Wisdom Tree
The Wisdom Tree mapper is a simple, cost-optimized one chip design consisting of a 74LS377 octal latch, aside from the ROM chip. Because the mapper consists of a single standard 74 series logic chip, it has two unusual properties:
First, unlike a usual MBC, it switches the whole 32 kiB ROM area instead of just the $4000-$7FFF area. If you want to use the interrupt vectors with this cart, you should duplicate them across all banks. Additionally, since the initial state of the '377 can't be guaranteed, the ROM header and some code for switching to a known bank should also be included in every bank. This also means that the Wisdom Tree mapper could be used as a multicart mapper for 32 kiB ROMs, assuming there was enough ROM space in each bank for some small initialization code, and none of the ROMs wrote to the $0000-$7FFF area. For example, if the last 5 bytes of all banks are unused, games can be patched as follows:
; At $0100 in all banks but the first
nop
jp $7FFB
; At $7FFB in all banks
ld hl, $0100
ld [hl], a
jp hl
Second, because the '377 latches data on the
positive edge, and the value on the Game Boy data bus is no longer valid when the positive edge of the write pulse arrives, the designer of this mapper chose to use the A7-A0 address lines for selecting a bank instead of the data lines. Thus, the value you write is ignored, and the lower 8 bits of the address is used. For example, to select bank $XX, you would write any value to address $YYXX, where $YY is in the range $00-$7F.
An emulator can detect a ROM designed for Wisdom Tree mapper in one of two ways:
- ROM contains "WISDOM TREE" or "WISDOM\x00TREE" (the space can be $20 or $00), $0147 = $00, $0148 = $00, size > 32k. This method works for the games released by Wisdom Tree, Inc.
- $0147 = $C0, $014A = $D1. These are the values recommended by beware for 3rd party developers to indicate that the ROM is targeting Wisdom Tree mapper hardware. (See below.)
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