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75 lines
3.3 KiB
Plaintext
75 lines
3.3 KiB
Plaintext
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REALTEC Cart Mapper - description v1 (2005.03.08)
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by Tasco Deluxe [tascoDLX(AT)hotmail(DOT)com]
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* Thanks to Mask Of Destiny for info about the boot state (from "The Earth Defend")
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The REALTEC cart mapper is found in unlicensed Genesis/MegaDrive game carts produced by REALTEC.
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Game carts known to utilize the mapper include "The Earth Defend", "Whac-A-Critter", and
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"Funnyworld/Balloon Boy 2-in-1". They all contain common code used to display the REALTEC logo
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and map a portion of the main ROM.
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When the cart is powered on, 8KB of boot code is mapped by default into the cart's ROM area and
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is mirrored throughout. This boot code is likely the last 8KB of the main ROM (all known carts
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are 4 Mbit [512KB] w/ boot code at $07E000). The boot code, after displaying the REALTEC logo,
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has the option to display a menu for game selection. After a selection is made, any neccessary
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initializations are performed and a portion of the main ROM is mapped.
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The code used to access the mapping registers is common amongst all known carts. A portion
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of code is copied to RAM and executed. This code writes to the mapping registers based on a
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selection number. Afterwards, the code clears the first 16 bytes of the I/O area ($A10000
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thru $A1000F) and resets the M68000 manually (stack address is read from $000000, code address
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is read from $000004).
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The mapping is performed by writing to 3 mapped-in registers -- $400000, $402000, $404000.
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Only one value is written per register. However, the same value is written 256 times in a row.
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It is unknown whether this is because the registers explicitly require 256 writes, or because
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the hardware is so crappy as to need multiple writes.
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The register descriptions below are listed in the order they are commonly written.
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All registers are byte-sized and only known to have write access.
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* $402000 - Size of ROM range to map (in 1Mbit [128KB] blocks)
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[maximum (theoretical) value of 32]
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* $400000 - Bits of the ROM address (lower)
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The bits (as written) are: ? ? ? ? ? c c c
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* $404000 - Bits of the ROM address (upper)
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The bits (as written) are: ? ? ? ? ? m m !
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'?' is an unknown bit that is clear (in all known cases)
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'!' is an unknown bit that is set (in all known cases)
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From the above registers, the resulting ROM address (binary) is:
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00mm ccc0 0000 0000 0000 0000
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The common code in all REALTEC mapper carts sets the mapper values based on a selection number
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(ROM address only -- the ROM size is fixed). The ROM addresses for these selections, numbered
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1 thru 8, are:
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1) $000000 [$00,$01]
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2) $040000 [$02,$01]
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3) $100000 [$00,$03]
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4) $180000 [$04,$03]
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5) $200000 [$00,$05]
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6) $280000 [$04,$05]
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7) $300000 [$00,$07]
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8) $380000 [$04,$07]
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The only cart known to use a selection other than #1 is "Funnyworld/Balloon Boy 2-in-1"
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("Balloon Boy" is #1, "Funnyworld" is #2). It is assumed that the included code is merely a
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suggestion and that any valid ROM address can be mapped.
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A ROM range that is mapped, in order to function properly, must include a M68000 vector table
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since the cart's entire ROM area is replaced by the mapped range. None of the known carts
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attempt to remap a different ROM range after the boot code has executed. It is unknown whether
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the mapper allows the boot code to be remapped, although it seems doubtful.
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