Intel IA-32 - Manual (Page 628)

17-36 Vol. 3A
IA-32 ARCHITECTURE COMPATIBILITY
The 32-bit processors also have descriptors for TSS segments, call gates, interrupt gates, and
trap gates that support the 32-bit architecture. Both kinds of descriptors can be used in the same
system.
For those segment descriptors common to both 16- and 32-bit processors, clear bits in the
reserved word cause the 32-bit processors to interpret these descriptors exactly as an Intel 286
processor does, that is:
•
Base Address — The upper 8 bits of the 32-bit base address are clear, which limits base
addresses to 24 bits.
•
Limit — The upper 4 bits of the limit field are clear, restricting the value of the limit field
to 64 KBytes.
•
Granularity bit — The G (granularity) flag is clear, indicating the value of the 16-bit limit
is interpreted in units of 1 byte.
•
Big bit — In a data-segment descriptor, the B flag is clear in the segment descriptor used
by the 32-bit processors, indicating the segment is no larger than 64 KBytes.
•
Default bit — In a code-segment descriptor, the D flag is clear, indicating 16-bit addressing
and operands are the default. In a stack-segment descriptor, the D flag is clear, indicating
use of the SP register (instead of the ESP register) and a 64-KByte maximum segment
limit.
For information on mixing 16- and 32-bit code in applications, see Chapter 16, “Mixing 16-Bit
and 32-Bit Code.”
17.32. SEGMENT AND ADDRESS WRAPAROUND
This section discusses differences in segment and address wraparound between the P6 family,
Pentium, Intel486, Intel386, Intel 286, and 8086 processors.
17.32.1 Segment Wraparound
On the 8086 processor, an attempt to access a memory operand that crosses offset 65,535 or
0FFFFH or offset 0 (for example, moving a word to offset 65,535 or pushing a word when the
stack pointer is set to 1) causes the offset to wrap around modulo 65,536 or 010000H. With the
Intel 286 processor, any base and offset combination that addresses beyond 16 MBytes wraps
around to the 1 MByte of the address space. The P6 family, Pentium, Intel486, and Intel386
processors in real-address mode generate an exception in these cases:
•
A general-protection exception (#GP) if the segment is a data segment (that is, if the CS,
DS, ES, FS, or GS register is being used to address the segment).
•
A stack-fault exception (#SS) if the segment is a stack segment (that is, if the SS register is
being used).