Intel ARCHITECTURE IA-32 User Manual download pdf (Page 582)

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16-2 Vol. 3A

MIXING 16-BIT AND 32-BIT CODE

16.1 DEFINING 16-BIT AND 32-BIT PROGRAM MODULES

The following IA-32 architecture mechanisms are used to distinguish between and support

16-bit and 32-bit segments and operations:

• The D (default operand and address size) flag in code-segment descriptors.

• The B (default stack size) flag in stack-segment descriptors.

• 16-bit and 32-bit call gates, interrupt gates, and trap gates.

• Operand-size and address-size instruction prefixes.

• 16-bit and 32-bit general-purpose registers.

The D flag in a code-segment descriptor determines the default operand-size and address-size

for the instructions of a code segment. (In real-address mode and virtual-8086 mode, which do

not use segment descriptors, the default is 16 bits.) A code segment with its D flag set is a 32-bit

segment; a code segment with its D flag clear is a 16-bit segment.

The B flag in the stack-segment descriptor specifies the size of stack pointer (the 32-bit ESP

for all data descriptors also controls upper address range for expand down segments.

When transferring program control to another code segment through a call gate, interrupt gate,

or trap gate, the operand size used during the transfer is determined by the type of gate used

(16-bit or 32-bit), (not by the D-flag or prefix of the transfer instruction). The gate type deter-

mines how return information is saved on the stack (or stacks).

For most efficient and trouble-free operation of the processor, 32-bit programs or tasks should

have the D flag in the code-segment descriptor and the B flag in the stack-segment descriptor

set, and 16-bit programs or tasks should have these flags clear. Program control transfers from

16-bit segments to 32-bit segments (and vice versa) are handled most efficiently through call,

interrupt, or trap gates.

Instruction prefixes can be used to override the default operand size and address size of a code

segment. These prefixes can be used in real-address mode as well as in protected mode and

virtual-8086 mode. An operand-size or address-size prefix only changes the size for the duration

of the instruction.

16.2 MIXING 16-BIT AND 32-BIT OPERATIONS WITHIN A CODE

SEGMENT

The following two instruction prefixes allow mixing of 32-bit and 16-bit operations within one

segment:

• The operand-size prefix (66H)

• The address-size prefix (67H)

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