I was just now trying to see which hardwareserial.h (there are 6 of them in my HDD- lets forget this) is used by the IDE.
So I have a mega2560, i try one after the other and see in compilation of specific sketch how much memory is used. ok i got it.
initial value was rx 256, tx 256 (it seems I had change in past) : Compilation message (I translate from greek) : global variables use 3117 bytes
change values to rx 64, tx 64 : Compilation message : global variables use 1965 bytes
That is a difference 3117-1965=1152bytes
256-64=192 bytes "each" buffer
1152/192 = 6 buffers
Mega 2560 has Serial, Serial1, Serial2, Serial3 : 4 ports *2 buffers =8
Am I missing something or one port is not affected (which one?)