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Genesis Programming FAQ by HRieke

Updated: 10/18/95

`
          How to program the Sega Genesis/Mega Drive
Sega Programming FAQ October 18, 1995, Sixth Edition - Final
=============================================================

This FAQ (Frequently Asked Questions) is about how to program
the Sega Genesis and Mega Drive Video Games Systems.

This file IS Public Domain, and can be used by all.
You may copy and post this file elsewhere. Share and enjoy.

If you want to add anything, especially game code or
information on the hardware please E-mail me. My E-mail
and real address are below and at the end of this FAQ.

Disclaimer : In no way I am indorsing any products, systems,
etc. What I am doing is providing the information that I
have found throughout internet and other network systems and
reporting them here, with out reverse engineering. All
information is reported as is- I will not accept any
responsibility if you fry your system trying something out.
All copyrights are owned by their respective owners, no
infringements intended.

Henry Rieke
[U6c16@Wvnvm.Wvnet.Edu]

=============================================================

1.0 Questions

Q) What are the differences between Mega Drive and Genesis?
   What are the differences between Mega CD and Sega CD?

A) Physically, on the outside, the only difference is the
   plastic case, the cartridge shape is slightly different on
   the American version, smaller in size. On the inside there
   are four jumpers labeled J1 though J4. J1 and J2 is the
   language switch, between English and Japanese, J3 and J4
   select the output between PAL and NTSC. You can build a
   switch that can allow you to select between these options.
   The diagram is included in this document (Chapter 5.0,
   called "Language Switch"). The reason for the language
   switch is to keep people from getting the game before it
   is officially released elsewhere in the world. The newer
   Sega Machines do not have the jumpers, and hence you can
   not build a switch (as of yet).

   The difference between Mega CD and Sega CD are internal.
   In side the machine there is a ROM BIOS chip which is changed
   for different markets. Sega has a version of the BIOS which
   will play any system's games, but this is for Game developing
   and testing. There is a device called Pro-CDX which will allow
   you to by-pass the lockout and play any country's games.
   How this works is on Track 0 of the CD, which the machine
   reads in first to get the CPUs in synch, the country code
   is checked and if it fails, then up pops the warring screen.

Q) Where to find files on the Sega Genesis/Mega Drive
   hardware? I have no idea if any of these sites are
   still valid.

A) You can find the files on the Sega hardware from one of
   the following FTP sites. A brief section describing memory
   and cartridge ROMs are included in this FAQ. The author
   Rick McTeague, is from Louisville and the most current copy
   can be found there first.

FTP ftp.spd.louisville.edu
    pub\sega\hardware.txt

FTP sunsite.unc.edu
    pub\micro\games\sega\faqs\...
    It's in there. I'm not sure of the exact location.
    There are also many other files here as well as a few
    68000 ASM.

FTP x2ftp.oulu.fi
    pub\console\sega\...
    pub\msdos\programming\faqs\sega1409.faq


Please note that the regular name for this document is Segafaq.doc
I have noted the change at the second listing at x2ftp.oulu.fi
to ease confusion.

Q) What does it take to program the Genesis?

A) Development kits can be bought from many companies, a list
   is included in the section called "Companies". These kits
   require an IBM or Amiga computer to compile the code,
   which is usually assembly, but can be in C as long as it
   can be compiled into 68000 binary. SEGA's own kit is extremely
   expensive and they are rumored to have slow response time
   to answer questions. The price of the kits cost from $1000
   to $4000.

Q) Is IBM or Amiga the only two platforms for game
   development?

A) No. The Apple Macintosh does have a system, which does allow
   you to compile games, but it is a do-it yourself job (See below
   for the whole scope). There also is a system for the Apple IIgs,
   called Sluggo from Parsons Engineering. As for the Mac, if you
   know, please pass it on. It is rumored that Electronic Arts uses
   a priority system on the Macintosh for porting the games.

-- This information comes from Landon Dyer [landon@apple.com],
-- You can generate 68K code for the Sega "trivially" with the
   C compiler and assembler in the Macintosh programmer's workshop
   (MPW). You'll need to hack up some transfer utilities, but this
   is not really hard. You can wire-wrap a ROMulator, or buy one;
   there are a bunch on the market that should work just fine.
   I had a ROMulator and a debugger up and running in about a
   week, working evenings. (Ed note: ROMular information can be found
   at the FTP sites)

Q) How do I program the SEGA CD Rom?

A) I am beginning to understand how to program this device,
   but more information is needed, so if you have any, please
   pass it along to me so I can include it in the next update.
   Look under Hardware 2.1, Sega CD for more information about
   the unit.

Q) What is Saturn, and how do I program that?

A) Saturn is Sega's next generation game machine, which
   is based on Sega's S-1 /S-2 arcade Machine's motherboard.
   The Saturn will allow programming in C as well as Assembly,
   and porting of arcade games that are
   based on the S-1 and S-2 boards will take only a week.

Q) What else is there on Internet?

A) There is Rec.Games.Video.Programmer and
   Rec.Games.Video.Sega where you can ask your questions.
   The list server at Busop.cit.wayne.edu died due to low
   usage.

   There are also files on how to use the old SEGA 3D glasses
   with your IBM compatible computer which can be found all
   over the place (including the above FTP sites).

=============================================================

2.0 Hardware : This is the inner workings of the Sega system
ROM and Memory. For a better understanding Sega Hardware I
suggest reading Hardware.txt. An up-to-date version of this
file can be gotten by anonymous ftp from Ftp.spd.louisville.edu,
in the file "Pub/sega/Hardware.txt".

Genesis - this information comes from Rick McTeague.

Sega Genesis Hardware Internals - Revised: 04/07/1993

Please send corrections or comments to:

   Rick McTeague
   Sysrick@starbase.spd.louisville.edu

Disclaimers:

The following information was extracted without the use of
official information from Sega Enterprises, Ltd. I therefore
offer no warranty on the correctness of this information.
Use it at your own risk.

This information was gathered as a personal project, and is
not the responsibility of my employer.

Different versions of the Genesis exist, so I won't suggest
that any of this information is right for anybody's Genesis
but my own.

Sega, Genesis, Sonic the Hedgehog, and Ecco the Dolphin are
trademarks of Sega Enterprises, Ltd. Other trademarks are the
property of their respective companies.


The Cartridge ROM
-----------------

As others on the net have observed, a standard Genesis
cartridge is nothing more than a small printed circuit board
with a ROM on it. There are some cartridges which have
battery-backed RAM for saving high scores, configurations,
etc., but I haven't seen these, so I don't know about them.
If you know of a particular cartridge that has RAM, let me
know so I can go rent one and take it apart (shhhh!) to see
what's in it.

A cartridge contains a word-wide (16 bit) ROM with up to 2
Meg words (4 Mbytes). This is based on the signal !C_CE, which
is used as the cartridge ROM's chip enable on the cartridges
I've looked at, and which is active at addresses $000000-$3fffff.

It may be possible to have more than 4 Mbyte cartridges; if
so, either they ignore the !C_CE line and do their own
address decoding, or there's a register somewhere in the
Genesis which changes the default address decoding scheme for
!_CE.

Note that cartridges are advertised as being "8 Meg"; as far
as I know, is 8 Meg _bits_, not bytes. 1 Mbyte is still
pretty impressive for a game machine.

SONIC uses a 40-pin 256Kx16 (512Kx8) ROM in word mode.

Ecco the Dolphin uses a 42-pin 512Kx16 (1024Kx8) ROM in word
mode. This one confused me because I'd never seen a 42-pin
DIP before, so I thought it was 40 pins. No wonder the
signals didn't look right...

Putting together a little bit of information from a Sharp
data sheet for a 512 Kword ROM with a bit of conjecture based
on the Sonic and Ecco cartridges, this is what I can come up
with for pinouts:

         Sonic                              Ecco
      (256 Kword)                       (512 Kword)

                                          ----u----
         ----u----               A18    1=|       |=42   VCC?
A17    1=|       |=40   A 8      A17    2=|       |=41   A 8
A 7    2=|       |=39   A 9      A 7    3=|       |=40   A 9
A 6    3=|       |=38   A10      A 6    4=|       |=39   A10
A 5    4=|       |=37   A11      A 5    5=|       |=38   A11
A 4    5=|       |=36   A12      A 4    6=|       |=37   A12
A 3    6=|       |=35   A13      A 3    7=|       |=36   A13
A 2    7=|       |=34   A14      A 2    8=|       |=35   A14
A 1    8=|       |=33   A15      A 1    9=|       |=34   A15
A 0    9=|       |=32   A16      A 0   10=|       |=33   A16
!CE   10=|       |=31   !BYTE    !CE   11=|       |=32   !BYTE
GND   11=|       |=30   GND      GND   12=|       |=31   GND
!OE   12=|       |=29   D15      !OE   13=|       |=30   D15
D 0   13=|       |=28   D 7      D 0   14=|       |=29   D 7
D 8   14=|       |=27   D14      D 8   15=|       |=28   D14
D 1   15=|       |=26   D 6      D 1   16=|       |=27   D 6
D 9   16=|       |=25   D13      D 9   17=|       |=26   D13
D 2   17=|       |=24   D 5      D 2   18=|       |=25   D 5
D10   18=|       |=23   D12      D10   19=|       |=24   D12
D 3   19=|       |=22   D 4      D 3   20=|       |=23   D 4
D11   20=|       |=21   VCC      D11   21=|       |=22   VCC
         ---------                        ---------

Note that the address line numbering is different than that
of the 68000; line A0 of the ROM is really connected to the
68000's A1. This is because of the way the 68000 handles
even/odd byte addressing using !UDS and !LDS.

