Add description for out of bounds MBC ram access (#594)

* Add description for out of bounds MBC ram access

This is based on:
- Source code of the SameBoy emulator
- Dissassembling Pokemon Pinball which depends on this behavior
- Mbc1 schematics - https://www.devrs.com/gb/files/mbc1.gif

* Change mbc ram access formula

* Rephrased explanation to be oriented to the CPU memory bus (instead the MBC)

* Fix internal address formula

* Update src/MBCs.md

Co-authored-by: Eldred Habert <me@eldred.fr>

* Update src/MBCs.md

---------

Co-authored-by: Antonio Vivace <avivace4@gmail.com>
Co-authored-by: Eldred Habert <me@eldred.fr>

Author: 3 people

src/MBCs.md

@@ -20,3 +20,10 @@ so, it might be nevertheless possible to use Double Speed during periods
 which use only code and data which is located in internal RAM. Despite the 
 above, a self-made MBC1-EPROM card appears to work stable and fine even in 
 Double Speed Mode.
+
+## MBC Unmapped RAM Bank Access
+
+In most MBCs, if an unmapped RAM bank is selected (which would be translate to an out of bounds RAM address by the MBC controller), 
+the MBC will simply wrap around the internal ram address and would access a valid RAM address.
+
+The MBC internal address being accessed can be calculated using this formula: `((address - external_ram_start_address) + (active_ram_bank * ram_bank_size)) % max_external_ram_size`.