pTAL Reference Manual (H06.08+)

Structures

HP pTAL Reference Manual—523746-006

9-9

Base Alignment

The base alignment of a structure is the alignment of the widest field in the structure.

The base alignment determines where the structure can be located in memory and be

well-aligned. A structure is well-aligned when the memory address at which it is located

is an integral multiple of its base alignment.

A structure is well-aligned if the address of the base of the structure in memory is a

multiple of its base alignment; otherwise, the structure is misaligned. If a structure is

misaligned, some or all of its fields will also be misaligned.

Structure Alignment Examples

The following examples illustrate how your structure data layout is affected by structure

alignment. Only SHARED8 structures are shown because SHARED2 structures are

not well-aligned. pTAL always generates conservative code for references to fields of a

SHARED2 structure that are more than 16 bits long.

Figure 9-1 on page 9-10 shows a structure, s1, that specifies

FIELDALIGN(SHARED8). Because the widest field in the structure, f, is a FIXED field,

the base alignment of s1 is 8. To be well-aligned, s1 must be allocated at a memory

address that is an integral multiple of eight. Filler is added as follows:

•

Before i32 so that i32 begins at an offset that is a multiple of four relative to the

beginning of the structure.

•

Before f so that f begins at an offset that is a multiple of eight relative to the

beginning of the structure.

•

At the end of the structure so that the total length of the structure is an integral

multiple of the widest field in the structure.

Table 9-3. Base Alignment and Field Alignment Relationships

Width of Widest Field in Structure FIELDALIGN(SHARED2) FIELDALIGN(SHARED8)

1 1 or 2* 1 or 2*

222

424

828

*Definition (inline) substructures have a base alignment of one. All other structures—definition structures,

referral structures, and referral substructures—have a base alignment of two.