4-2 Vol. 1

DATA TYPES

4.1.1 

Alignment of Words, Doublewords, Quadwords, and Double Quadwords

Words, doublewords, and quadwords do not need to be aligned in memory on natural boundaries. The natural 
boundaries for words, double words, and quadwords are even-numbered addresses, addresses evenly divisible by 
four, and addresses evenly divisible by eight, respectively. However, to improve the performance of programs, data 
structures (especially stacks) should be aligned on natural boundaries whenever possible. The reason for this is 
that the processor requires two memory accesses to make an unaligned memory access; aligned accesses require 
only one memory access. A word or doubleword operand that crosses a 4-byte boundary or a quadword operand 
that crosses an 8-byte boundary is considered unaligned and requires two separate memory bus cycles for access.
Some instructions that operate on double quadwords require memory operands to be aligned on a natural 
boundary. These instructions generate a general-protection exception (#GP) if an unaligned operand is specified. 
A natural boundary for a double quadword is any address evenly divisible by 16. Other instructions that operate on 
double quadwords permit unaligned access (without generating a general-protection exception). However, addi-
tional memory bus cycles are required to access unaligned data from memory.

4.2 NUMERIC 

DATA 

TYPES

Although bytes, words, and doublewords are fundamental data types, some instructions support additional inter-
pretations of these data types to allow operations to be performed on numeric data types (signed and unsigned 
integers, and floating-point numbers). Single-precision (32-bit) floating-point and double-precision (64-bit) 
floating-point data types are supported across all generations of SSE extensions and Intel AVX extensions. Half-
precision (16-bit) floating-point data type is supported only with F16C extensions (VCVTPH2PS, VCVTPS2PH). See 
Figure 4-3. 

Figure 4-2.  Bytes, Words, Doublewords, Quadwords, and Double Quadwords in Memory

EH

DH

7AH

CH

FEH

BH

06H

AH

36H

9H

1FH

8H

A4H

7H

23H

6H

0BH

5H

4H
3H

74H

2H

CBH

1H

31H

0H

Quadword at Address 6H

Contains 

Doubleword at Address AH

Contains 7AFE0636H

Word at Address BH

Contains FE06H

Byte at Address 9H

Contains 1FH

Word at Address 6H

Contains 230BH

Word at Address 1H

Contains CB31H

Word at Address 2H

Contains 74CBH

Double quadword at Address 0H

45H
67H

12H

Contains

12H

7AFE06361FA4230BH

4E127AFE06361FA4230B456774CB3112

4EH

FH