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Document address wrapping behaviour of MBC2 RAM, and note questionable source of $52-$54 ROM size tags #106

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Mar 7, 2021
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Document address wrapping behaviour of MBC2 RAM, and note questionable source of $52-$54 ROM size tags #106

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Document address wrapping behaviour of MBC2 RAM, and note questionabl…
TheThief Jan 26, 2021
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Copy notice about 4 bit values to the echo section
TheThief Feb 28, 2021
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15 changes: 12 additions & 3 deletions content/MBC2.md
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Expand Up @@ -16,15 +16,24 @@ The MBC2 doesn't support external RAM, instead it includes 512x4 bits
of built-in RAM (in the MBC2 chip itself). It still requires an external
battery to save data during power-off though. As the data consists of
4bit values, only the lower 4 bits of the "bytes" in this memory area
are used.
are used. The upper 4 bits of each byte are undefined and should not be
relied upon.

### A200-BFFF - 15 "echoes" of A000-A1FF

Only the bottom 9 bits of the address are used to index into the internal
RAM, so RAM access repeats. As with the A000-A1FF region, only the lower
4 bits of the "bytes" are used, and the upper 4 bits of each byte are
undefined and should not be relied upon.

## Control Registers

### 0000-3FFF - RAM Enable and ROM Bank Number (Write Only)

This address range is responsible for both enabling/disabling the RAM
and for controlling the ROM bank number. Bit 8 of the address determines
whether to control the RAM-enable flag or the ROM bank number.
and for controlling the ROM bank number. Bit 8 of the address (the least
significant bit of the upper address byte) determines whether to control
the RAM enable flag or the ROM bank number.

#### When Bit 8 is Clear

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18 changes: 14 additions & 4 deletions content/The_Cartridge_Header.md
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Expand Up @@ -192,24 +192,34 @@ Specifies the ROM Size of the cartridge. Typically calculated as "N such that 32
| $53 | 1.2 MByte | 80 *
| $54 | 1.5 MByte | 96 *

\* Only listed in unofficial docs
\* Only listed in unofficial docs. No cartridges or ROM files using these sizes are known.
As the other ROM sizes are all powers of 2, these are likely inaccurate. The source for these
values is unknown.

### 0149 - RAM Size

Specifies the size of the external RAM in the cartridge (if any).

| Code | Size | Comment
|------|--------|---------
| $00 | 0 | No RAM
| $01 | - | Unused (but unofficial docs list it as 2 KB)
| $00 | 0 | No RAM *
| $01 | - | Unused **
| $02 | 8 KB | 1 bank
| $03 | 32 KB | 4 banks of 8 KB each
| $04 | 128 KB | 16 banks of 8 KB each
| $05 | 64 KB | 8 banks of 8 KB each

When using a MBC2 chip, $00 must be specified in this entry, even though
\* When using a MBC2 chip, $00 must be specified as the RAM Size, even though
the MBC2 includes a built-in RAM of 512 x 4 bits.

\** Listed in various unofficial docs as 2KB. However, a 2KB RAM chip was never used in a cartridge.
The source for this value is unknown.

Various "PD" ROMs ("Public Domain" homebrew ROMs generally tagged "(PD)"
in the filename) are known to use the $01 RAM Size tag, but this is believed
to have been a mistake with early homebrew tools and the PD ROMs often don't use
cartridge RAM at all.

### 014A - Destination Code

Specifies if this version of the game is supposed to be sold in Japan,
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