Defined illegal instruction

[Andrew Waterman]

If memory serves, the reason we didn't define unimp to be 0x00000000
is that 0x0000 is a 16-bit-long instruction (because the two LSBs are
zero).  When targeting RISC-V variants that lack support for 16-bit
instructions (via the C extension), the inclusion of a 16-bit-long
instruction (or, I suppose, a pair of 16-bit-long instructions) can
confuse both humans and disassemblers.  unimp does map to 0x0000 when
the C extension is provided; it only maps to the
illegal-write-of-read-only-CSR when the C extension is not provided.

On Fri, Feb 9, 2024 at 9:48 AM Peter Bergner <bergner@linux.ibm.com> wrote:
>
> On 2/9/24 11:03 AM, jacob navia wrote:
> > The riscv processor defines an illegal instruction (all zeroes). I do not
> > find the mnemonic used by gas for this. As far as I remember, the x86 also
> > has a defined illegal instruction.
>
> I can't speak for riscv or x86, but Power also defines a 32-bit all zero
> instruction as an illegal instruction.  We do not have a mnemonic for it
> though.  When we want to emit that into the instruction stream, we just
> emit a ".long 0" assembler directive.  Maybe the other architectures do
> the same thing?
>
> Peter
>