The !BYTE line is held high. This puts the ROM chip into
"word" mode, where data is accessed 16 bits at a time.

The convenient correspondence between ROM and 68000 data and
address lines (ROM D0 = 68K D0, ROM D1 = 68K D1, etc..., and
ROM A0 = 68K A1,  ROM A1 = 68K A2, etc...) may not exist in
all cartridge designs. This is sometimes difficult to
visualize, but keep in mind that the ROM doesn't care whether
its D0 is really D0, or if its A3 is really A3, etc. As long
as each data line is used as the same D## line for both
programming and reading, and each address line is used as the
same A## for both programming and reading, the ROM will
function properly and the 68000 will see the right
instructions. Think about it...

Why bring this up? If you remove the ROM from the cartridge
PCB and try to read it with a EPROM programmer, there may be
situations where you won't see valid 68000 instructions, or
things won't be in the right place... Assuming your
programmer isn't messed up, this would be because the address
and data lines on the ROM aren't assigned the way you'd
expect.

Why would someone make a cartridge like this? To make your
life difficult, if you're wanting to hack their ROMs, or to
make their life easy, by making the cartridge PCB easier to
design.

The easiest way to avoid this potential problem is to build
an adapter from your programmer's EPROM socket to an edge
card connector into which you plug the cartridge. This is a
bit safer, too, so you don't have to remove the ROM from the
cartridge (or even open it up).

The Memory Map
--------------

$000000 $3fffff Cartridge ROM, when enabled by cartridge
control register and !CART_IN.

$000000 $0007ff Internal OS ROM, when enabled by cartridge
control register and !CART_IN.

$ff0000 $ffffff 64Kbytes scratchpad RAM
Partially decoded; RAM actually appears repeated between
$e00000 through $ffffff.

$a14101 cartridge control register
bit 0 = 0: cartridge disabled, OS ROM enabled
bit 0 = 1: cartridge enabled, OS ROM disabled

$a00000 $a????? Unknown. Audio? Controller I/O?

$c00000 $c????? Video display controller registers

The OS puts the top of its stack at $ffff00, and defines a
"user stack pointer" at $000000 (building down through the
top of RAM memory).

The Z80 processor is used for sound and has 8Kbytes of RAM.
I don't know anything about how the Z80 does its work, as I
haven't looked.

Acknowledgements
----------------
Thanks to:

dt93tn@pt.hk-r.se (Tore Nestenius) for info on older Genesis
systems.

lewism@rpi.edu (Michael Lewis) for info on the Sony CXA1145
and Samsung KM6264 chips.

crs@crs-sys.uucp (Chris Gregors) for posting a version of the
Genesis cartridge port pinouts.

merlyn@digibd.com (Brian Westley) for Game Genie information
and decoder program.
------------------------------------------------------------

Brief technical description of the Genesis:

---
The genesis graphics hardware consists of 2 scrollable planes.
Each plane is made up of tiles. Each tile is an 8x8 pixel square
with 4 bits per pixel. Each pixel can thus have 16 colors. Each
tile can use 1 of 4 color tables, so on screen you can get 64
colors at once, but only 16 in any specific tile. Tiles require
32 bytes. There is 64K of graphics memory. This would allow
for 2048 unique tiles if memory were used for nothing else.

Each plane can be scrolled independently in various ways.
Planes consist of tables of words, where each word describes a
tile. The word contains 11 bits for describing which tile, 2 bits
for flip x and flip y, 2 bits for the selection of the color table,
and 1 bit for a depth selector. Sprites are composed of tiles also.
A sprite can be up to 4 tiles wide by four tiles high. Since each
tile is 8x8, this means sprites can be anywhere from 8x8 pixels to
32x32 pixels. There can be 80 sprites on screen at one time. On a
scan line you can have 10 32 pixel wide sprites or 20 16 pixel wide
sprites. Each sprite can only have 16 colors but they are out of the
4 different color tables. Color 0=transparent.

Colors are 3 bits for each gun, so 512 colors are possible.

There is a memory copier that is in hardware. This does fast copies from
the 68000 ram into the graphics ram.

The 68000 runs at about 8 mhz. It has 64K of memory devoted to it. The
ROM cartridge appears at 0.

The Z80 has 8K of ram. The 68000 can download programs to the z80 and
let them go. The z80 can access the graphics chips or the sound synth
chips but usually those things are controlled by the 68000.

The sound chips consist of a Yamaha synthesis chip and a TI programmable
sound generator. The PSG has 3 square wave tones and 1 white noise tone.
Each tone/noise channel can have its own frequency and volume.

The Yamaha chips are based on FM synthesis. There are 6 voices with 4
operators each. The chips are similiar to those used in the Yamaha
DX27 and DX100 synthesizers. By setting up registers a rich variety of
sounds can be created.

------------------------------------------------------------

2.1 Sega CD Rom. The following information came from
Leyland@island.COM (Robert Leyland), who was a programmer for the
most excellent Sega CD Rom game - Flying Aces. (Plug!Plug!
Plug!Plug!Plug!Plug!)

-- The Sega CD is ISO 9660 format, with proprietary start-up code
that must be licensed from SEGA in order to work. For programming
you use two PC's, one connected to a debugging system and the other
emulated an CD-ROM drive.
The trickiest part of programming the Sega CD/Genesis combo is
keeping the two CPU's synchronized. For some reason the single
hardest thing to do with any computer is get it to talk reliably
with another computer.
--
Sega uses track 0 on the CD as check program. This is where
you can find the boot code to synch the CPUs as well as the
language code.
This is the same for the Saturn.

------------------------------------------------------------

2.2 Joystick - 3 button version - Could some please post a 6
button version?

By: nhowland@matt.ksu.ksu.edu (Neal Howland) for information
on the Joystick port.

First some background info:  The chip inside the controller
is a 74HC157.  This is a high-speed cmos quad 2-line to
1-line multiplexer. Basically how this works is there are two
inputs ( A and B ) for every output ( Y ).  There are four
groups like this.  There is one select signal for the whole
chip.  When the select signal is low, the output ( Y ) is the
same as input A.  When the select signal is high, the output
Y is the same as input B.  The pinout for the chip is as
follows:

   Pin 1 - SelectPin 16 - Vcc (+5V)
   Pin 2 - 1APin 15 - G (? must be low)
   Pin 3 - 1BPin 14 - 4A
   Pin 4 - 1YPin 13 - 4B
   Pin 5 - 2APin 12 - 4Y
   Pin 6 - 2BPin 11 - 3A
   Pin 7 - 2YPin 10 - 3B
   Pin 8 - GndPin 9 - 3Y


All the controls are done with switches.  Up is a switch,
Down is a switch, etc. Now, I will be referring to the output
of these switches later on. What I mean is that the output is
usually high, that is when the switch isn't pressed.

When the button is pushed, the output goes low.  This is
accomplished by connecting the output to +5V through a 10k
resistor. The button is then attached between the output and
ground. It looks like this:

     +5V -----/\/\/------+--------- Output
               10k       |
                         |
                /        |
   Ground -----/  -------+
              button
              (normally open)

For all of those who could actually decipher the above
schematic, congratulations!


I will now run down what lines from the plug are connected to
what. The line numbers are determined as follows, looking
straight at the plug on the front of the Genesis the numbers
are:

   1 2 3 4 5
    6 7 8 9

  (For those of you who buy a joystick cable from radio shack
the pin #'s to wire colors are as follows: 1-white 2-blue
3-green 4-brown 5-yellow 6-orange 7-red 8-black 9-gray )

anyway, line connections:

Line 1 - Up output      These are the only two direct
Line 2 - Down output             connections

Line 3 - Pin 4 of the chip output 1Y
Line 4 - Pin 7 of the chip output 2Y

Line 5 - This line carries in +5V. It is connected to the
         +5V bus line.

Line 6 - Pin 9 of the chip output 3Y
line 7 - Pin 1 of the chip this carries in a select signal
         from the Genesis. This is a signal which varies
         rapidly and controls which input goes through the
         output.

