Flash Write Using Code

"...working fine when iam using Level 8 Optimization in Keil Compiler but when i use constant folding (Level 0) it was not"

Remember that each higher optimisation level includes all the lower-level optimisations: http://www.keil.com/support/man/docs/c51/c51_optimize.htm

Therefore, Level-8 includes constant folding.

You need to reduce the optimisation a step at a time to determine where it actually stops working - and then examine the differences between the generated code at that point.

Could you be using software delay loops, and these end up just being too slow at the lower optimisation?
(this is why you should never write software timing loops in a high-level language)

"...working fine when iam using Level 8 Optimization in Keil Compiler but when i use constant folding (Level 0) it was not"

Remember that each higher optimisation level includes all the lower-level optimisations: http://www.keil.com/support/man/docs/c51/c51_optimize.htm

Therefore, Level-8 includes constant folding.

You need to reduce the optimisation a step at a time to determine where it actually stops working - and then examine the differences between the generated code at that point.

Could you be using software delay loops, and these end up just being too slow at the lower optimisation?
(this is why you should never write software timing loops in a high-level language)

"you should never write software timing loops in a high-level language"

That is, not if you want any sort of guarantee of the timing- see: www.8052.com/.../read.phtml