Attribute 'pack_mode'

The 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 whole structure.

Syntax: {attribute 'pack_mode' := '<pack mode value>'}

Possible values for <pack mode value>:

<pack mode value>	Associated package type	Description
0	aligned	All variables are assigned to byte addresses; no memory gaps occur.
1	1-byte aligned
2	2-byte aligned	There are 1-byte variables assigned to byte addresses 2-byte variables assigned to addresses that are divisible by 2. The maximum gap that may occur is 1 byte 4-byte variables assigned to addresses that are divisible by 2. The maximum gap that may occur is 1 byte 8-byte variables assigned to addresses that are divisible by 2. The maximum gap that may occur is 1 byte Strings always at byte addresses. No gap is created.
4	4-byte aligned	There are 1-byte variables assigned to byte addresses 2-byte variables assigned to addresses that are divisible by 2. The maximum gap that may occur is 1 byte 4-byte variables assigned to addresses that are divisible by 4. The maximum gap that may occur is 3 byte 8-byte variables assigned to addresses that are divisible by 4. The maximum gap that may occur is 3 byte Strings always at byte addresses. No gap is created.
8	8-byte aligned	There are 1-byte variables assigned to byte addresses 2-byte variables assigned to addresses that are divisible by 2. The maximum gap that may occur is 1 byte 4-byte variables assigned to addresses that are divisible by 4. The maximum gap that may occur is 3 byte 8-byte variables assigned to addresses that are divisible by 8. The maximum gap that may occur is 7 byte Strings always at byte addresses. No gap is created.

Insertion location: Line above the declaration of the data structure

	Depending on the configuration of the structure, there may be no difference in the memory split between the individual modes. In this case the memory distribution of a structure with pack_mode = 4 can correspond to that of pack_mode = 8.

	Arrays of structures: If the structures are grouped into arrays, bytes are inserted at the end of the structure so that the next structure is aligned again.

Example 1

{attribute 'pack_mode' := '1'} 
TYPE ST_MyStruct:  
STRUCT 
    bEnable         : BOOL;
    nCounter        : INT; 
    nMaxSize        : INT;
    bMaxSizeReached : BOOL;
    bReset          : BOOL;
END_STRUCT
END_TYPE

The memory area for a variable of data type ST_MyStruct is allocated "aligned: If the memory address of your component bEnable is 0x0100 for example, then the component nCounter follows at address 0x0101, nMaxSize at address 0x0103, bMaxSizeReached at address 0x0105 and bReset at address 0x0106. If 'pack_mode' := '2', then nCounter would be at 0x0102, nMaxSize at 0x0104, bMaxSizeReached at 0x0106 and bReset at 0x0107.

Example 2

STRUCT 
    bVar1 : BOOL  := 16#01;
    nVar2 : BYTE  := 16#11;
    nVar3 : WORD  := 16#22;
    nVar4 : BYTE  := 16#44;
    nVar5 : DWORD := 16#88776655;
    nVar6 : BYTE  := 16#99;
    nVar7 : BYTE  := 16#AA;
    nVar8 : 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 
    nVar1 : BYTE  := 16#01;
    nVar2 : LWORD := 16#11;
    nVar3 : BYTE  := 16#22;
    nVar4 : BYTE  := 16#44;
    nVar5 : DWORD := 16#88776655;
    nVar6 : BYTE  := 16#99;
    nVar7 : BYTE  := 16#AA;
    nVar8 : 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