Line 8 - Ground  This is connected to the Ground bus line.
Line 9 - Pin 12 of the chip output 4Y

Now for the chips pin connections:

Pin 1 - Line 7    (select)
Pin 2 - Ground    (1A)  Don't ask me why they do this. Maybe
Pin 3 - Left      (1B)  future expansion
Pin 4 - Line 3    (1Y)
Pin 5 - Ground    (2A)  Again, possibly future expansion
Pin 6 - Right     (2B)
Pin 7 - Line 4    (2Y)
Pin 8 - Ground    (GND)
Pin 9 - Line 6    (3Y)
Pin 10 - Button B (3B)
Pin 11 - Button A (3A)
Pin 12 - Line 9   (4Y)
Pin 13 - Button C (4B)
Pin 14 - Start    (4A)
Pin 15 - Ground   (G)   This must be connected to ground
Pin 16 - +5V      (Vcc) Power source for the chip

Anyway that's all the info needed to build your own joystick.

Now as an added bonus, additional information!

A simple source for a joystick cable is the Radio Shack
joystick extension cable. It is around $5 and is 10 ft. long.
Just snip off the connector that won't plug into the Genesis,
strip the wires back, and use the color pinout list I gave
above.

I went to my local arcade game repair company today and
purchased the supplies I needed.  They were much cheaper than
I expected.  Things you would need to buy from them would be:

   an 8-way joystick     this ran me $15
        3 buttons             $2.50 apiece

I only bought three buttons because I am going to use a Radio
Shack push button switch for my start button. You can purchase
4 arcade game buttons if you wish.

Interesting ideas for extras on the joystick:

1.  Autofire can be achieved with a simple 555 timer circuit.

    For anyone wanting a challenge it should be possible to
build a digital autofire using a divide-by-n counter with the
select signal as clock input.

2.  Slow down mode is just autofire for the start button.

=============================================================

3.0 Companies - these companies produce the hardware that is
required to write games for the Genesis. The original
document came from: Scott Golby [sgolby@st.nepean.uws.edu.au]

   Hi Everyone,

   Here is the current list of info I have about the Sega.

   I have had about 10 replies so far asking to pass my
info on.  If people want I will pass their EMail address on
to everyone else so we can work as a group.  Drop me a line
if you want to be added to the 'group' list.

*****    [----- Five astrict like this means info from
different people

   Sega Genesis Developer Resources

   Dan Chang April 18, 1993

Sega of America Inc
Consumer Products Division
125 Shoreway Road
San Carlos, California, 94070
Tel : (415) 802-4400
Fax : (415) 802-4458

Stuart Kosoy is the person to contact at Sega of America
at (415)802-4407.

Products : Detailed SEGA Genesis technical information.  GEMS
SEGA music development package.
Solution : Music and Sound effects package for the Sega. SOA
can provide detailed information on programming the Genesis;
however they will want to know more about your company first.

Western Technologies, Inc.
12057 Jefferson Blvd
Culver City, California, 90230
Tel : (310) 821-7880
Fax : (310) 306-1739

Products : SegaDev Card. PC-Hosted Genesis development
hardware, which includes SEGADEV.EXE, a Windows-hosted
source-level debugger, and SEGALOAD.EXE, a PC_hosted 68000
object code downloader.
Solutions : SEGA Genesis Hardware, WT recommends Sierra
Systems 68000 Assembler.


Sierra Systems
6728 Evergreen Ave
Oakland California 94611
Tel : (510) 339-8200  or (800) 776 4888
Fax : (510) 339 3844

Products : PC-hosted Sierra C cross-compiler, which included
68000 C compiler, assembler, linker.

Contact person at Sierra Systems is Larry Rosenthal
E-Mail Sierra@netcom.com


Motorola, Literature Dist

This entry basically has a list of 68000 reference and
programming books.


Echidna
1101 W Stevens Ave, Suite 232
Santa Ana California 92707
Tel : (714) 545-2662
Fax : (714) 545-3705

Products : tUME - the Universal Map Editor. PC-Hosted tool to
edit tile-maps.
Solutions : Saves time in designing game maps and placing
level information. Downloads maps to Western Techs SegaDev
card.

A tUME demo can be found by FTP busop.cit.wanye.edu under
/pub/megadev/incoming/tumedemo.zip
Well worth a look.


Cross Products Limited
23 The Calls
Leeds LS2 7EH, England
Tel 0532 429814
Fax 0532 426163   (dial 011 44 532 426163 from USA)

Product : SNASM68K : PC-Hosted SEGA Genesis development
system, which includes a 68000 assembler, linker, debugger.


Parsons Engineering
Imhurst Ave
Covina California 91724
Tel (818) 966 5538

Products : PC/Apple IIgs hosted Sluggo Genesis development
hardware.


Advantech
1333 E.9400 South, Suite 160
Sandy Utah, 84092
Tel : (801) 572-5410
Fax : (810) 572-5674

Products : blank SEGA Genesis cartridge PC boards, blank
EPROMS.

*****
Genesis Development System Frequently Asked Questions

Q:What is the Genesis Development System?

A:The GDS is a cross development system that lets you develop games and other
  software to run on the Sega Genesis. The system consists of a 4 inch by 8
  inch printed circuit board that plugs in the cartridge slot of the Genesis,
  and has a cable going to the parallel port of the host computer. A complete
  software set is included consisting of an integrated editor/assembler, a
  stand alone assembler, a symbolic debugger, a music and sound effect 
editor,
  a utility to break standard 16 color IFF images into tiles to be downloaded
  to the Sega, a utility to capture screen graphics on the Genesis, a
  utility to send an LBM image to the Genesis for preview, a linker for
  joining multiple object modules, and several example files.

Q:Does it come with any source files?

A:You get source to the music editor, source to a Z80 program to run on the
  genesis to interpret the music files, source to a demo program that plays
  music and has sprites bouncing around behind the Sega logo, source to the
  download utilities. Complete source is provided on communicating between
  the host machine and the Genesis.

Q:How much memory does it have on the board?

A:It comes with 2.25 or 4.25 megabytes of memory. Older versions had 1.25
  megabytes. 256K is reserved for use by the debugger for variable
  storage and symbols.

Q:Can you program CD games with it?

A:It is not specifically designed for a CD. You could program as if you're
  creating a cartridge then modify the code later to make it work on the
  CD. I provide no information on the Genesis CDROM drive. I don't own one
  myself. The Genesis CDROM market is perhaps 10% of the cartridge market.

Q:Does it include technical specifications for the Genesis?

A:Yes, it comes with descriptions of the memory map, graphics registers, the
  Z80's functions, reading the joysticks, sprites, scrolling, tiles, the
  DMA copier, the audio chips-in short everything required to program the
  genesis. Also provided are several working demos with source.

Q:Is this proprietary information stolen from Sega?

A:No, the information on the Genesis came from reverse engineering it,
  completely independent of Sega. There are no copyright violations in the
  purchase or sale of this development system, nor is there anything illegal
  about any use of it.

Q:How does it work?

A:The board has ROMs on it which contain the monitor and debugger that runs
  on the Genesis. When the Genesis is reset, the ROMs take over and wait for
  commands to come from the host machine-commands like download bytes, send
  bytes back, set up registers, define a symbol, or enter the interactive
  debugger. When in the interactive debugger the host machine acts as a
  terminal, and characters are sent over the cable. Full 68000 debugging
  is possible, including disassembly, single step, register view and modify,
  breakpoints, expression evaluation, memory view, modify, search and copy,
  symbol definition, memory compare. There is also a z80 disassembler built
  in.

Q:What about the assembler?

A:The assembler is a full macro 68000 assembler, and includes the full 8080
  and parts of the Z80 instruction set. It generates symbol table information
  that can be downloaded to the Genesis to allow for symbolic debugging.
  On a 66 Mhz DX2 the assembler is about 550,000 lines/minute. There is also
  a linker which combines multiple object files.

Q:Can I program in C?

A:There is no C compiler included in the package. In the future there
  may be.

Q:How do I make artwork? Is there a sprite editor?

A:Originally I planned on including a simple graphic editor, but I found that
  no one used it-instead they used Dpaint. The best way is to use dpaint to
  create graphics in lo-res 16 color mode, then to use the utility I provide
  to convert them to raw data to be sent to the Sega. Animation can be done
  within Dpaint. Unfortunately there is no editor for manipulating giant
  figures made out of sprites, but in principle it would be possible to do. 
So
  probably you will use dpaint or some other painting program to create the
  artwork.

A:Can you backup existing cartridges like the game backup systems?

Q:The system is intended for game development. However if you were able
  to obtain ROM images, they can be downloaded just like original code.
  When running a ROM under the system, you can tinker with memory, modify
  gameplay, add lives, etc. You can do all the normal debugging functions,
  like single step, breakpoints, etc, to see how the program is working.
  In the future (Mid March, 1994) a reader will be available for $100 which
  will let you read out the contents of your cartridges.

