11.2.IDENTIFICATION OF EARLIER INTEL ARCHITECTURE
PROCESSORS
The CPUID instruction is only available in the Pentium
®
Pro, Pentium
®
, and recent Intel486
processors. For the earlier IA processors (including the earlier Intel486 processors), several
other architectural features can be exploited to identify the processor.
The settings of bits 12 and 13 (IOPL), 14 (NT), and 15 (reserved) in the EFLAGS register (refer
to Figure 3-7, Section 3.6.3., EFLAGS Register, in Chapter 3, Basic Execution Environment)
is different for Intels 32-bit processors than for the Intel 8086 and Intel 286 processors. By
examining the settings of these bits (with the PUSHF/PUSHFD and POP/POPFD instructions),
an application program can determine whether the processor is an 8086, Intel286, or one of the
Intel 32-bit processors:
8086 processor Bits 12 through 15 of the EFLAGS register are always set.
Intel 286 processor Bits 12 through 15 are always clear in real-address mode.
32-bit processors In real-address mode, bit 15 is always clear and bits 12 through 14
have the last value loaded into them. In protected mode, bit 15 is always clear, bit 14 has
the last value loaded into it, and the IOPL bits depends on the current privilege level
(CPL). The IOPL field can be changed only if the CPL is 0.
Other EFLAG register bits that can be used to differentiate between the 32-bit processors:
Bit 18 (AC) Implemented only on the Pentium
®
Pro, Pentium
®
, and Intel486
processors. The inability to set or clear this bit distinguishes an Intel386 processor from
the other Intel 32-bit processors.
Bit 21 (ID) Determines if the processor is able to execute the CPUID instruction. The
ability to set and clear this bit indicates that the processor is a Pentium
®
Pro, Pentium
®
, or
later version Intel486 processor.
To determine whether an FPU or NPX is present in a system, applications can write to the
FPU/NPX status and control registers using the FNINIT instruction and then verify the correct
values are read back using the FNSTENV instruction.
After determining that an FPU or NPX is present, its type can then be determined. In most cases,
the processor type will determine the type of FPU or NPX; however, an Intel386 processor is
compatible with either an Intel 287 or Intel 387 math coprocessor. The method the coprocessor
uses to represent
?
(after the execution of the FINIT, FNINIT, or RESET instruction) indicates
which coprocessor is present. The Intel 287 math coprocessor uses the same bit representation
for +
?
and
??
; whereas, the Intel 387 math coprocessor uses different representations for +
?
and
??
.
