Intel IA-32 Manuale Utente

4-32 Vol. 3A

Some of the system instructions (called “privileged instructions”) are protected from use by
application programs. The privileged instructions control system functions (such as the loading
of system registers). They can be executed only when the CPL is 0 (most privileged). If one of
these instructions is executed when the CPL is not 0, a general-protection exception (#GP) is
generated. The following system instructions are privileged instructions:

•

LGDT — Load GDT register.

•

LLDT — Load LDT register.

•

LTR — Load task register.

•

LIDT — Load IDT register.

•

MOV (control registers) — Load and store control registers.

•

LMSW — Load machine status word.

•

CLTS — Clear task-switched flag in register CR0.

•

MOV (debug registers) — Load and store debug registers.

•

INVD — Invalidate cache, without writeback.

•

WBINVD — Invalidate cache, with writeback.

•

INVLPG —Invalidate TLB entry.

•

HLT— Halt processor.

•

RDMSR — Read Model-Specific Registers.

•

WRMSR —Write Model-Specific Registers.

•

RDPMC — Read Performance-Monitoring Counter.

•

RDTSC — Read Time-Stamp Counter.

Some of the privileged instructions are available only in the more recent families of IA-32
processors (see Section 17.12., “New Instructions In the Pentium and Later IA-32 Processors”).

The PCE and TSD flags in register CR4 (bits 4 and 2, respectively) enable the RDPMC and
RDTSC instructions, respectively, to be executed at any CPL.

When operating in protected mode, the processor validates all pointers to enforce protection
between segments and maintain isolation between privilege levels. Pointer validation consists
of the following checks:

1.

Checking access rights to determine if the segment type is compatible with its use.

2.

Checking read/write rights.