Q:How fast are downloads?

A:The current board has transfer rates of up to 105K bytes per second. Older
  versions of the board had a 49K bytes per second transfer rate.

Q:What host machines is this available on?

A:Currently the complete system is available on the IBM PC. There is also
  an Amiga version but it has fewer features than the PC version.

Q:Why should I buy your system and not go through Sega?

A:My system is better and cheaper than those provided by Sega. I sell to
  anyone. Sega sells only to large companies. I require no non-disclosure
  statements to be signed. Sega requires you to sign extensive non-disclosure
  statements, and you essentially have to be subservient to them forever.
  If you buy my system you immediately have the ability to create software
  for the Genesis. If you create a game, you can then sell it outrifht or
  attempt to publish it yourself. You wouldn't have to pay Sega any of their
  rediculous royalties on each unit, regardless of how well your program
  sells.

Q:How long has this system been available?

A:It first became available in April 1991. The 2 and 4 megabyte versions
  first became available in February, 1994.

Q:How many have already been sold?

A:I've sold around 30 units so far.

Q:How much does it cost?

A:For the 4 megabyte version the cost is $1600. For the 2 megabyte version
  the cost is $1500. For the duration of March, 1994 there is a special
  introductory price of $1000 for the 2 megabyte version and $1100 for the
  4 megabyte version. After March, 1994 the price will be raised by $500.

Q:Will it be ported to other platforms?

A:There are no plans to port the software to any other platforms.

Q:What about technical support? How about software upgrades?

A:You can send me email and I will answer questions and help solve your
  problems. If I'm home you can call me. Concerning software support: I want
  you to be happy with this. If you want some program written, and it seems
  like a good idea, chances are I'll do it for free and include it with the
  system. Currently if the software seems sparse it's because no one has
  complained so I guess I include enough. There's no point in writing
  software if it isn't going to be used. In any case software upgrades are
  free unless it's some major upgrade like including a C compiler-I don't
  believe in selling something for hundreds of dollars then nickel and diming
  you on upgrades. EPROM upgrades are free if you send back your originals.

Q:Do we need to pay you any royalty on games developed with the system?

A:No. Once you bought the system you can do anything you want with it-except
  copy it and sell it yourself...

Q:How can I contact you?

A:Send me mail, email or call.
  818-584-0357
  INTERNET: dash@netcom.com
  David Ashley
  395 Sierra Madre Villa
  Pasadena, CA 91107
  USA

*****

=============================================================

4.0 Code samples. The following code was graciously given to
me by David Ashley, inventor of the GDS. We all owe him a big
Thanks!

Here's sample code for reading the joystick:
------------
;jsinit must be called to set up the joystick, otherwise
reading back will ;have errors. This need only be called
once.
jsinit:     moveq #$40,d0
      move.b      d0,$a10009
      move.b      d0,$a1000b
      move.b      d0,$a1000d
      rts
;The following two routines can be used for reading from the
joystick
;ports.
;porta and portb return a byte in d0. 0 bits mean button is
not down,
;1 mean button is down.
;The order is:
;76543210
;SACBRLDU
;meaning Start, A, C, B, Right, Left, Down, Up
porta:      move.b      #$40,$a10003
      nop
      nop
      move.b      $a10003,d1
      andi.b      #$3f,d1
      move.b      #$00,$a10003
      nop
      nop
      move.b      $a10003,d0
      andi.b      #$30,d0
      lsl.b #2,d0
      or.b  d1,d0
      not.b d0
      rts
portb:      move.b      #$40,$a10005
      nop
      nop
      move.b      $a10005,d1
      andi.b      #$3f,d1
      move.b      #$00,$a10005
      nop
      nop
      move.b      $a10005,d0
      andi.b      #$30,d0
      lsl.b #2,d0
      or.b  d1,d0
      not.b d0
      rts
-----------------------

Graphics code is another matter. Since I sell a development
system, and one of the features is I provide technical info
on the Genesis hardware, it would sort of be a conflict of
interest for me to give things away :^).

-----------------------

The whole sample is not inlcuded because of it's size.
However you can find the rest of the code, about 5 pages worth, at the
same locations as this file. Look for a file called code.txt.
These files will always be posted together.

        dc.l $FFFE00,$D7E,$200,$200,$200
        dcb.l $17,$200
        dc.l $E9E,$200,$EAE,$200,$20E
        dcb.l $F,$20E
        dcb.l $F,$200
        dc.b "SEGA MEGA DRIVE (C)SEGA "
        dc.b "1989.AUG _____",$87,"V        "
        dc.b "                        "
        dc.b "              ",$87,"V        "
        dc.b "                        "
        dc.b "        GM 00054010-01",$93,"G"
        dc.b "JD              ",$0,$0,$0,$0,$0,$3,"",$FF
        dc.b $0,"",$0,$0,$0,"",$FF,"                "
        dc.b "                        "
        dc.b "                        "
        dc.b "JUE             "
        move #$100,$A11100

=============================================================

4.11 Four Way Player Tap from Electonic Arts. While no code
is prodived, here is a hack of sorts. I am honoring the request
of anonymity from the sender. The $__ are ports calls by the
software to control the hardware (of couse).

With the EA 4 way play adapter, all the controllers are read through
controller 1's port after sending the selection data out of controller
2's port. The selection data is $0C for controller 1, $1c for controller
2, $2c for controller 3, and $3c for controller 4.

The adaptor is detected by setting the registers in the following way.
(This is from G______ C____ code, so don't ask me why they did some of
this stupid stuff.)

$40-]CTRL1
$40-]CTRL2
$43-]CTRL2 (This doesn't make sense... why set it twice?)
$7C-]DATA2 (This is probably masked off by the $43 above...)
$7F-]CTRL2
$7C-]DATA2

If you read DATA1 and either of the low 2 bits is set, there isn't a 4
tap. You must enable writing to port two after that so that you can
select the port.

Well this is the view from inside the machine... hope this helps you
somewhat.

=============================================================

4.2 Graphics, Color Palette and sprit control


Hi,

I found a file called 'MegaDrive Documentation' on a BBS in Holland.
I dunno which format it use (maybe an Amiga one) but it's easily redable.
You'll maybe find some info in it for your faq. So here it is...

Regards,

David Delabassee [delabass@nic.INbe.net]

==============================================================

DATABASE
NODE MAIN "Table of Contents"


Table of Contents:

{"About" LINK About }
{"MegaDrive MAP" LINK Megarivemap}
{"The VRAM" LINK TheVRAM }
{"The Tiles 8*8" LINK TheTiles8*8}
{"Horizontal Scrolls" LINK HorizontalScrolls}
{"The CRAM" LINK TheCRAM}
{"The Screens Maps" LINK TheScreensMaps}
{"Priority of Plans" LINK Priority}
{"The SRAM" LINK TheSRAM}
{"Sprites List" LINK SpritesList}
{"Access to the VRAM" LINK AccesstoVRAM}
{"Video Registers" LINK Video-Registers}
{"Interruptions" LINK Interruptions}
{"Exceptions of 68000 of the GENESIS" LINK Exceptions}


{"More InFormAtions" LINK Informations}



NODE  About "About"

      *     MEGADRIVE DOCUMENTATION VERSION 1.0 *

      Date   : 09-02-94
      Author : All Writen by MuMBlY / {"MYSTIC" LINK Informations}


ENDNODE

NODE  Megarivemap "megadrive map"

----------------------------------------------------------------------
                  MEGADRIVE MAP
----------------------------------------------------------------------

            000000-3FFFFF : 4 Megas Rom Cardtrige
            400000-9FFFFF : Reserved
            A00000-AFFFFF : I/O Ports
            B00000-BFFFFF : Reserved
            C00000-DFFFFF : Vdp
            E00000-FEFFFF : Ram Image
            FF0000-FFFFFF : 64kb RAM

ENDNODE

NODE TheVRAM "The VRAM"

The VRAM
----------------------------------------------------------------------

The 'RAM video' (64 kb) is used for countaining all informations of
the screen.

- Caracters (tiles) of 8*8 pixels in 16 colors (32 bytes per caracters).
- Screens (2 screen of 32*32 to 128*128 in 64 colors).
- Attributs of Sprites (maximum of 80 sprites).
- Horizontal pointers of scrollings (1 pointer per line).

ENDNODE

NODE TheCRAM "The CRAM"

----------------------------------------------------------------------
                  The CRAM
----------------------------------------------------------------------

The 'RAM color' (128 bytes) is used for containing the 4 palettes of 16
colors (1 word per color).

