Hmmm.... Just discovered something weird trying to boot Windows NT 4.0 to verify CPU behviour to be working properly still.

Somehow it reaches what seems to be opcode 84h (TEST r/m8,r8), but fails to fetch the modr/m parameters for some weird reason?
Looking at the fetching precalcs, it looks like the settings aren't properly retrieved from the instruction parameter lookup table, if at all (perhaps it points to another opcode, so never fetched properly)?
Edit: Hmmm... Now after some more testing (Windows 95 again), I see it failing on a opcode seemingly page faulting and failing somehow (proceeding to execute an invalid instruction).
Edit: After improving that, now Windows 95 fails to boot, telling there's not enough memory to load the registry?

Modified immediate parameter reading faulting/aborting(including paging locks) to check for active instruction by checking the parameter passed to the immediate read function(indicating checking(0)/loading from instruction(1) or main fetching routine(3, for 286+ CPUs only)) instead of directly checking the CPU's fetching/EU execution status (which is reset on any fault or execution phase change).

Also fixed resetting EIP for faults not to manually just load the CS selector, to keep CS selector/descriptor relationship consistent.

Now Hiren's BootCD 7.8 changes once again, now trying to execute an invalid LXS (LDS,LES etc.) instruction with register parameter, in real mode?

Just improved two things on the x86 segment descriptor handling:
- Make RSM properly set the CS segment descriptor mode to real or protected-mode/V86 mode on Pentium and up (applying the CS real mode limit behaviour on Pentium properly).
- Make segment descriptor read/write/execute permission precalcs point directly into the global precalcs lookup table, instead of copying all 256 bytes over, for increased performance (as the data in the table never changes), thus reducing the 256-byte memcpy to a simple 64-bit or 32-bit assignment (depending on the executable address size being 64-bit or 32-bit).