> I think what he really meant was real >time<.

On "normal" x86 processors (say: the CPU that will be inside your PC/laptop if you buy one) it is really hard (I don't want to claim "impossible", but at least really hard) to do real-time stuff. Intel knows that and this is also among the reasons why they released their Intel Quark SoC/micro controller, which is perfectly suitable for real-time stuff.

Why is this the case? One obvious reason is that the caches (D$, I$, micro op cache etc.) und the pipeline stages (EDIT: including out-of-order execution) make it really hard to predict/prove/test strict upper bounds on the performance of some code fragment on "normal" x86 processors.

Another also well-known "issue" is that many modern x86 clock up or down depending on core temperate (I think this behavior can at least partly controlled by the firmware (UEFI)).

But there is a much more subtle reason, too: It is SMM (system management mode): https://en.wikipedia.org/w/index.php?title=System_Management...

For backward compatibility a lot of legacy functions are implemented via SMM calls such as emulating a PS/2 mouse/keyboard (while really a USB mouse/keyboard is connected). As long as you cannot guarantee that the code of your OS will not trigger such a function that is emulated via SMM, it is very hard to ensure real-time guarantees.

EDIT: To quote directly from the Wikipedia article: "Operations in SMM take CPU time away from the applications, operating system kernel and hypervisor, with the effects magnified for multicore processors since each SMI causes all cores to switch modes. There is also some overhead involved with switching in and out of SMM, since the CPU state must be stored to memory (SMRAM) and any write-back caches must be flushed. This can destroy real-time behavior and cause clock ticks to get lost. The Windows and Linux kernels define an ‘SMI Timeout’ setting a period within which SMM handlers must return control to the operating system or it will ‘hang’ or ‘crash’.

The SMM may disrupt the behavior of real-time applications with constrained timing requirements."