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Managing Memory and Data Access
16.4 Using Alignment Directives, Qualifiers, and Flags
Byte Layout for Example 16–5:
|REC1 |
|FLD1 |FLD2 |FLD3 |
|FLD1-1 |FLD1-2 | |*| |FLD3-2 |FLD3-3 |
| | | | |f f f| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 5 12 17 21 25
Begin byte number (starting with 0)
Record length is 29 bytes.
Notes:
The asterisk (*) designates FLD3-1.
The letter f designates internal filler bytes produced because the field
FLD3-2 has to start on the boundary that is the next higher multiple of
4 because it is longword-aligned. The intervening three bytes following
FLD3-1 are skipped over.
Example 16–6 Data Map for /ALIGNMENT=PADDING, -align pad
Source Listing
Data Names in Declared Order
Line Level Name Location Size Bytes Usage Category
---- ----- ------- ------------ ----- ----- -------- --------
7 01 REC1 2 00000000 36 36 DISPLAY Group
8 02 FLD1 2 00000000 12 12 DISPLAY Group
9 03 FLD1-1 2 00000000 9 4 COMP N
10 03 FLD1-2 2 00000004 7 7 DISPLAY N
11 02 FLD2 2 0000000C 5 5 DISPLAY AN
12 02 FLD3 2 00000014 16 16 DISPLAY Group
13 03 FLD3-1 2 00000014 1 1 DISPLAY AN
14 03 FLD3-2 2 00000018 9 4 COMP N
15 03 FLD3-3 2 0000001C 5 5 DISPLAY N
Byte Layout for Example 16–6:
|REC1
|FLD1 |FLD2 |FLD3 |
|FLD1-1 |FLD1-2 (1)| | (2) |*| |FLD3-2 |FLD3-3 | (3) |
| | |p| |p p p| |f f f| | |p p p|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 5 13 18 21 25 29 34 36
Begin byte number (starting with 0)
Record length is 36 bytes.
1. This pad byte brings substructure FLD1 up to:
12 = 3 * 4 bytes - multiple of longword alignment
2. These three pad bytes are skipped over. They are required because FLD3, the
next substructure, has to start on a longword boundary because it contains
FLD3-2.
FLD2 contains five bytes and is padded three bytes out to eight.
16–12 Managing Memory and Data Access