Intel IA-32 - Manual (Page 583)

Vol. 3A 16-3
MIXING 16-BIT AND 32-BIT CODE
These prefixes reverse the default size selected by the D flag in the code-segment descriptor. For
example, the processor can interpret the (MOV mem, reg) instruction in any of four ways:
•
In a 32-bit code segment:
— Moves 32 bits from a 32-bit register to memory using a 32-bit effective address.
— If preceded by an operand-size prefix, moves 16 bits from a 16-bit register to memory
using a 32-bit effective address.
— If preceded by an address-size prefix, moves 32 bits from a 32-bit register to memory
using a 16-bit effective address.
— If preceded by both an address-size prefix and an operand-size prefix, moves 16 bits
from a 16-bit register to memory using a 16-bit effective address.
•
In a 16-bit code segment:
— Moves 16 bits from a 16-bit register to memory using a 16-bit effective address.
— If preceded by an operand-size prefix, moves 32 bits from a 32-bit register to memory
using a 16-bit effective address.
— If preceded by an address-size prefix, moves 16 bits from a 16-bit register to memory
using a 32-bit effective address.
— If preceded by both an address-size prefix and an operand-size prefix, moves 32 bits
from a 32-bit register to memory using a 32-bit effective address.
The previous examples show that any instruction can generate any combination of operand size
and address size regardless of whether the instruction is in a 16- or 32-bit segment. The choice
of the 16- or 32-bit default for a code segment is normally based on the following criteria:
•
Performance — Always use 32-bit code segments when possible. They run much faster
than 16-bit code segments on P6 family processors, and somewhat faster on earlier IA-32
processors.
•
The operating system the code segment will be running on — If the operating system is
a 16-bit operating system, it may not support 32-bit program modules.
•
Mode of operation — If the code segment is being designed to run in real-address mode,
virtual-8086 mode, or SMM, it must be a 16-bit code segment.
•
Backward compatibility to earlier IA-32 processors — If a code segment must be able
to run on an Intel 8086 or Intel 286 processor, it must be a 16-bit code segment.
16.3 SHARING DATA AMONG MIXED-SIZE CODE SEGMENTS
Data segments can be accessed from both 16-bit and 32-bit code segments. When a data segment
that is larger than 64 KBytes is to be shared among 16- and 32-bit code segments, the data that
is to be accessed from the 16-bit code segments must be located within the first 64 KBytes of
the data segment. The reason for this is that 16-bit pointers by definition can only point to the
first 64 KBytes of a segment.