Defender Questions

YellowDog

In Memoriam

Donor 2011, 2013
Joined
Apr 12, 2008
Messages
9,851
Reaction score
158
Location
Humble, Texas
I swear I ran across this but can't find it now. Did somebody hack the Defender ROMs to work with a later MPU board? By later MPU board, I am talking about the Stargate, Joust or later MPU board.

I know the watchdog address need to be moved and a couple of other minor tweaks are necessary, but I would like to avoid re-inventing the wheel.

Basically there are a lot of issues with keeping Defender MPU boards running and I would like to be able to find a way to migrate the ROMs to a later MPU board that is more stable.

If anybody knows where those ROM images are, please let me know.

Thanks.

ken
 
Thanks Ian. That is a start. I had that link, but it was pointing to Sean's old website so the link was broken.

There was also a version for a multigame system, either Sean's or Clay's possibly that took things that one step further to get rid of the bank switching so that the ROMs could get mounted directly into a Stargate ROM board.

ken
 
Thanks Ian. That is a start. I had that link, but it was pointing to Sean's old website so the link was broken.

There was also a version for a multigame system, either Sean's or Clay's possibly that took things that one step further to get rid of the bank switching so that the ROMs could get mounted directly into a Stargate ROM board.

ken

Ken-

I know this is kind of an old post, but I just found it. I considered patching out the Defender bank switching way back when, but burned out and never did it. Just the other day I reconsidered doing it, but after a few hours thinking about it, I don't see how to do it.

The hardware register differences can be patched out, and I think the bankswitching of ROM into $C000 could be undone with some hard work and help from the excellent debugger in MAME, but Defender always has RAM from $0000-$BFFF, whereas the later games can bankswitch ROM into those locations. On the newer games, writes always go to RAM, but reads come from RAM or ROM depending on the last value written to $C900. You need some of that address space to put the ROM contents that are no longer bankswitched into $C000, so you'd have to add code to Defender to select ROM/RAM as needed. Even if you did all that work, the added cycles would kill the game play, like my old software patch for playing Splat on special chip 1s.

So I think some hardware hack will always be necessary to run Defender on a later boardset. If that is the case, then it is by far easier to replicate the Defender bankswitching hardware and leave the code alone.

Sean
 
Back
Top Bottom