Guys,
Is there a simple way to determine how many milliseconds I need to "Sleep" for in order to "emulate" a 2 mhz speed. In other words, I want to execute an instruction, call System.Threading.Thread.Sleep() function for an X amount of milliseconds in order to emulate 2 mhz. This doesn't need to be exact to the millisecond, but is there a ball park I can get? Some forumlate that divides the PC clock speed by the 2 mhz or something?
Thanks
A 2 MHz clock has a 500 ns period. Sleep's argument is in milliseconds, so even if you used Sleep(1), you would miss 2,000 cycles.
Worse, Sleep does not promise that it will return after X milliseconds, only that it will return after at least X milliseconds.
Your best bet would be to use some kind of Timer with an event that keeps the program from consuming or producing data too quickly.
The problem you will have is that the minimum sleep on windows seems to be about 20-50ms, so though you may put that you want to sleep for 1ms, it will wake up later, due to the fact that other processes will be running, and the time slice is quite large.
If you must have a small time such as 500ns (1/2e06 * 1000) then you will want to use DirectX, as it has a high-resolution timer, so that you can just loop until the pause is done, but, you will need to take over the computer, not allow other processes to interrupt what is going on.
For the user, a pause of less than 100 ms or so will generally be imperceptible. Based on that, instead of attempting to sleep after each instruction, you'd be much better off executing for something like 50 ms, then sleeping for an appropriate length of time, then executing for another 50 ms.
Also note, however, that most processors with a 2 MHz clock (e.g. a Z80) did not actually execute 2 million instructions per second. A 2 MHz Z80 took a minimum of four processor clocks to fetch one instruction giving a maximum instruction rate of 500 KHz.
Note that sleeping is not at all a good proxy for running code on a less capable CPU. There are many things that affect computational performance other than clock rate. In many cases, clock rate is a second or third (or 10'th) order determinate of computational performance.
Also note that QueryPerformanceCounter() while high resolution is expensive on most systems (3000 to 5000 CPU clocks in many cases). The reason is that it requires a system call and several reads from the HPET in the system's south bridge. (note, this varies by system).
Could you help us better understand what you are trying to do?
As I mentioned in my comment on James Black's answer: do not poll a timer call (like QPC or the direct X stufF). Your thread will simply consume massive amounts of CPU cycles and not let ANY thread at a lower priority run, and will eat up most of the time at its priority. Note that the NT Scheduler does adjust thread priorities. This is called 'boosting'. If your thread is boosted and hits one of your polling loops, then it will almost assuredly cause perf problems. This is very bad behavior from a system perspective. Avoid it if at all possible.
Said another way: Windows is a mult-tasking OS and users run lots of things. Be aware that your app is running in a larger context and its behavior can have system wide implications.