[muricula]

Processors have "traps" or "exceptions" which work kind of like interrupts. They are utterly unlike exceptions in say C++. They transfer control to another location you specify in an interrupt table. The most well known of these is the "page fault" which occurs when you write/read/fetch from memory you're not allowed to.

Say you have a userland process which dereferences a NULL pointer in C. In the CPUs page table the NULL virtual address is not mapped to a physical address, so a page fault occurs. Control transfers to the OS page fault handler which then delivers the SIGSEGV signal to the process.

There's also a division by zero exception, and several more. You can read more about them here: http://wiki.osdev.org/Exceptions

[SilasX]

That much makes sense, but the original framing of the opcode as illegal still comes off as a category error to me (though perhaps that phrasing is common and excepted). The opcode itself isn't illegal, as opcodes are a CPU-level concept, where nothing is illegal.

Rather, the opcode may bring the memory state to something that might violate some OS's security model. But the opcode still does something to the CPU (Boolean) circuit state.

[sand500]

You could think of it as a switch case statement where the valid opcodes are the cases and there is a default that catches all non defined opcodes and throws an error. Its illegal in the sense you should never expect to run an instruction with those opcodes.