Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1670

SLDT—Store Local Descriptor Table Register (Continued)
Real Address Mode Exceptions
#UD
Virtual 8086 Mode Exceptions
#UD
4:368
The SLDT instruction is not recognized in real address mode.
The SLDT instruction is not recognized in virtual 8086 mode.
Volume 4: Base IA-32 Instruction Reference