Attribute ‘pack_mode’

This pragma defines how a data structure is packed during the allocation. The attribute must be inserted above the data structure and affects the packing of the entire structure.

Syntax:

{attribute 'pack_mode' := '<Value>'}

Insert location: above the declaration of the data structure

Possible values for <Value>:
pack_mode Associated packing method Description
0 aligned, All variables are allocated to byte addresses. There are no memory gaps.
1 1-byte-aligned
2 2-byte-aligned

There are

  • 1-byte variables at byte addresses
  • 2 byte variables at addresses divisible by 2. A maximum gap of 1 byte results.
  • 4 byte variables at addresses divisible by 2. A maximum gap of 1 byte results.
  • 8 byte variables at addresses divisible by 2. A maximum gap of 1 byte results.
  • Strings are always at byte addresses. No gaps result.
4 4-byte-aligned

There are

  • 1-byte variables at byte addresses
  • 2 byte variables at even addresses. A maximum gap of 1 byte results.
  • 4 byte variables at addresses divisible by 4. A maximum gap of 3 byte results.
  • 8 byte variables at addresses divisible by 4. A maximum gap of 3 byte results.
  • Strings are always at byte addresses. No gaps result.
8 8-byte-aligned

There are

  • 1-byte variables at byte addresses
  • 2 byte variables at addresses divisible by 2. A maximum gap of 1 byte results.
  • 4 byte variables at addresses divisible by 4. A maximum gap of 3 byte results.
  • 8 byte variables at addresses divisible by 8. A maximum gap of 7 byte results.
  • Strings are always at byte addresses. No gaps result.

Note

Depending on the structure, there may be no difference in the memory mapping of the individual modes. Therefore, the memory allocation of a structure with pack_mode =4 can correspond to that of pack_mode = 8.

Note

Arrays of structures: If the structures are combined in arrays, then bytes are added at the end of the structure so that the next structure is aligned.

Example 1

Example

{attribute 'pack_mode' := '1'}

TYPE myStruct:
STRUCT
  Enable: BOOL;
  Counter: INT;
  MaxSize: BOOL;
  MaxSizeReached: BOOL;
  END_STRUCT
END_TYPE

The memory range for a variable of the data type myStruct is allocated ‘aligned’: If the storage address of its component Enable is 0x0100, for example, then the component Counter follows at the address 0x0101, MaxSize at address 0x0103 and MaxSizeReached at address 0x0104. In the case of 'pack_mode':=2, Counter would be at 0x0102, MaxSize at 0x0104 and MaxSizeReached at 0x0106.

Example 2

STRUCT
  Var1 : BOOL  := 16#01;
  Var2 : BYTE  := 16#11;
  Var3 : WORD  := 16#22;
  Var4 : BYTE  := 16#44;
  Var5 : DWORD := 16#88776655;
  Var6 : BYTE  := 16#99;
  Var7 : BYTE  := 16#AA;
  Var8 : DWORD := 16#AA;
END_TYPE
  pack_mode = 0 pack_mode = 1 pack_mode = 2 pack_mode = 4 pack_mode = 8
  Variable Value Variable Value Variable Value Variable Value Variable Value
0 Var1 01 Var1 01 Var1 01 Var1 01 Var1 01
1 Var2 11 Var2 11 Var2 11 Var2 11 Var2 11
2 Var3 22 Var3 22 Var3 22 Var3 22 Var3 22
3 … 00 … 00 … 00 … 00 … 00
4 Var4 44 Var4 44 Var4 44 Var4 44 Var4 44
5 Var5 55 Var5 55            
6 … 66 … 66 Var5 55        
7 … 77 … 77 … 66        
8 … 88 … 88 … 77 Var5 55 Var5 55
9 Var6 99 Var6 99 … 88 … 66 … 66
10 Var7 AA Var7 AA Var6 99 … 77 … 77
11 Var8 AA Var8 AA Var7 AA … 88 … 88
12 … 00 … 00 Var8 AA Var6 99 Var6 99
13 … 00 … 00 … 00 Var7 AA Var7 AA
14 … 00 … 00 … 00        
15         … 00        
16             Var8 AA Var8 AA
17             … 00 … 00
18             … 00 … 00
19             … 00 … 00
20                    
21                    
22                    
23                    
24                    
25                    
26                    
27                    
28                    
29                    
30                    
31                    

Example 3

STRUCT
  Var1 : BYTE  := 16#01;
  Var2 : LWORD := 16#11;
  Var3 : BYTE  := 16#22;
  Var4 : BYTE  := 16#44;
  Var5 : DWORD := 16#88776655;
  Var6 : BYTE  := 16#99;
  Var7 : BYTE  := 16#AA;
  Var8 : WORD  := 16#AA;
END_TYPE
  pack_mode = 0 pack_mode = 1 pack_mode = 2 pack_mode = 4 pack_mode = 8
  Variable Value Variable Value Variable Value Variable Value Variable Value
0 Var1 01 Var1 01 Var1 01 Var1 01 Var1 01
1 Var2 11 Var2 11            
2 … 00 … 00 Var2 11        
3 … 00 … 00 … 00        
4 … 00 … 00 … 00 Var2 11    
5 … 00 … 00 … 00 … 00    
6 … 00 … 00 … 00 … 00    
7 … 00 … 00 … 00 … 00    
8 … 00 … 00 … 00 … 00 Var2 11
9 Var3 22 Var3 22 … 00 … 00 … 00
10 Var4 44 Var4 44 Var3 22 … 00 … 00
11 Var5 55 Var5 55 Var4 44 … 00 … 00
12 … 66 … 66 Var5 55 Var3 22 … 00
13 … 77 … 77 … 66 Var4 44 … 00
14 … 88 … 88 … 77     … 00
15 Var6 99 Var6 99 … 88     … 00
16 Var7 AA Var7 AA Var6 99 Var5 55 Var3 22
17 Var8 AA Var8 AA Var7 AA … 66 Var4 44
18 … 00 … 00 Var8 AA … 77    
19         … 00 … 88    
20             Var6 99 Var5 55
21             Var7 AA … 66
22             Var8 AA … 77
23             … 00 … 88
24                 Var6 99
25                 Var7 AA
26                 Var8 AA
27                 … 00
28                    
29                    
30                    
31