MIPS-X, while designed by the same team and architecturally very similar, is not instruction-set compatible with the mainline MIPS R-series processors. The processor is obscure enough that (as of this writing) support for it is provided only by specialist developers (such as Green Hills Software), and is notably missing from GCC.
MIPS-X has become important among DVD player firmware hackers, since many low-end DVD players use chips based on the IIT design (and produced by ESS Technology Inc.) as their central processor and MPEG-2 decoder.
The Programmer's Manual described an instruction called hsc, which means halt and spontaneously combust. The manual claimed that on the NSA versions of the processor, this instruction was executed when a protection violation was detected. On other platforms, this type of instruction is known as Halt and Catch Fire.