NES 2.0

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NES 2.0 is an extension to the iNES executable format, proposed by Kevin Horton on 2006-09-18. He has implemented it in his FPGA NES.

Unwikified portion

                     NES 2.0 File Addition Specification
                     -----------------------------------


Version 1.00
09/18/06
Written by K.Horton
Thanks to Quietust for ideas and proofing and help
----


This is the tentative addition to the standard .NES file format that
most emulators use.  This addition is designed to disambiguate certain
ROMs that currently can only be discerned via a CRC-32 or similar hash
check.  Naturally, this causes problems for new ROMs that are not in the
database, but need special handling.

There are four goals for this specification.

1) Retain 100% backwards compatibility with existing emulators/ROMs/etc.
   (*this includes "dirty ROMs" with crap such as "DiskDude!" in the header
   and other atrocities*)

2) The format must be "future proof".

3) The changes made must be VERY CAREFULLY documented and make sense.

4) Said changes must make sense from both a hardware and software
   standpoint.

* * *

The standard iNES specification is presented below:

 0-3: string    "NES"<EOF>
   4: byte      Number of 16K byte program ROM pages
   5: byte      Number of 8K character ROM pages (0 indicates CHR RAM)
   6: bitfield  flags byte 0
   7: bitfield  flags byte 1
8-15: byte      These bytes are not used, and should be 00h.

Flags byte 0:

7       0
---------
NNNN FTBM

N: Lower 4 bits of the mapper number
F: Four screen mode. 0 = no, 1 = yes. (When set, the M bit has no effect)
T: Trainer.  0 = no trainer present, 1 = 512 byte trainer at 7000-71FFh
B: SRAM at 6000-7FFFh battery backed.  0= no, 1 = yes
M: Mirroring.  0 = horizontal, 1 = vertical.

Flags byte 1:

7       0
---------
NNNN xxPV

N: Upper 4 bits of the mapper number
P: Playchoice 10.  When set, this is a PC-10 game
V: Vs. Unisystem.  When set, this is a Vs. game
x: these bits are not used.

* * *

For NES 2.0, none of the above will be changing, except for the two unused
bits on flags byte 1.

To indicate that this is a NES 2.0 file, bit 2 will be clear, and bit 3 will
be set.   i.e.

7       0
---------
NNNN 10PV

Same as above, but the 1 and 0 pattern will denote an NES 2.0 file.  This
neatly gets around the "diskdude!!!" problem, because those headers do not
have the correct bits set on this byte, and will thus be treated as a
regular iNES file.

That is the "how" of determining whether we are dealing with a valid NES 2.0
file.  Now that that is done, the desired aspects of such a thing need to
be considered.  I have tested over 4000 ROMs and have dumped at least a
thousand more, and reverse engineered probably 60-70 different mappers.
This has given me a front row seat into the shortcomings of the original,
and a good idea of where additional information is needed.

* * *

The new things we need to know are thusly:

1) Vs. Unisystem

The Vs. Unisystem is one of the two Nintendo arcade machine series produced
which use "mostly NES/famicom" hardware.  These games will run fine on
emulators if a little extra things are stored in the header.

Nintendo wanted to make life difficult for arcade operators when it came to
copy protection.  Three main schemes were devised. (See the "Vs. system byte"
description below for a detailed analysis)

2) PRG ROM in excess of 2Mbytes, CHR ROM in excess of 1Mbyte

This has already occured, and has been causing trouble for some ROMs. So
far, the hack has been to set PRG ROM to 00h to indicate 4Mbytes of ROM
(since FFh is 16K short of 4Mbytes), and in the case of exceeding the
2Mbyte-8K CHR barrier, ROMs have been allocating the CHR in the PRG space,
and the emulator has to sort this out. Very messy.

3) "Sub-mappers"

Some of the allocated mappers are actually multiple mappers with 1 number.
Examples include mapper 34 (Nina/BNROM), mapper 83 (two styles of CHR ROM
banking), mapper 1 (various ROM/RAM hacks), mapper 16 (EEPROM/WRAM/light pen/
etc).

Currently, the only fix for this is to CRC the games and then hack the mapper
if the CRC or other hash matches.  This of course fails if the game is not
in the database.

4) Mapper numbers

Face it, we're running out of mapper numbers.  16 seemed like it would be
enough, but they were quickly exhausted.  Then 256 mappers seemed like a
vast space to work on.  But now, we are getting near the end of the line,
and running out of mapper numbers.  I have personally assigned at least 50
or 60 of them, which is almost 1/4th of the total mapper space.

5) WRAM

Not all carts that support WRAM support 8K of it. Some support less,
some support more, and some even have EEPROM!   Heck, some carts even
battery backed the stupid CHR RAM.  This last one was a very recent
find and goes to show that a workable extention needs to reasonably
cover all possible bases.

* * *

The proposed solution:

Byte 8:

7       0
---------
SSSS xxxM

S: Sub-mapper number.

This specifies the submapper for this ROM.  If no submapper mode is
needed, set this nybble to 0000b.

M: Mapper number extension.

