LLDT instruction (System)

Forms

Every encoding of LLDT that Intel XED knows, one row per XED instruction form (iform). How to read the notation.

Instruction	Encoding	Requires
`LLDT m16` LLDT_MEMw	`0F 00 /2`	Base instruction set
`LLDT r16` LLDT_GPR16	`0F 00 /2`	Base instruction set

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

if CPL > 0:
    fault(GP, 0)
s = src.read()                   # 16 bits at any operand size
if s & ~3 == 0:
    ldtr = s                     # null: no LDT, so that a later use of
                                 # one raises #GP
else:
    if s & 4:
        fault(GP, s & ~3)        # TI set: not a selector of the GDT
    d = descriptor(s)            # #GP(sel) past the GDT's limit
    h = descriptor(s + 8)        # the second 8 bytes
    if d >> 40 & 0x1F != 2:
        fault(GP, s & ~3)        # S clear and type 2: an LDT
    if h >> 40 & 0x1F:
        fault(GP, s & ~3)
    b = d >> 16 & 0xFFFFFF | d >> 56 << 24
    b |= (h & 0xFFFFFFFF) << 32  # bits 63:32 of the base
    if not canonical(b):
        fault(GP, s & ~3)
    if not d >> 47 & 1:
        fault(NP, s & ~3)
    ldtr = s                     # with the LDT's base and limit
# the LDTR field of the TSS does not change

Run on AMD Zen 5 (EPYC 9645): 4 cases. Every one raised an exception, at CPL 3, where the probe runs.

Operands

What each form reads and writes, as Intel XED records it: the operands of its syntax, in order, and the implicit ones that the syntax does not name. How to read.

Operand	Kind	Action
`LLDT r/m16`
`r/m16`	explicit	read
`LDTR`	implicit	write

Flags

None: XED records no flag that the forms test or change.

Attributes

What else XED records about the forms. How to read.

All forms: privileged: CPL 0 only; not in real-address mode; may abort a TSX transaction

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, whose AMD processors end with Zen 2, and the CPU data of this site for Zen 3 to Zen 6.

ISA set	CPUID	Processors
`I286PROTECTED` System	No CPUID bit: part of the base instruction set.	Every x86-64 processor in XED’s model and Zen 3 to Zen 6

LLDT