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Trying to understand how storage is allocated with COMP-5

 
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misi01
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PostPosted: Wed Jun 02, 2021 4:21 am    Post subject: Trying to understand how storage is allocated with COMP-5 Reply with quote

I'm racking my brains trying to see some sort of logic to how storage is allocated to an OCCURS field containing COMP-5 fields.
Here's the compiler listing
Code:

    111       3  SOKTALAN2-NUM . . . . . . . . . . . . .   0000001B0   DS 1F             Binary                         
    113       3  SOKTALAN. . . . . . . . . . . . . . . . . 0000001B4   DS 0CL60          Group          O           <--- occurs 5       
    115         4  BELOPP-NUM. . . . . . . . . . . . . . . 0000001B4   DS 1F             Binary                         
    118         4  BELOPP. . . . . . . . . . . . . . . . . 0000001B8   DS 7C             Disp-Num                       
    120         4  DELPRODUKTKOD-NUM . . . . . . . . . .   0000001C0   DS 1F             Binary                         
    123         4  DELPRODUKTKOD . . . . . . . . . . . . . 0000001C4   DS 20C            Display                         
    125         4  SYFTE-NUM . . . . . . . . . . . . . . . 0000001D8   DS 1F             Binary                         
    128         4  SYFTE . . . . . . . . . . . . . . . . . 0000001DC   DS 3C             Disp-Num                       
    130         4  AMORTERINGSTID-NUM. . . . . . . . . . . 0000001E0   DS 1F             Binary                         
    133         4  AMORTERINGSTID. . . . . . . . . . . . . 0000001E4   DS 2C             Disp-Num                       
    135         4  RANTESATS-NUM . . . . . . . . . . . . . 0000001E8   DS 1F             Binary                         
    138         4  RANTESATS . . . . . . . . . . . . . . . 0000001EC   DS 3P             Packed-Dec                     
    139         4  FILLER. . . . . . . . . . . . . . . . . 0000001EF   DS 1C             Display                         
    141       3  BEFINTLIGALAN2-NUM. . . . . . . . . . . . 0000002E0   DS 1F             Binary                         
    143       3  BEFINTLIGALAN . . . . . . . . . . . . . . 0000002E4   DS 0CL160         Group          O   


I appreciate that every COMP-5 variable is placed on a full-word boundary, but this is how I'm analyzing it (incorrectly it would seem).
1B4 (hex) is 436 (dec) which is exactly divisible by 4, which means that SOKTALAN is already on a full-word boundary.
The total number of bytes in the group is 56 bytes which is also divisble by 4, so this means that EVERY row of the 5 occurs will start on a full-word boundary. So WHY is the length of each row defined as 60 bytes as it is ?
At the same time, I have another occurs defined as follows: -

Code:

     43       3  KAMPANJKODER2-NUM . . . . . . . . . . . .             0000000A8   DS 1F             Binary                         
     44       3  KAMPANJKODER. . . . . . . . . . . . . . .             0000000AC   DS 0CL12          Group          O               <---- occurs 5
     45         4  KOD-NUM . . . . . . . . . . . . . . . .             0000000AC   DS 1F             Binary                         
     47         4  KOD . . . . . . . . . . . . . . . . . .             0000000B0   DS 6C             Display                         
     48         4  FILLER. . . . . . . . . . . . . . . . .             0000000B6   DS 2C             Display                         
     51       3  SOKANDEN2-NUM . . . . . . . . . . . . . .             0000000E8   DS 1F             Binary             


In this example, KAMPANJKODER starts at offset AC (hex) or 172 (dec) which is ALSO exactly divisible by 4 which means that it TOO starts on a full-word boundary.
The length of each row here is 12, so again, all rows will start on a full-word boundary.
So why does SOKTALAN have an extra 4 bytes added to each row's length but not KAMPANJKODER?

BTW. We're on Cobol 6.3.0.
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Spolacek
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PostPosted: Wed Jun 02, 2021 1:57 pm    Post subject: Reply with quote

Hello Michael

The answer is slack bytes. The compiler is inserting slack bytes between some fields in order that the binary fields align on the proper boundary.

For example, field SYFTE is 3 character positions beginning at displacement 1DC. So it occupies bytes 1DC, 1DD, and 1DE. The next field is AMORTERINGSTID-NUM, which is a 4 byte binary field. Under the rules of boundary alignment, it must begin on a full-word boundary. However, the first byte available after field SYFTE is 1DF, which is not a full-word boundary. Therefore, the compiler inserted a slack byte at 1DF in order to cause AMORTERINGSTID-NUM to begin on a full-word boundary at 1E0.
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PostPosted: Wed Jun 02, 2021 2:58 pm    Post subject: Reply with quote

misi01,

As Spolacek pointed out it is the slack bytes. Your table definitions have probably SYNC coded.

Check this link which explain in detail about it. Read the section "Slack bytes within records"

https://www.ibm.com/docs/en/cobol-zos/6.3?topic=entry-synchronized-clause
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misi01
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PostPosted: Thu Jun 03, 2021 12:44 am    Post subject: Reply with quote

Thanks to both of you. I as aware of slack bytes, but it must have go to the stage where I couldn't see the wood for the trees.
Anyway, thought I'd include a "layout" of of the SOKTALAN layout so the slack bytes become obvious.
Code:

                                             Offset    Length   Start offset  End offset
                                             (hex)              (dec)         (dec)
3  SOKTALAN. . . . . . . . . . . . . . . . . 0000001B4 0CL60                 
  4  BELOPP-NUM. . . . . . . . . . . . . . . 0000001B4 1F       436           439
  4  BELOPP. . . . . . . . . . . . . . . . . 0000001B8 7C       440           446 
                                                                                   1 slack byte needed here 
  4  DELPRODUKTKOD-NUM . . . . . . . . . . . 0000001C0 1F       448           451         
  4  DELPRODUKTKOD . . . . . . . . . . . . . 0000001C4 20C      452           471         
  4  SYFTE-NUM . . . . . . . . . . . . . . . 0000001D8 1F       472           475         
  4  SYFTE . . . . . . . . . . . . . . . . . 0000001DC 3C       476           478         
                                                                                   1 slack byte needed here
  4  AMORTERINGSTID-NUM. . . . . . . . . . . 0000001E0 1F       480           483         
  4  AMORTERINGSTID. . . . . . . . . . . . . 0000001E4 2C       484           485 
                                                                                   2 slack bytes needed here   
  4  RANTESATS-NUM . . . . . . . . . . . . . 0000001E8 1F       488           491         
  4  RANTESATS . . . . . . . . . . . . . . . 0000001EC 3P       492           494         
  4  FILLER. . . . . . . . . . . . . . . . . 0000001EF 1C       495           495         


Now we can see that the "original" row length of 56 needs to be increased by 4 bytes making the length 60 bytes (which of course is exactly what is in the compiler listing).
Thanks again for forcing me to do my homework better.
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