- Each Color is coded on 12 bits:
  Bit 11-8 : Bleue Value
  Bit 7-4  : Green Value
  Bit 3-0  : Red   Value

THe least signicative bit of each value is ignored, that is given a palette
of 8*8 values =3D 512 differents colors

ENDNODE

NODE TheSRAM "The SRAM"

----------------------------------------------------------------------
                  The SRAM
----------------------------------------------------------------------

The 'Scroll RAM' (128 Bytes ?)is used for countaining values of vertical
scrolling.

ENDNODE

NODE TheTiles8*8 "The Tiles 8*8"

----------------------------------------------------------------------
                  The Tiles 8*8
----------------------------------------------------------------------

The Tiles of 8*8 pixels in 16 colors.
Each tile is longer of 32 bytes.
The Format is simple : 1 longer word per line*8lines.
Each Pixel is represented in an half-byte.

Example : dc.1$01111100 ; letter A in Color 1 !
        dc.1$11000110
        dc.1$11000110
        dc.1$11111110
        dc.1$11000110
        dc.1$11000110
        dc.1$11000110
        dc.1$00000000
ENDNODE

NODE TheScreensMaps "The Screens Maps"

----------------------------------------------------------------------
                  The Screens Maps
----------------------------------------------------------------------

The GENESIS is able to manage 2 SUperposed screens (Playfields).
Each palyfield can do of 32*32 tiles until 128*128 tiles.
Each tiles is coded on 1 word and are consecutive in the screen map.

The Format is
      Bit 15    : {"Priority" LINK Priority}
        Bit 14-13 : Palette  ( 0 to 3 )
        Bit 12    : Vertical Flipping
        Bit 11    : Horizontal Flipping
        Bit 10-0  : Tile number ( 0 to 2047 )

Example :
    dc.w $8001 ; Priority =3D 1, Pallette =3D 0, no Flipping, Tile number #1

ENDNODE

NODE Spriteslist "Sprites LIst"

----------------------------------------------------------------------
                  Sprites List
----------------------------------------------------------------------

The GENESIS is able to manage until 80 sprites on the screen.
The only limit is of 20 sprites per line of pixel and of 320 pixels
of sprites per line. ( ya can display 20 sprites of 16 pixels of large
with the same Y-axis but which 10 sprites of 32 pixels of large )

The sprites used the same tiles that the screen.
THose lastest can do of 1*1 tiles to 4*4 tiles.

Format of the list of sprites to display (8 bytes per sprite) :

byte 0-1 : Y-axis (0 to 511)+$80
byte 2   : SIze    (bit 0-1 : height 0-3+1 , Bit 2-3 : width 0-3+1)
byte 3   : Link Data ( next sprite in the list or 0 for end of list)
byte 4-5 : Fisrt number tile sprite (See farther !)
bute 6-7 : X-axis (0 to 511)+$80

It indicate that the first tile of sprite b'coz the GENESIS deduces
automaticaly the next as that :

      Tile #1:Tile #4 ; Sprite of 16 pixels on 24 pixels
      Tile #2:Tile #5
      Tile #3:Tile #6
=09
The format of tile number is same at format of screens
That's limit the sprites to 16 colors (maxi!).
The Flipping is indicated for the complete sprite.

example List :
=09
 dc.w $0+$80  ; Y-axis in the top of screen (0)
 dc.b %0101   ; Size 16*16 pixels
 dc.b 1       ; Number of the next sprite in the List for the priority
 dc.w $4001   ; Palette =3D 2, First tile =3D 1, No Flipping=
 (Tiles#1,#2,#3,#4)
 dc.w $0+$80  ; X =3D 0
 dc.w $40+$80 ; Y =3D 64
 dc.b %0001   ; Size 8*16 pixels
 dc.b 0       ; Last sprite
 dc.w $5001   ; Palette 2, Vertical Flipping, Tile #1 (Tile #1,#2)
 dc.w $40+$80 ; X =3D 64

 the order of sprites in the list indicates the priority of sprites between
 them : the first sprite is First Plan.
ENDNODE

NODE HorizontalScrolls "Horizontal Scrolls"

----------------------------------------------------------------------
                  Horizontal Scrolls
----------------------------------------------------------------------

The GENESIS is able to manage an horizontal offset for each line.

The Format for each horizontal offset :

 1 word for each offset of playfield A
 1 word for each offset of playfield B
 ...

 Example :

 dc.w $0001 ; A move of 1 pixel about right for playfield A
 dc.w $ffff ; A move of 1 pixel about left for playfield B
 dc.w $0002 ; 2 pixels to right for A (second line)
 dc.w $0003 ; 3 pixels to right for B (second line)
 ...

 it's possible to have a pointer which functions for all screen or for all
 tiles or for all lines.
 The numbber of pointer is in function of choose mode.
ENDNODE

NODE Priority "Priority of Plans"

----------------------------------------------------------------------
                  Priority of Plans
----------------------------------------------------------------------

Each Tiles of each screen and each sprite countains a bit of priority.
That settles the priorities of plans between them and the sprites with
plans.
When all bits are to 0 : the sprites are on the screen A which is on the
screen B. Each bit that we have putting to 1, does moving the plan.

Example :
 Screen B=3D1, Screen A=3D0, Sprite=3D0
 The screen B pass in front of the sprites which goes in front of the
 screen A.
ENDNODE

NODE AccesstoVRAM "Access to the VRAM"

----------------------------------------------------------------------
                  Access to the VRAM
----------------------------------------------------------------------

On MAnual Mode (without SRAM) :
2 ports are used : 1 for the address and 1 for the data.
the two ports are on 32 bits and the address increases itself
automaticaly.
the only prolem is that it must to code the address which follows the type
of RAM that we want to address.

For the VRAM : Address 0-FFFF

 ((address and $3fff)+$4000)*$10000+(($1c000 and address)shr 14)

For the CRAM : Address 0-7F

 ($c000+address)*$10000

For the SRAM : Address 0-7F ?

 (($3fff and address)+$4000)*$10000+$10

 The address is putting in the vdp_cmd ($c00004)

 The Data is putting in the vdp_dat ($c00000)

Example :

 move.1#$c0000000,vdp_cmd ; Address 0 of the CRAM
 move.1#$c0000eee,vdp_dat ; color 0 and 1 of the first palette (black&white)
 move.w#$0444,vdp_dat ; color 2 of the first palette (Dark Grew)
ENDNODE

NODE Video-Registers "Video Registers"

----------------------------------------------------------------------
                  Video Registers
----------------------------------------------------------------------

the write access to video regiters does by the vdp_cmd
it codes on a word :

      Bit 15=3D1 : indicates that's a register
      Bit 14-8 : number of register
      Bit 7-0  : Value to write in the register

Example :

 move.w#$8000,vdp_cmd ; Register #0=3D0
 move.1#$81ff8200     ; Register #1=3Dff, register #2=3D0

Register #0

 Bit 7 : 0
 Bit 6 : 0
 Bit 5 : 0
 Bit 4 : Control interrupt H(Niv 4) 1:ON 0:OFF
 Bit 3 : 0
 Bit 2 : 1
 Bit 1 : Control mode screen ?
 Bit 0 : 0

Register #1

 Bit 7 : 0
 Bit 6 : TV 1:ON 0:OFF
 Bit 5 : Control Interrupt V(Niv 6) 1:ON 0:OFF
 Bit 4 : DMA 1:ENABLE 0:DISABLE
 Bit 3 : 0:NTSC(28 lines Cell) 1:PAL(30 lines Cell)
 Bit 2 : 1
 Bit 1 : 0
 Bit 0 : 0

Register #2

 Address of base of the Screen MAP A/$400 (that all the $2000).

Register #3

 Address of base of the window/$400 (Third Screen)   (D000)

Register #4

 Address of base of Screen MAP B/$2000               (E000)

Register #5

 Address of base of the list of sprites/$200         (BC00)

Register #6

 Not used (0)

Register #7

 Background Color (0 to 63) : 'Border' color.

Register #8 and #9

 Not used (0)

Register #10

 Line counter per H interrupt (0 to 255).

