Intel IA-32 Manuale Utente

Vol. 3A 5-57

Fault.

Indicates that the processor detected an unaligned memory operand when alignment checking
was enabled. Alignment checks are only carried out in data (or stack) accesses (not in code
fetches or system segment accesses). An example of an alignment-check violation is a word
stored at an odd byte address, or a doubleword stored at an address that is not an integer multiple
of 4. Table 5-7 lists the alignment requirements various data types recognized by the processor.

Note that the alignment check exception (#AC) is generated only for data types that must be
aligned on word, doubleword, and quadword boundaries. A general-protection exception (#GP)
is generated 128-bit data types that are not aligned on a 16-byte boundary.

To enable alignment checking, the following conditions must be true:

•

AM flag in CR0 register is set.

•

AC flag in the EFLAGS register is set.

•

The CPL is 3 (protected mode or virtual-8086 mode).

Word

2

Doubleword

4

Single-precision floating-point (32-bits)

4

Double-precision floating-point (64-bits)

8

Double extended-precision floating-point (80-bits)

8

Quadword

8

Double quadword

16

Segment Selector

2

32-bit Far Pointer

2

48-bit Far Pointer

4

32-bit Pointer

4

GDTR, IDTR, LDTR, or Task Register Contents

4

FSTENV/FLDENV Save Area

4 or 2, depending on operand size

FSAVE/FRSTOR Save Area

4 or 2, depending on operand size

Bit String

2 or 4 depending on the operand-size attribute.