I am wondering how long it takes to read whole 256 kb flash and SRAM in milliseconds?
I am asking it because I wondering if the Arduino MEGA2560 can calculate 256kb flash/SRAM each 33 milliseconds?
What does "calculate 256kb"
What kind of memory?
I'm not optimistic though, you only have like 4 clock cycles per byte to do that with...
Why not write a quick sketch and find out? 
By my calculation, a 16Mhz processor will be hard pressed to iterate over 256KB of any sort of memory in 33ms anyway no matter how you optimize it. 16000000 cycles/s / (256*1024) bytes * 0.033s=2.014 clock cycles/byte. I think we can say this is not going to happen.
A CPLD or a small FPGA could be made to do this, however, especially if the calculation on that data is straightforward, assuming the memory can keep up.
Also, you talk about the 256KB on the 2560 as if it is SRAM. It isn't, it's FLASH and meant to be used for program space. Now, you can put constant data there but keep in mind that you never get the full 256K for constant data because you need space for your program and it has to be constants written at compile time and cannot be changed at runtime as it can if it was stored in SRAM. The chip has only 8K of real SRAM. However, there are a lot of IOs so you can pretty much add as much external memory as you like, though there is no dedicated memory interface so you have to do the overhead of your own memory access which is very inefficient compared to a dedicated memory interface. Other Atmel chips like the XMEGA have a dedicated memory interface, they call it the EBI (http://www.atmel.com/images/doc8058.pdf). No such luck on the 2560 though.
So it actually read roughly 128kb each 33 ms, now that is clearly!
I have other question.
Does PORT give light continuously or flashing a bit each loop?
If it flashing, then I need to have dedicated microcontroller connect to the port to give continously LED lamp so it does not flashing.
I have one more question.
If I use for example eight pins that give up to 256 values to select, then connect them to many arduino boards then programming each to receive a few fixed values; for example Arduino 1 receives 0000 0000, 000 0001, 0000 0002, and 0000 0003 (four values), and so on with 31 other boards. Or do I need to use Serial1, Serial2, and Serial3 which is included in MEGA2560 ?