Aligning Data Structure Subfields
Alignment of subfields may be necessary. In some cases it is done automatically; in others, it must be done manually.
For example, when defining subfields of type basing pointer or procedure pointer using the length notation, the compiler will automatically perform padding if necessary to ensure that the subfield is aligned properly.
When defining float, integer or unsigned subfields, alignment may
be desired to improve run-time performance. If the subfields are defined
using length notation, you can automatically align float, integer
or unsigned subfields by specifying the keyword ALIGN on the data
structure definition. However, note the following exceptions:
- The ALIGN keyword is not allowed for a file information data structure or a program status data structure.
- Subfields defined using the keyword OVERLAY are not aligned automatically, even if the keyword ALIGN is specified for the data structure. In this case, you must align the subfields manually.
Automatic alignment will align the fields on the following boundaries.
- 2 bytes for 5-digit integer or unsigned subfields
- 4 bytes for 10-digit integer or unsigned subfields or 4-byte float subfields
- 8 bytes for 20-digit integer or unsigned subfields
- 8 bytes for 8-byte float subfields
- 16 bytes for pointer subfields
If you are aligning fields manually, make sure that they are aligned
on the same boundaries. A start-position is on an n-byte
boundary if ((position - 1) mod n) = 0. (The
value of "x mod y" is the remainder after dividing x by y in integer
arithmetic. It is the same as the MVR value after X DIV Y.)
Figure 1 shows a sequence of bytes and identifies the different boundaries used for alignment.
Note the following about the above byte sequence:
- Position 1 is on a 16-byte boundary, since ((1-1) mod 16) = 0.
- Position 13 is on a 4-byte boundary, since ((13-1) mod 4) = 0.
- Position 7 is not on a 4-byte boundary, since ((7-1) mod 4) = 2.