Im using an ARM Cortex-M3(LPC1768).I would like to know how many oscillator periods make up 1 instruction cycle for arm.Like for example,12 oscillator periods make up 1 instruction cycle for 8051.

LOL. You are perfectly correct! For some very short delays, for instance where a device you're driving just needs to get its wits together to reply, it can be a saving to do a quick delay then check if the expected  result comes back. If it does then one is on track and knows exactly what to do next. Otherwise the easiest thing is to just return and handle it as a general interrupt whenever the device responds. This can give a useful increase in speed sometimes. However it should only be done when the speed is very important.

If substituting an M3 for an 8051 I'd have though the best thing to do would be to use interrupts properly to start with and the speed would more than adequate. Longer times can be got using the system clock.

LOL. You are perfectly correct! For some very short delays, for instance where a device you're driving just needs to get its wits together to reply, it can be a saving to do a quick delay then check if the expected  result comes back. If it does then one is on track and knows exactly what to do next. Otherwise the easiest thing is to just return and handle it as a general interrupt whenever the device responds. This can give a useful increase in speed sometimes. However it should only be done when the speed is very important.

If substituting an M3 for an 8051 I'd have though the best thing to do would be to use interrupts properly to start with and the speed would more than adequate. Longer times can be got using the system clock.