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Add description for out of bounds MBC ram access #594

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Add description for out of bounds MBC ram access #594

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e9dedf0
Add description for out of bounds MBC ram access
alloncm Mar 22, 2025
10955e0
Change mbc ram access formula
alloncm Apr 5, 2025
a254c59
Rephrased explanation to be oriented to the CPU memory bus (instead t…
alloncm May 17, 2025
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7 changes: 7 additions & 0 deletions src/MBCs.md
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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

On most of the MBC's in case an unmapped RAM bank is selected (which would be translate to an out of bounds RAM address by the MBC controller),
the MBC controller will simply wrap around the internal ram address and would access valid RAM address.

The MBC internal address being accessed can be calculated using this formula: `((address - external_ram_start_address) * ram_bank_size) % max_external_ram_size`.
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I'd have expected (active_ram_bank % nb_ram_banks) * 0x1000 + (address - 0xA000). Am I missing something?

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