Register #11

 Bit 7-4 : 0
 Bit 3   : Level 2 Interrupt 1:ON 0:OFF
 Bit 2   : Vertical Scroll Control 1: An offset per 16 pixels
                           0: An Screen offset
 Bit 1-0 : Horizontal Scroll Control
            00 : An Screen offset
            01 : Not Used
            10 : An offset per cellule
            11 : An offser per line

Register #12

 Bit 7 : Must be as the bit 0
 Bit 6 : 0
 Bit 5 : 0
 Bit 4 : 0
 Bit 3 : Shadow effect between Playfields and the sprites 1:ON 0:OFF
 Bit 2 : Vertival Resolution (224 or 448 pixels)
 Bit 1 : Entrelaced ( only on entrelaced : 448 pixels are possible)
 Bit 0 : 32 Tiles per line or 40 tiles per line (320 or 256 pixels)

Register #13

 Address of base for Horizontal Scrolls/$400         (B800)

Register #14

 Not used (0)

Register #15

 Increase offset of vdp_cmd in word

Register #16

 Bit 7-6 : 0
 Bit 5-4 : Size Vertical Screen Maps A and B 00:32 01:64 10:USED 11:128
 Bit 3-2 : 0
 Bit 1-0 : SIze Horizontal Screen Maps A and B ( Same format as Vertical)

Register #17

 Horizontal Dimension Window

Register #18

 Vertical Dimension Window

Register #19                                         (93)

 Less weight longer transfert DMA

Register #20                                   (94)

 Strong Weight longer transfert DMA

Register #21                                         (95)

 DMA Source Less Weight

Register #22                                         (96)

 DMA Source Strong Weight for VRAM or Middle Weight for RAM

Register #23                                         (97)

 DMA Control

 Bit 7   : Source 1:VRAM 0:RAM
 Bit 6   : Source 1:ON 0:OFF (or Bit 23 Source RAM)
 Bit 5-0 : RAM Source ( Bit 22-17)
 In thiz case of RAM Source, the bits 6-0 are considerated as the strong
 weight of the source.
ENDNODE

NODE Interruptions "Interruptions"

----------------------------------------------------------------------
                  INTERRUPTIONS
----------------------------------------------------------------------

 The interruptions are of the 68000 : (!!!!!!)

 Level 2 : External Interrupt (???)
 Level 4 : H interrupt (Interruption Raster Runs by the VDP)
 Level 6 : V interrupt (Interruption Vertical Blanck)

 ...
ENDNODE

NODE Exceptions "Exceptions of 68000 of the GENESIS"

*****************************************************************
            EXCEPTIONS OF 68000 OF THE GENESIS
*****************************************************************

      $00   SYSTEM SSP
      $04   SYSTEM PC
      $08   BUS ERROR
      $0C   ADDRESS ERROR
      $10   ILLEGAL INSTRUCTION
      $14   DIVISION BY ZERO
      $18   CHK EXCEPTION
      $1C   TRAPV EXCEPTION
      $20   PRIVILEGE VIOLATION
      $24   TRACE EXCEPTION
      $28   LINE-A EMULATOR
      $2C   LINE-F EMULATOR
      $30   RESERVED BY MOTOROLA
      $34   RESERVED BY MOTOROLA
      $38   RESERVED BY MOTOROLA
      $3C   RESERVED BY MOTOROLA
      $40   RESERVED BY MOTOROLA
      $44   RESERVED BY MOTOROLA
      $48   RESERVED BY MOTOROLA
      $4C   RESERVED BY MOTOROLA
      $50   RESERVED BY MOTOROLA
      $54   RESERVED BY MOTOROLA
      $58   RESERVED BY MOTOROLA
      $5C   RESERVED BY MOTOROLA
      $60   SPURIOUS EXCEPTION
      $64   INTERRUPT REQUEST LEVEL 1
      $68   INTERRUPT REQUEST LEVEL 2
      $6C   INTERRUPT REQUEST LEVEL 3
      $70   INTERRUPT REQUEST LEVEL 4 (VDP INTERRUPT)
      $74   INTERRUPT REQUEST LEVEL 5
      $78   INTERRUPT REQUEST LEVEL 6 (VERTICAL BLANK)
      $7C   INTERRUPT REQUEST LEVEL 7
      $80   TRAP #00 EXCEPTION
      $84   TRAP #01 EXCEPTION
      $88   TRAP #02 EXCEPTION
      $8C   TRAP #03 EXCEPTION
      $90   TRAP #04 EXCEPTION
      $94   TRAP #05 EXCEPTION
      $98   TRAP #06 EXCEPTION
      $9C   TRAP #07 EXCEPTION
      $A0   TRAP #08 EXCEPTION
      $A4   TRAP #09 EXCEPTION
      $A8   TRAP #10 EXCEPTION
      $AC   TRAP #11 EXCEPTION
      $B0   TRAP #12 EXCEPTION
      $B4   TRAP #13 EXCEPTION
      $B8   TRAP #14 EXCEPTION
      $BC   TRAP #15 EXCEPTION
      $C0   RESERVED BY MOTOROLA
      $C4   RESERVED BY MOTOROLA
      $C8   RESERVED BY MOTOROLA
      $CC   RESERVED BY MOTOROLA
      $D0   RESERVED BY MOTOROLA
      $D4   RESERVED BY MOTOROLA
      $D8   RESERVED BY MOTOROLA
      $DC   RESERVED BY MOTOROLA
      $E0   RESERVED BY MOTOROLA
      $E4   RESERVED BY MOTOROLA
      $E8   RESERVED BY MOTOROLA
      $EC   RESERVED BY MOTOROLA
      $F0   RESERVED BY MOTOROLA
      $F4   RESERVED BY MOTOROLA
      $F8   RESERVED BY MOTOROLA
      $FC   RESERVED BY MOTOROLA
ENDNODE

NODE InFormAtions "More InFormAtions"

=============================================================

The 68000 keeps all of it's interrupt vectors in the first 1k of memory
(this is cartridge ROM!) The first 10 longwords are:
Stack pointer after reset
Program counter after reset
Bus Error
Address Error
Illegal Instruction
Division by Zero
CHK instruction
TRAPV instruction
Priviledge violation
Trace

The external interrupts start at $60 (vector 25-31) and end at $7F. Now
all videogame units have a variety of interrupts of VBlank and HBlank. If
you poke arount the interrupt table with the hints I've given you, and
follow those addresses into the code, you can probably find the VBLANK
and HBLANK routines for any given game. The vblank routines often read
the controllers, do DMA, access video RAM, etc. so this is a good short
cut to find meaty code within the machine! (Look for nice round addresses
close together... wouldn't those be VDP registers?) HBLANK is even better
if a game uses it (look at any sonic game!) because they must be short
(and are thus easily identifiable when traced from the interrupt table
vectors), generally they only have one or two instructions to load their
data into the VDP before it will be displayed on the screen, so you
should immediately be able to figure out how to load palettes into the
Genesis.

Look atSonic the Hedgehog 1 dissassembly, following the interrupt vectors
to find the HBLank routine. (All interrupt routines should end in RTE.)

Notice there is one branch and then some set up and a lot of longword
writes. Try changing these writes to write constant data and then look at
some section of the game which uses Hblank interrupts. Betcha the colors
have changed...(oops... the game will hang because the checksum is
no longer good. You must find the checksum and step over it... trace
through initialization till you find it... I think it should end on a
nice round address, but it's been a while so I am not sure. Look for a
loop with an add in it and nothing else. The checksum is stored in a
place where changing it will not change the checksum, lots of other data
like that is there... good luck!) Anyway, if you disable the checksum and
change the data in hblank then play a board with water I think you will
understand how to change color palettes on the Genesis!

If you can figure this out, you can probably guess that writing other
data to the VDP is very similar. Look at the VBlank code to see what it
does with similar addresses to figure out how to write characters and
stuff. This is pretty convoluted, unfortunately. I can only suggest that
you look at very old games for this information as they are programmed
more simply.

I hope this helps you somewhat. I ask that you don't print this as I have
written it, and what you do print please don't mention me as I could get
in trouble. If you do some hacking and get stuck, let me know and I'll
give you some more hints. I hate not being able to say things directly,
because I think the Genesis is a wonderful piece of hardware and wish
everyone could know how great it is.

P.S. don't look at Road Rash, it will drive you INSANE! It seems to run
entirely in VBLANK and HBLANK! I figured out the 4-TAP by looking for
Joystick accesses in the VBLANK code... easy!

=============================================================

5.0 Language Switch, from Rec.Games.Video FAQ

How to make a language switch (Genesis/MD).

On a Genesis/MD, there are jumpers labelled JP1, JP2, JP4,
and JP3.  The Genesis has a capacitor on JP1 and a trace on
JP2; the Mega Drive has a capacitor on JP2 and a trace on
JP1.  The bottom ends of JP1 and JP2 are connected together.
So if you cut the trace and the top end of the capacitor, and
install a DPDT switch between them which reconnects them
either unchanged or swapped left to right, you have a
language switch. You'll need some wire, a soldering iron,
solder, and a DPDT switch.

Some machines have an open circuit instead of the capacitor.
Also, I've been told that even if there is a capacitor, you
can throw it out and leave an open circuit. Either way, the
switch is a lot simpler, requiring a SPDT switch and less
wire and solder.

Several people have told me that you could just cut both JP1
and JP2 and put a SPST switch on JP1. This is even simpler,
but I'm not sure it really works, as opposed to putting your
machine in an intermediate state that only sort-of works.

The redesigned Genesis 2 machines don't appear to have either
the capacitor or circuit. Nobody yet knows how to make the
language switch for one, though there are language switch
cartridges you can buy to act as one.

