Windows 95 EMM386.EXE V86 monitor does not allow RDTSC instruction about doslib

[joncampbell123]

I can confirm that if I manually enter the RDTSC opcode in DOS DEBUG and try to single-step it, that the system hangs in the same way.

[joncampbell123]

Because of this issue, the CPU library now has a flag to indicate not to use the RDTSC instruction. For now, that flag is set if the build is 16-bit and virtual 8086 mode is active.

[joncampbell123]

I will update that code to not set the flag if it can recognize whether or not the v86 monitor permits RDTSC. Perhaps FreeDOS has a better implementation. Perhaps MS-DOS 6.22 and earlier don't have this issue, or perhaps Microsoft fixed this in Windows 98 DOS mode. I'll need to test this.

[joncampbell123]

This was observed on an old Pentium 133MHz test system with Windows 95 DOS mode on a CF card.

[joncampbell123]

TODO: Which MSR controls whether user-space can access RDTSC, by the way? I understand there's another one for whether or not user-space can use CPUID.

[joncampbell123]

A-ha:

https://en.wikipedia.org/wiki/Control_register#CR4

[darkstar]

The P133 does not support rdtsc in v86 mode: http://www.os2museum.com/wp/undocumented-rdtsc/