KanedaFr wrote:- why SMS mode ? why not a plain genny code ?
Well it could be because they knew Z80/SMS better than they knew 68K/Genesis. But it's more likely because it was easier to work with their hardware in SMS mode.
The things all use 8 bit RAM. This is the reason for the slightly odd format that is .SMD. One bank of RAM (8KB) is banked in at a time and the .SMD data copied directly to it - later on, odd and even banks will appear on the Genesis side as high and low bytes of the 16 bit data bus. This is also the reason that earlier models of these devices maxed out at 16Mbit, since 8KB*256 banks == 16Mbit.
KanedaFr wrote:- how did they reset the genny from software ?
They didn't... It's all hardware.
KanedaFr wrote:and reset from SMS to Genny mode !?
The Genesis boots into SMS mode via a pin on the cart port. All they need to do is switch that pin and hit the reset line - voila - rebooted into Genesis mode.