IF YOUR MACHINE HAS NO CAPACITOR (or if you want to cross
your fingers and throw away your capacitor) and is not a
Genesis 2:

Cut JP2.  The trace might be covered with paint and hard to
see.  (If you started with a Mega Drive, JP2 is open and you
have to cut JP1 instead.) If you aren't sure which end I mean
by "bottom", just check the back of the board to see which
end is connected together.
<CODE>
   Original state of machine:             After cutting:

       JP2 top     JP1 top              JP2 top     JP1 top
          |           |                    |           |
          |           |                    |           |
          |
          |
          |
          |
          |           |                    |           |
           \         /                      \         /
            \_______/                        \_______/
          bottom of both                   bottom of both

Add a SPDT switch which can be in one of two positions:
              ._ _ _ _ _ _ _ _ _ _ _ _
             .                        .
            .           . _ _ _ _ _ _ _ _ _ _
           .           .                .    .
       JP2 top     JP1 top               .    .
          |           |                  .    .
          |           |                 _________
                                        | o   o |
                                        | \     |
                                        |  \    |
                                        \___o___/
          |           |                     .
           \         /                      .
            \_______/ - - - - - - - - - - - -
          bottom of both

              ._ _ _ _ _ _ _ _ _ _ _ _
             .                        .
            .           . _ _ _ _ _ _ _ _ _ _
           .           .                .    .
       JP2 top     JP1 top               .    .
          |           |                  .    .
          |           |                 _________
                                        | o   o |
                                        |     / |
                                        |    /  |
                                        \___o___/
          |           |                     .
           \         /                      .
            \_______/ - - - - - - - - - - - -
          bottom of both
-------------------------------------------------------------

IF YOUR MACHINE DOES HAVE THE CAPACITOR:

Cut both sides.  (Note: if you started with a Japanese Mega
Drive the capacitor will be on the side labelled X instead)

   Original state of machine:             After cutting:

       JP2 top     JP1 top              JP2 top     JP1 top
          |           |                    |           |
          |           |                    |           |
          |           |
          |           |
          |           |                    |
        X |          ###                 X |          ###
          |          ###                   |          ###
          |          ###                   |          ###
           \         /                      \         /
            \_______/                        \_______/
          bottom of both

add switch which can be in one of two positions:

       JP2 top     JP1 top
(Connect 2 to 2
          |           |
and 1 to 1)
          |           |
          2           1                   2  1  1  2
                                        ______________
           _ _ _ _ _ _ _ _ _ _          | o  o  o  o |
          |                   `         |  \  \      |
        X |          ###`      `        |   \  \     |
          |          ### `      `       \____o__o____/
          |          ###  `      `           '  '
           \         /     `      `- - - - -'  '
            \_______/       ` _ _ _ _ _ _ _ _ '
          bottom of both

       JP2 top     JP1 top
(Connect 2 to 2
          |           |
and 1 to 1)
          |           |
          2           1                   2  1  1  2
                                        ______________
           _ _ _ _ _ _ _ _ _ _          | o  o  o  o |
          |                   `         |      /  /  |
        X |          ###`      `        |     /  /   |
          |          ### `      `       \____o__o____/
          |          ###  `      `           '  '
           \         /     `      `- - - - -'  '
            \_______/       ` _ _ _ _ _ _ _ _ '
          bottom of both

</CODE>
50/60 hertz switch.

You can build a 50/60 hertz switch on a Genesis/Mega Drive
like a language switch, but using jumpers JP3 and JP4.  The
standard setting is 50 in PAL areas such as Europe, and 60 in
NTSC areas like the US and Japan. In the 60 hertz mode, the
game is faster and the screen taller; however, not all TVs
and monitors in Europe can display this mode.


Some American/Japanese games are protected to keep Europeans
]from playing them; this protection checks the 50/60 hertz
setting.  You can usually get around it by installing the
switch and switching when starting the game, then switching
back afterwards.

Many European games are simple ports of American or Japanese
games and are not redesigned for 50 hertz, so work faster and
with "better" screen proportions if played at 60 hertz.


Genesis/Mega Drive dual version (language switch) list:

   *Battle Maina 2: The game plays in both positions. However
     with both controlers removed, a system info screen appears
     which informs about system version, langauge mode.
    Bonanza Brothers: Game plays in Japanese.  (Maybe. There
     seems to be more than one version floating around.)
    Columns: Game plays in Japanese.
   *Cyberball: Japanese version has a modem option.
    Dragon's Fury: Works only with language set to English.
     The original, Devil's Crush MD, works either way.
    Dynamite Duke: Harder on the Mega Drive.
    Elemental Master: Harder on the Mega Drive.
    Fatal Labyrinth: Game plays in Japanese.
    Fire Shark: Different title screen with Kanji.
    Flicky: Characters have Japanese names and instructions
     are in Japanese.
    Forgotten Worlds: Game plays in Japanese.
    Gaiares: only mentions the Japanese licensee on the title
     screen, and has Japanese text; you can also select
     Japanese text from the option screen.
    Ghostbusters: Game plays in Japanese.
    Ghouls and Ghosts: Different title screen with Kanji.  To
     see it on a Genesis, select the last music and sound (26
     and 56) from the options screen, then press lower left;
     A, B, or C; and Start all at the same time.  (I never
     tried this, but Gamepro magazine claimed it works.) The
     game shows some other Japanese text, and when you die
     during a boss you start out earlier.
    Insector X: Title screen refers to company as Hot-B, not
     Sage's Creation. The MD version shoots more slowly. The
     ending text is still English.
   *Marvel Land: The Japanese version says "for Mega Drive"
     or "for Genesis" but the language stays Japanese. (What
     does an English version do?)
    Monaco GP: Game plays in Japanese (also an option on the
     option screen).
    Mystic Defender: This game is actually the anime-based
     Kujaku-Ou (Peacock King) 2 game. In Japanese mode, the
     opening text is replaced by a graphics screen (never
     seen in the US version) with Japanese.  The levels have
     names, the main character wears a white robe, the
     lightning magic effect is different, and the character
     is named Kujaku in the ending (which is still English).
    Outrun: The attract mode lacks sound, the startup screen
     says "push" (not "press") start button, and "(C) Sega
     1986, 1991" is printed in reverse order. The default
     options are KM/H and a different button selection (but
     can still be changed on the option screen).
    Quackshot: Game plays in Japanese.
    Raiden Trad: The "licensed to Sega" line is absent on
     both title screens, and the second title screen includes
     only the Japanese part instead of the non-Japanese part
     of the first one.
    Revenge of Shinobi: Title changes to Super Shinobi;
     credits show at the end.
    Rolling Thunder II: The Japanese version only works on a
     Japanese setting. The US version works either way (and
     isn't bilingual).
    Sonic the Hedgehog II: Tails is renamed to "Miles".
    Streets of Rage: Title screen changes to Bare Knuckle,
     and all text is in Japanese, including the introduction.
     The clock resets when you encounter the bosses.
    Streets of Rage II: Turns to Bare Knuckle II, and renames
    Skate to Sammy --_if_ you change the setting sometime
     after turning the machine on (to skip the lockout).
    Thunder Force II: Title screen has "MD" on it, and
     company name is "Tecnosoft".
    Thunder Force III: company's name is spelled "Tecnosoft".
   *Thunder Storm FX (CD): Turns to Cobra Command in US mode.
    Truxton: Japanese title is Tatsujin.
    Twin Hawk: Different title screen with Kanji.
   *Wrestle War: The wrestler is blond on a Genesis and
     black-haired on a MD.

