Summary

This fixes an issue in the FE opcode (referred in Intel docs as "INC/DEC Grp 4") where any ModR/M Opcode bits other than 000 act as DEC r/m8. All other values than 000 or 001 are invalid and should throw #UD on processors that support that.

This also shows on actual hardware: similarly-configured Cyrix 5x86 and 486DX systems throw a #UD exception, but not in 86Box. (Thanks to @goshhhy for testing this!)

Checklist

• Closes #xxx
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/
• This pull request requires changes to the asset set
  • I have opened an assets pull request - https://github.com/86Box/assets/pull/changeme/

References

Intel® 64 and IA-32 Architectures Software Developer's Manual Combined Volumes 2A, 2B, 2C, and 2D, Appendix A.4.2.

Tested using the boot sector program here: https://datagirl.xyz/pub/grp4test.bin (source code: https://datagirl.xyz/pub/grp4test.asm)