CAUTION: DO NOT USE THIS BIT YET.  There are still some existing
numbers left in the current iNES mapper space. (Around 30-40 or so by
my last count).  The other three bits marked with "x" are also slated
to be used if more than 512 mappers are required.  This would allow
support for 4096 mappers all together.  This should hold us until the
next iceage.  I repeat: do not designate mappers above 256 yet.

-- 

Byte 9:

7       0
---------
CCCC PPPP

C: 4 more CHR ROM size bits
P: 4 more PRG ROM size bits

These combine with the existing 8 bits of each to form 12 bits total
for the number of PRG and CHR banks... this is enough for 64Mbytes-16K
of PRG data and 32Mbytes-8K of CHR data.

-- 

For the following two bytes, this table defines the size of the RAM
segments:

Each Nybble:
------------
0 - no RAM of this type is present.
1 - 128 bytes of RAM
2 - 256 bytes of RAM
3 - 512 bytes of RAM
4 - 1K of RAM
5 - 2K of RAM
6 - 4K of RAM
7 - 8K of RAM
8 - 16K of RAM
9 - 32K of RAM
10 - 64K of RAM
11 - 128K of RAM
12 - 256K of RAM
13 - 512K of RAM
14 - 1M of RAM
15 - reserved, do not use


Byte 10:
7       0
---------
pppp PPPP

p: Quantity of PRG RAM which is battery backed (or serial EEPROM, see below)
P: Quantity of PRG RAM which is NOT battery backed

-- 

Byte 11:
7       0
---------
cccc CCCC

c: Quantity of CHR RAM which is battery backed (yes it exists! see below)
C: Quantity of CHR RAM which is NOT battery backed

-- 

A note about serial EEPROMs and battery backed CHR RAM...
Some mapper 16 (Bandai) games use serial EEPROMs to store the game data,
rather than a battery backed SRAM.  These can be as small as 128 bytes or
as large as 512 bytes.  They tended to use 24C01 (128 bytes) 24C02 (256
bytes) or another semicustom chip I cannot find a workalike for.  The
interface for the 24Cxx parts is I^2E which is a Philips specification.
The workalike chip is very similar, but the address and data are clocked
in backwards from the I^2C parts.

As for battery backed CHR RAM, the Racermate cartridge has 64K of CHR RAM
total:  32K is battery backed, and 32K of it is not.  They store all the
stats and such in it.  Why you would do such a thing, I do not have a
clue... but they did!  I traced out the circuit myself and couldn't
believe it.

-- 

Byte 12:
7       0
---------
xxxx xxBP

P: This is a PAL ROM.  When set, indicates PAL mode.
B: When set, indicates this ROM works on both PAL and NTSC machines.
Some of the Codemasters games actually will adjust the game depending
on if it detects you running on a PAL or NTSC machine - it adjusts the
timing of the game, and fixes the music.

Not many games would have this B flag set.

x: These bits are not used yet.  They shall be maintained clear.


byte 13:
7       0
---------
MMMM PPPP

This byte is reserved for the Vs. Unisystem only.  If this is not a Vs.
Unisystem ROM, then this byte shall be all 0's.

P: PPU.  There are 13 Vs. PPUs that can be found on the games:

 0 - RP2C03B     (bog standard RGB palette)
 1 - RP2C03G     (similar pallete to above, might have 1 changed colour)
 2 - PR2C04-0001 (scrambled palette + new colours)
 3 - RP2C04-0002 (same as above, different scrambling, diff new colours)
 4 - RP2C04-0003 (similar to above)
 5 - RP2C04-0004 (similar to above)
 6 - RC2C03B     (bog standard palette, seems identical to RP2C03B)
 7 - RC2C03C     (similar to above, but with 1 changed colour or so)
 8 - RC2C05-01   (all five of these have the normal palette...
 9 - RC2C05-02   (...but with different bits returned on 2002)
10 - RC2C05-03
11 - RC2C05-04
12 - RC2C05-05
13 - not defined (do not use these)
14 - not defined
15 - not defined


I have dumped the palettes from ALL of these PPUs, and have exact bit for
bit copies of them.  The last 5 PPUs (RC2C05) have the standard NES
palette in them, however they return a specific word in the lower 5 bits
of 2002h, and registers 2000h and 2001h are flipped around.  I'm fairly
certain that these are all the PPU's that exist.  I have a good cross
section of games now.


M: Vs. mode:

 0 - Normal- no special mode(s)
 1 - RBI Baseball
 2 - TKO Boxing
 3 - Super Xevious
 4 - ...


This section is a tad bare right now... I'm still trying to figure out
exactly how to flesh this out.  This should be a good start, however.

If anyone is interested in the things nintendo did to make your life
difficult as an arcade operator, here it is:

 a) Different PPUs.  There are 13 different PPU chips made that you
    can find on Vs. arcade boards.

 b) Different controller pinouts.  Some games came with new control panels
    you had to install with the game.  This was pretty basic stuff and just
    remapped a few of the buttons.

 c) Atari/Namco/Tengen came up with at least three different protection chips
    which map in the 5000-5FFFh area that the game checks. If the chip
    does not return the correct data, the game hangs or fails to start.