    * Information from testing a Japanese game

=============================================================

6.0 Sega Internet Information - This information came from
Bernd P. Ziller. Mailing to any of these site (or FTP, gopher,
etc.) will not work due to a firewall. They are looking out, but
we can't look in. I strongly suggest that you don't try anything
stupid.
<code>
 segaoa.com       198.176.13.13      130slmail        198.176.14.65
 loghost          198.176.13.13      localhost        127.0.0.1
 400mail          198.176.14.57      pd275            198.176.14.68
 trixie           198.176.9.40       torcable         198.176.14.53
 pd130            198.176.14.61      storm            198.176.11.47
 segaintera       198.176.14.52      segaoa           198.176.13.13
 sonic            198.176.13.13      blackhole        198.176.14.69
 multimedia       198.176.14.63      pd1              198.176.14.62
 segard           198.176.14.51      segaeurope       198.176.14.50
 tormail          198.176.14.54      si32             198.176.14.55
 sb1mail          198.176.14.60      macpo            198.176.14.67
 pluto            198.176.9.52       fremontml        198.176.14.56
 racerx           198.176.9.41       hummer           198.176.11.55
 303tdmail        198.176.14.64      sb2mail          198.176.14.58 
 ftp              198.176.13.13      gambit           198.176.11.46
 Emerald_Dragon   198.176.8.22
</code>
=============================================================

A Very Special Thanks to:

David Ashley [dash@netcom.com]

Rober Leyland [Leyland@island.COM]

David Delabassee [delabass@nic.INbe.net]

Special Thanks to :

Scott Golby [sgolby@st.nepean.uws.edu.au]

Rick McTeague [sysrick@starbase.spd.louisville.edu]

Landon Dyer [landon@apple.com]

Neal Howland [nhowland@matt.ksu.ksu.edu]

Bernd P. Ziller [bziller@ba-stuttgart.de]

=-=-=-=-=-=-=-=--=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

KX-P118017 Feb 94 09:30:55 +0100
From:
Subject: Re: What Language are games programmed in?
To: U6C16@WVNVM.BITNET
Message-id: [9402170830.AA16515@leda.pt.hk-r.se]
X-Envelope-to: U6C16@WVNVM.BITNET

Heres an extract from the game _____. I just included some of the beginning
of the code and some ASCII segments. I hope you are satisfied with this.
<code>



        dc.l $FFFE00,$D7E,$200,$200,$200
        dcb.l $17,$200
        dc.l $E9E,$200,$EAE,$200,$20E
        dcb.l $F,$20E
        dcb.l $F,$200
        dc.b "SEGA MEGA DRIVE (C)SEGA "
        dc.b "1989.AUG _____",$87,"V        "
        dc.b "                        "
        dc.b "              ",$87,"V        "
        dc.b "                        "
        dc.b "        GM 00054010-01",$93,"G"
        dc.b "JD              ",$0,$0,$0,$0,$0,$3,"",$FF
        dc.b $0,"",$0,$0,$0,"",$FF,"                "
        dc.b "                        "
        dc.b "                        "
        dc.b "JUE             "
        move #$100,$A11100
        nop
        rte
        bra.s $20C
        bra.s $20E
        move.l a6,d5
        lea $C00004,a5
        move.l #$940000,d4
        move d6,d4
        lsl.l #8,d4
        move #$9300,d4
        move.b d6,d4
        move.l d4,(a5)
        lsr.l #1,d5
        and.l #$7FFFFF,d5
        move.l #$960000,d4
        move d5,d4
        lsl.l #8,d4T        move.b d5,d4
        move.l d4,(a5)
        swap d5
        move #$9700,d4
        move.b d5,d4
        move d4,(a5)
        or #$80,d7
        swap d7
        move d7,(a5)
        swap d7
        move d7,-(a7)
        move (a7)+,(a5)
        rts
        move.l #$7C000002,$C00004
        move.l #$0,$C00000
        rts
        bsr $3BCC
        lea $FFFFC800.w,a6
        moveq #$0,d6
        move #$13F,d7
        move d6,(a6)+
        dbf d7,$282
        move.l #$78000002,d7
        move #$13F,d6
        bra $2B8
        move.l #$40000003,d7
        bra $2A6
        move.l #$60000003,d7
        move #$1FFF,d6
        bra $2B8
        move.l #$60000002,d7
        move #$DFF,d6
        moveq #$0,d5
        move #$8F01,$C00004
        lea $C00004,a5
        move.l #$940000,d4
        move d6,d4
        lsl.l #8,d4
        move #$9300,d4
        move.b d6,d4
        move.l d4,(a5)
        move #$9780,(a5)
        or.l #$40000080,d7
        move.l d7,(a5)
        move.b d5,$C00000
        move (a5),d4
        btst #$1,d4
        bne.s $2EC
        move #$8F02,(a5)
        rts
        move.l #$40000010,d7
        moveq #$28,d6
        moveq #$0,d5T        move d5,$C00000
        dbf d6,$30A
        rts
        lea $C00000,a5
        move.l #$800000,d3
        move.l d7,$C00004
        move d6,d2
        move (a6)+,(a5)
        dbf d2,$32A
        add.l d3,d7
        dbf d5,$322
        rts
        move d6,d2
        lea (a5),a4
        move d4,(a5)+
        addq #1,d4
        dbf d2,$33C
        lea $80(a4),a5
        dbf d5,$338
        rts
        bset #$6,$FFFFC127.w
        bra.s $36E
        move.l #$C0200000,$C00004
        move #$0,$C00000
        bclr #$6,$FFFFC127.w
        move $FFFFC126.w,$C00004
        rts
        bset #$0,$FFFFC15E.w
        moveq #$0,d0
        moveq #$0,d1
        move.b (a6)+,d0
        lea $FFFFC080.w,a1
        add d0,a1
        move.b (a6)+,d0
        move (a6)+,(a1)+
        dbf d0,$38C
        rts
        bclr #$0,$FFFFC15E.w
        beq.s $3CE
        btst #$6,$A10001
        beq.s $3AE
        move #$6EE,d0
        dbf d0,$3AA
        lea $C00004,a5
        move.l #$94009340,(a5)
        move.l #$96E09540,(a5)
        move #$977F,(a5)
        move #$C000,(a5)
        move #$80,-(a7)
        move (a7)+,(a5)
        rts
        moveq #$7,d1
        lea $FFFFC080.w,a6T        moveq #$E,d7
        move (a6),d4
        move d4,d2
        and d7,d2
        beq.s $3E4
        subq #2,d2
        asl #4,d7
        move d4,d3
        and d7,d3
        beq.s $3F0
        sub #$20,d3
        asl #4,d7
        and d7,d4
        beq.s $3FA
        sub #$200,d4
        or d2,d3
        or d3,d4
        move d4,(a6)+
        dbf d0,$3D8 
        bset #$0,$FFFFC15E.w
        moveq #$7,d2
        moveq #$1,d0
        jsr $F80(pc)
        dbf d2,$40C
        dbf d1,$3D2
        rts
        moveq #$1F,d0
        lea $FFFFC080.w,a2
        lea -$80(a2),a1
        move.l (a2)+,(a1)+
        dbf d0,$426
        moveq #$1F,d0
        clr.l (a1)+
        dbf d0,$42E
        rts
        moveq #$E,d1
        lea $FFFFC080.w,a6
        lea -$80(a6),a1
        moveq #$3F,d0
        move d1,d5
        moveq #$E,d7
        move (a1)+,d4
        move d4,d2
        and d7,d2
        sub d5,d2
        bpl.s $452
        moveq #$0,d2
        asl #4,d7
        asl #4,d5
        move d4,d3
        and d7,d3
        sub d5,d3
        bpl.s $460T        asl #4,d7
        asl #4,d5
        and d7,d4
        sub d5,d4
        bpl.s $46C
        moveq #$0,d4
        or d2,d3
        or d3,d4
        move d4,(a6)+
        dbf d0,$442
        cmp #$8020,$FFFFC142.w
        bne.s $48E
        lea $FFFFC0A0.w,a2
        lea -$80(a2),a1
        moveq #$7,d0
        move.l (a1)+,(a2)+
        dbf d0,$488
        moveq #$7,d2
        bset #$0,$FFFFC15E.w
        jsr $F7E(pc)
        dbf d2,$496
        subq #2,d1
        bpl.s $438
        rts


........


        move #$B4,$FFFFC150.w
        clr $FFFFC152.w
        moveq #-$79,d7
        jsr $748.w
        lea $16F8,a0
        jsr $14F4(pc)
        lea $1700,a0
        jsr $14F4(pc)
        lea $1296(pc),a6
        lea $FF03A0,a5
        jsr $197A
        lea $FF25A0,a5
        lea $1274(pc),a6
        bsr $19E8
        lea $FF359A,a5
        lea $127E(pc),a6
        bsr $19E8
        lea $FF37A2,a5
        lea $128E(pc),a6
        bra $19E8
        dc.b "GAME OVER",$0,"CONTINUE PLAY "
        dc.b "^",$0,"YES NO",$0,$0,"PLAYER 1",$0,$0,$0,$0,$0,$0

        dc.b "NUS@@@@@@@@@@@@@@@",$0,"MISSI"
        dc.b "ON COMPLETED BONUS@@",$0,"MIN"
        dc.b "//SEC",$0,"OVER/////SEC///NO/"
        dc.b "BONUS",$0,"UNDER////SEC",$0,"PTS",$0,"P"
        dc.b "TS",$0,"PTS",$0,$0
        dc.b "INFILTRATE ENEMY LINES?",$0
        dc.b $12,"RESCUE SECRET AGENT?",$0,$0,$14
        dc.b "GO TO THE ARSENAL?",$0,$8,"BLOW"
        dc.b " UP ALL THE ENEMY WEAPON"
        dc.b "S?",$0,$10,"PENETRATE THE FORTRE"
        dc.b "SS?",$0,$E,"RESCUE COLONEL TRAU"
        dc.b "TMAN?",$0,$0,"PLAYER 1 START",$0,$0

--EOF--

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