how to generate random number?

As you just want numbers 0..127, why not just use a lookup table - so each "call" to rand() just returns the next entry?

But, Ivan said that he was out of code memory.

Ivan: if you are really desparate for space, it would be possible to implement an 8 or 7 bit LFSR. The advantage of a 7-bit LFSR is that it would give you values in the range 1..127 without any further processing. On the other hand the sequence would not be very random - you will get all the values 1..127 exactly once before the sequence repeats. If that is what you want, I can probably write a generator in about five minutes!

As you just want numbers 0..127, why not just use a lookup table - so each "call" to rand() just returns the next entry?

But, Ivan said that he was out of code memory.

Ivan: if you are really desparate for space, it would be possible to implement an 8 or 7 bit LFSR. The advantage of a 7-bit LFSR is that it would give you values in the range 1..127 without any further processing. On the other hand the sequence would not be very random - you will get all the values 1..127 exactly once before the sequence repeats. If that is what you want, I can probably write a generator in about five minutes!