [AR2,#P0.0]? Or must the area bits always be set?

I think you'll find he's pointing to the first word in the Instance DB prior to adding the pointer as below

[FONT=monospace]
  L     #i_Offset_write; 
  SLD   4; 
  +AR2  ;[/FONT]

this is the best way to do this.

 

[Peter Nachtwey]

Yes, but why not simply

  L     #i_Offset_write; 
  SLD   4; 
  LAR2 ;

unless the upper area bits must be set to point to the DI area?

The question is must the area bits always be set or can one override the area bits in the AR registers by specifying the what DI or DB register to use in the instruction such as L DBW [AR2,P#0.0].

I have not been able to find any documentation on what takes precedence. The instruction or the area bits in the address register.

BTW, I would have saved the DI and AR2 away and restored them at the end. This way the FC can be called from a FB and not mess up the DI and AR2 register.

 

[TurpoUrpo]

Peter, no need to save AR2 when using it inside FC, it is saved automatically by compiler when you call the FC (again LD or s7guy has shown that in some post).

It should work also:

L plaa
LAR2

L DIW[plaa]

Have not tested it with DI but with DB it works, so i see no reason why would it not work with DI too.

I think its about how one likes to do it.

 

[L D[AR2P#0.0]]

The instruction overrides the area specified in the address register: