Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 4-3
DATA TYPES
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, additional 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 instruc-
tions support additional interpretations of these data types to allow operations to be
performed on numeric data types (signed and unsigned integers, and floating-point
numbers). See Figure 4-3.