Another Siemens Indirect Addressing Question

L D[AR2P#0.0] · Apr 22, 2012

rikcando said:
That is extremely helpful. I can see that my pointer is now 3.1 instead of 4. I realize that I need to increment the original offset instead of the current.

I prefer to use the Address register AR1 for this type of access - you can specify an offset so there is no need for any programmatic arithmetic:

e.g.

Code:

      L     #Pnt_QB3                    //Load command byte
      SLD   3
      TAR1                              //tranfer to output byte 
      L     #CmdByte
      T     QB [AR1,P#0.0]              //e.g if AR1=3.0
      T     QB [AR1,P#1.0]              //then this will access QB4

rikcando · Apr 22, 2012

I see the pionter to teh pionter to QB3 and understand how this would be helpful. I do need to look into AR1, AR2. I do not know what these address registers hold the addresses of.

rikcando · Apr 22, 2012

Ok, I missed the TAR1. You are putting the pointer into the address register.

rikcando · Apr 22, 2012

No, I do not see it.
First you get the pointer to my QB then mask out the high bytes.

Then you 'TAR2 ; //current instance address'.
How did the value get to AR2?

PeterW · Apr 22, 2012

AR2 always holds the address of the first data of the current FB.

rikcando · Apr 22, 2012

The pointer to QBx is masked out and added to the instance address. This gives us a relative address, no?

Then you open the instance DB.

How is this the pointer to QBx?
L D [AR1,P#2.0];

L D[AR2P#0.0] · Apr 22, 2012

A pointer consists of 6 bytes. The first two bytes contain the DB number (not applicable to a QB), the second 4 contain the area pointer which contains the value QB3 coded as 82000018H (82=Q area 18 = 3 SLD 3)

rikcando · Apr 22, 2012

A pointer consists of 6 bytes. The first two bytes contain the DB number (not applicable to a QB), the second 4 contain the area pointer which contains the value QB3 coded as 82000018H (82=Q area 18 = 3 SLD 3)

That cetainly helps make sense of what I am monitoring. It works but I cannot admit to understanding it. I just cannot see how the pointer to QBx got into [AR1,P#2.0].

L D[AR2P#0.0] · Apr 22, 2012

When FB4 is called, the editor detects that the input parameter is a pointer and generates code to transfer the db number and area pointer into the instance DB - that is how the numbers got there.

L D[AR2P#0.0] · Apr 22, 2012

This may be too much detail, but here is the call to FB4 with the behind the scenes code shown:
(P#Q3.0=82000018)

Code:

      Call
      BLD   3
      =     L     34.0
      CDB   
      OPN   DI     4
      TAR2  LD    30
[COLOR=Red]      L     0
      T     DIW    0
      L     P#Q 3.0
      T     DID    2[/COLOR]
      L     MB     4
      T     DIB    6
      LAR2  P#DBX 0.0
      UC    FB     4
      LAR2  LD    30
      CDB   
      BLD   4
      End Call

rikcando · Apr 22, 2012

L P##pCmdAddress; //pointer ro pointer
AD DW#16#FFFFF; //mask off area
This get the pointer to QB3, masking out the DB number by only looking at first 5 bytes. Stroed in ACCU1.

TAR2 ; //current instance address
Transfers address of first data point of FB into ACCU1. ACCU1 moves to ACCU2.

+D ; //add offset to give absolute address
Added the two together and is stored in ACCU1. ACCU2 remains unchanged.

LAR1 ; //use AR1 to read pointer to Q
Stores ACCU1 into AR1 (I see this now). ACCU1 and ACCU2 unchanged.
AR1 now holds a relative address to QB3?

L DINO; //get instance DB number
T #wDBNo;
OPN DB [#wDBNo]; //open instance DB as global DB
L D [AR1,P#2.0]; //finally get pointer to Q area
Gets the current instance DB number and opens it.
Why?

L D[AR2P#0.0] · Apr 22, 2012

rikcando said:
Gets the current instance DB number and opens it.
Why?

You have to open the DB before you can read from it.

L D[AR2P#0.0] · Apr 22, 2012

rikcando said:
LAR1 ; //use AR1 to read pointer to Q
Stores ACCU1 into AR1 (I see this now). ACCU1 and ACCU2 unchanged.
AR1 now holds a relative address to QB3?

AR1 contains the absolute address in the instance DB of the location containing QB3 in pointer format (word+dword = DB+area pointer)

L D[AR2P#0.0] · Apr 22, 2012

rikcando said:
L P##pCmdAddress; //pointer ro pointer
AD DW#16#FFFFF; //mask off area
This get the pointer to QB3, masking out the DB number by only looking at first 5 bytes. Stroed in ACCU1.

No, the area pointer is coded [xx][unusedbits][Address] where xx=area, the above code is masking off the area coding. Area pointers do not contain DB numbers.

L D[AR2P#0.0] · Apr 22, 2012

For reference, here is the pointer format:

Another Siemens Indirect Addressing Question

L D[AR2P#0.0]

Lifetime Supporting Member

rikcando

Member

rikcando

Member

rikcando

Member

PeterW

Member

rikcando

Member

L D[AR2P#0.0]

Lifetime Supporting Member

rikcando

Member

L D[AR2P#0.0]

Lifetime Supporting Member

L D[AR2P#0.0]

Lifetime Supporting Member

rikcando

Member

L D[AR2P#0.0]

Lifetime Supporting Member

L D[AR2P#0.0]

Lifetime Supporting Member

L D[AR2P#0.0]

Lifetime Supporting Member

L D[AR2P#0.0]

Lifetime Supporting Member

Similar Topics