Help... Regarding the PWM speed measurement and 7 segment by using 8051

please help me to carry out the project.
i have the some questions.
first, when the fan is operated. i want the speed signal input to 8051 (yellow cable signal send to 8051),
and it is output show by 7 segment to display (r.p.m)
how to using the 8051 to detect the speed(r.p.m)???
and would you tell me the about the 8051 instruction of them?

thank you so much
FAN detail as below

Take a look at this project... It is very similar to what you need. It calculates pulses per second. Just convert it to pulses per minute.
Frequency Counter using AT89C2051 and LCD (Assembly)