How to get the relevant instruction code from the instruction byte?

Question content

When I run a Go program that contains some Go assembly code, I receive the following error message:

SIGILL: illegal instruction
PC=0x508c04 m=0 sigcode=2
instruction bytes: 0xc4 0x42 0x68 0xf2 0xea 0xc5 0xf9 0xef 0xc1 0x41 0x9 0xfd 0x45 0x1 0xee 0xc1

How to get the relevant instruction code from the instruction byte? I tried the https://github.com/golang/arch Decode method but I only got opcode 0.

Starting from the first byte 0xc4, it should be the VEX prefix.

CPU：Intel(R) Xeon(R) CPU E5-2620)

Operating system: Linux

Correct answer

This is ANDN, which is part of the BMI1 instruction set. On Intel, Haswell and newer support BMI1, but your chip is Sandy Bridge, which is a generation (or two, depending on how you count) too old.

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