you wrote:
<<
>From what I can work out the equation gives the number of instructions
which are executed. I am using a PIC16F84 (10Mhz) and want to know who
to convert instructions to time. I have gone through th F84 data sheet
and it doesn't specificly say how many clock cycles are used per
instruction. It says 'cycles' but this is 1 or 2 and 0.1uS sounds a bit
small.
>>

Don't forget that a crystal is not *that* accurate... a regular ±50ppm
per ¡C is expected from commercial crystals, so a 4MHz unit can oscilate
from 3,999,800 to 4,000,200 Hz, inside the same temperature degree.  It
means that each 20 thousand oscillations, it can gain or lose one count,
per temperature Celsius degree change.  Moving the unit from 20 to 28¡C,
it can slide the frequency from 4,000,000 to 4,001,600 Hz.  ECS produce
some crystal and oscillators that goes up to ±100ppm/¡C...

It is hard to find information about the stated frequency precision, but
experience shows that no one 4MHz crystal oscillates at 4MHz, except
inside of a calibrated thermal chamber.

Wagner.