Instruction Set (Continued)
Instruction Issues for Instruction Set Summary
k. JMP, CALL, INT, RET, and IRET instructions referring to
another code segment will cause an exception 13, if an appli-
cable privilege rule is violated.
Issues a through c apply to real address mode only:
a. This is a protected mode instruction. Attempted execution in
real mode will result in exception 6 (invalid opcode).
l. An exception 13 fault occurs if CPL is greater than 0 (0 is the
most privileged level).
b. Exception 13 fault (general protection) will occur in real mode
if an operand reference is made that partially or fully extends
beyond the maximum CS, DS, ES, FS, or GS segment limit
(FFFFH). Exception 12 fault (stack segment limit violation or
not present) will occur in real mode if an operand reference is
made that partially or fully extends beyond the maximum SS
limit.
m. An exception 13 fault occurs if CPL is greater than IOPL.
n. The IF bit of the Flags register is not updated if CPL is greater
than IOPL. The IOPL and VM fields of the Flags register are
updated only if CPL = 0.
o. The PE bit of the MSW (CR0) cannot be reset by this instruc-
tion. Use MOV into CR0 if desiring to reset the PE bit.
c. This instruction may be executed in real mode. In real mode,
its purpose is primarily to initialize the CPU for protected
mode.
p. Any violation of privilege rules as apply to the selector oper-
and does not cause a Protection exception, rather, the zero
flag is cleared.
d.
-
q. If the processor’s memory operand violates a segment limit or
segment access rights, an exception 13 fault will occur before
the ESC instruction is executed. An exception 12 fault will
occur if the stack limit is violated by the operand’s starting
address.
Issues e through g apply to real address mode and protected
virtual address mode:
e. An exception may occur, depending on the value of the oper-
and.
f. LOCK# is automatically asserted, regardless of the presence
or absence of the LOCK prefix.
r. The destination of a JMP, CALL, INT, RET, or IRET must be in
the defined limit of a code segment or an exception 13 fault
will occur.
g. LOCK# is asserted during descriptor table accesses.
Issues h through r apply to protected virtual address mode
only:
Issue s applies to National Semiconductor-specific SMM in-
structions:
h. Exception 13 fault will occur if the memory operand in CS,
DS, ES, FS, or GS cannot be used due to either a segment
limit violation or an access rights violation. If a stack limit is
violated, an exception 12 occurs.
s. All memory accesses to SMM space are non-cacheable. An
invalid opcode exception 6 occurs unless SMI is enabled and
SMAR size > 0, and CPL = 0 and [SMAC is set or if in an SMI
handler].
i. For segment load operations, the CPL, RPL, and DPL must
agree with the privilege rules to avoid an exception 13 fault.
The segment’s descriptor must indicate “present” or exception
11 (CS, DS, ES, FS, GS not present). If the SS register is
loaded and a stack segment not present is detected, an
exception 12 occurs.
Issue t applies to cache invalidation instruction with the
cache operating in write-back mode:
t. The total clock count is the clock count shown plus the num-
ber of clocks required to write all “modified” cache lines to
external memory.
j. All segment descriptor accesses in the GDT or LDT made by
this instruction will automatically assert LOCK# to maintain
descriptor integrity in multiprocessor systems.
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