Suppose you have an application like SimpliciTi that says:" low memory needs(<8kB flash and 1kB RAM depending on the configuration)", you also have in your program the Petit FAT file system: "Very small RAM consumption (44 bytes work area + certain stack), Very small code size (2K-4K bytes)", finally you have some inertial sensors to calculate some angles, like in Nine-Axis Sensor Fusion Using the Direction Cosine Matrix Algorithm on the MSP430F5xx Family AppNote, where you need RAM ~ 0.75 kB, Flash ~ 11.7 kB.
Summarizing:
flash 8kB + RAM 1 kB
flash 4kB + RAM 44 B
flash 11.7kB + RAM 0.75 kB
How do you choose the memory requirements for the microcontroller. The flash should be big enough to acomodate all the applications. Is the same case with the RAM? should I add together those values? Is the RAM used only when I'm using that particular app, for example I need 0.75kB only when I'm doing the math for the sensor fusion?
Thanks!