I have 3 instructions here.

(a) mov.b @Data, R1L
(b) mov.w @Data, R1
(c) mov.l @Data, ER1

@Data is 32 bits, the CPU bus is 16 bits.

So they each take 4,4,6 execution cycle to execute respectively.

So how was the cycle computed? I know I can see from the data sheet, but I don't understand how.

I am guessing :

Step 1 : A cycle is used to fetch instruction.
Step 2 : I have no idea actually, I am guessing 2 Cycles are spend fetching @Data for (a) and (b), and 4 Cycles are spend fetching @Data for (c)
Step 3 : Is the actual move

But I think I am wrong. X_X

 

Keep studying the data sheet, especially the overall timing info.

If you tell us what processor it is, and come up with an answer, people on here can check your conclusions.