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Add description for out of bounds MBC ram access #594

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May 20, 2025
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7 changes: 7 additions & 0 deletions src/MBCs.md
Original file line number Diff line number Diff line change
Expand Up @@ -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`.