Software code to activate SMS mode in Genesis 1

TmX

New Member
There are two titles that I know of that will work only on the Genesis Model 1. Ishido and Budokon - The Martial Spirit. As far as I know Ishido fails to send the proper checks for the later model genesis to run it. So this is most likely not a game that enables the SMS mode. However, Budokon appears to work differently than Ishido and I am curious if this is a game that switches the Genesis into SMS mode. I did a hex edit scan on the budokon SMD file and right after the SMD header there is what looks like an old dos style run command. Now of course this would be most unusual to have this be the few lines of code to do the mode switch and also unlikely to be able to see it in a hex edit prg. But is there a way to track down the bit code if the image could be disasembled or could it be seen in a hex edit prg like headers are and reproduced and pasted on othe SMS images? Thanks.
 

mal

Member
I was under the impression that SMS mode was enabled through hardware rather than software. ???
 

TmX

New Member
Do you know what type of hardware and what games had this hardware? Im thinking Phantasy Star MD, but are there others that use hardware as well? I also thought that it was called or considered to be Mode 7 or something like that that enabled SMS. Again, is this software or now that you mention it is it hardware that does this?
 

mal

Member
I'm thinking it's hardware because you can use a Powerbase convertor to play all(?) SMS games on a Geneisis. The first official ones have a cart slot, card slot and I believe a pause button as well.

I don't know of any games other than Phantasy Star that use SMS mode directly though.
 

antime

Extra Hard Mid Boss
The Budokan cart was also an unlicensed release, thus it won't work with those Megadrives that check the licensing information. AFAIK SMS mode is really only the graphics mode, as the hardware is otherwise compatible. SMS mode can be enabled by toggling bit 3 in VDP register 2, or through pin M3 on the cart connector (see Charles MacDonald's VDP documentation). The Power Base Converter contains the boot ROM necessary to get the games working, but it's mainly a cart converter.
 

ExCyber

Staff member
I did a hex edit scan on the budokon SMD file
SMD files are not very useful for any kind of direct manipulation, as they're not straight ROM images.

AFAIK SMS mode is really only the graphics mode, as the hardware is otherwise compatible.
SMS mode requires disabling the 68000 and enabling an SMS-compatible memory map for the Z80.

SMS mode can be enabled by toggling bit 3 in VDP register 2
I'd imagine this only changes to the SMS-compatible video mode rather than actually putting the system into SMS mode.

or through pin M3 on the cart connector
IIRC, this is pin B31 on the cart slot, and probably causes the system to be put into SMS mode as well as forcing "mode 4".

The Power Base Converter contains the boot ROM necessary to get the games working
As far as I can tell, the PBC does not contain a ROM. The only major IC on mine is an SPLD, and Charles MacDonald's pics of another converter design show no ICs at all.
 

Vic Viper

New Member
BTW, that PAL (SPLD) is there on purpose of enabling the card slot. If you got no card slot, there's no need of it. That's why the brazilian clone adapter (Megavision) had no PAL chip.
 

antime

Extra Hard Mid Boss
Sort of on-topic, during the Sega vs. Accolade trial one of Sega's engineers demonstrated two ways of booting unlicensed carts. Does anyone know how this was accomplished? The court decision hints that it was done through a hardware modification (eg. blanking the screen while the Sega logo is displayed) but offers no technical details.
 

ExCyber

Staff member
Sort of on-topic, during the Sega vs. Accolade trial one of Sega's engineers demonstrated two ways of booting unlicensed carts. Does anyone know how this was accomplished?
The theory I remember hearing is that hardware is added to the cartridge to pull one of the data lines high/low for some period of time (possibly triggered by one or more of the address bits); supposedly this manages to wreck the palette, making the text invisible. Even if this particular information is not correct, it's probably close to the truth. However, Sega could probably have foiled these methods in a future model of the Genesis by buffering the appropriate lines...
 

Nemesis

New Member
SMS mode can be enabled by toggling bit 3 in VDP register 2
As Excyber mentioned, that only switches the graphics mode the VDP is in. It switches the screen to the SMS resolution, but nothing more.

You might want to ask this qustion over at the sega emulation forums. There are a lot of people ther qutie diverse in the ways of the genesis, and IIRC, a topic related to the SMS mode was brought up a month or so ago, so do a search first.
 
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