NO NO... RAM was just for an analogy.
Like a system, that needs to be run independently using the subsystem.
As said, the main arduino CPU I want to be free from extra codes and to be more busy with processing, not on graphics display. But as I need graphics display, a separate m2560 as a gpu can handle that easily.
the same way, gsm module needs some time to init and also some time to finish other tasks. Like deleting a sms for sim800c takes 5 sec as per the reference manual. I dont want to keep it waiting for 5 sec untill finish. Send the command of delete SMS to controller subsection and keep going, as subsection will do that and will notify to main CPU.
Multitasking Simultaneously is a need here. I know RPi can do that... even a panel PC (as it mus be stable and RPi is good ony if your SD card is good) can do that. But for pricing issues, i have selected arduino and also for stability.
Again saying, RAM and etc is just as an analogy. Basically there will be a seperate subsystems to do the works independently. One is display (like i2c displays, and probably i will actually use that). Other one is SD card via SPI. These 2 are one way... always slave. But for GSM and also for motor controlling issues, there needs both way communication and they MUST run independently with decisions from main CPU.
The brigadier will pass order and make calls, but battalion commanders will work independently based on the wish of the brigadier.