How can CAS simulators like PTLsim achieve cycle accurate simulation of x86 hardware?

Question

Can somebody enlighten me how does CAS software like http://www.ptlsim.org/ work? How do they achieve cycle accuracy if there is neither information how many cycles are used

Answer 1

On the site you linked there are statements such as;

PTLsim is a state of the art cycle accurate microprocessor simulator and virtual machine for the x86 and x86-64 instruction sets.

and

It runs directly on the same platform it is simulating (an x86-64 or x86 machine running Linux)

It is not clear to me then how this differs from any other x86 Virtual machine technology such as QEMU, VirtualBox, VMWare or Virtual PC, which would be cycle accurate by virtue of actually directly running instructions on the hardware (as well as running at core speeds). Is it a simulator or a VM? In my mind they are not the same thing; bochs for example is a simulator rather than a VM, PTLsim appears to be somewhere in-between perhaps?