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I've often pondered what the purpose of ROM bank shifting is. The linked wiki page says "No known software uses this feature".

Recently it popped into my head that perhaps it was designed to facilitate multicarts.

The shift, perhaps better described as a rotate, has the effect of re-mapping a 128KB chunk of ROM into one of four "super-banks". This should mean that one could put four standalone 128KB games on a cartridge and switch between them without needing for them to be aware of what's happening from the perspective of memory addressing. The only thing that would be needed is some way to facilitate the switching, so potentially the first 128KB game could contain an initial switching menu. This could probably be achieved with a pretty minimal hack to that original ROM, but maybe there's an even more elegant way to do it.

Anyone else had any other ideas about the purpose of ROM bank shifting, would love to hear them!

I assumed it was to facilitate multicarts. It doesn’t seem useful for dev work, and the only true multicart Sega made was an old fashioned power-cycle-to-increment hack just like pirates.

I do agree with Maxim - I suspect they were planning to create n-in-1 games cartridge using a single 512 KiB ROM chip with this integrated mapper. Maybe a 3-in-1 with a nice menu at boot, or a 4-in-1 hacking the menu into some free space in the 'first' game ROM.
Also it would work with one 256 KiB and two 128 KiB games I think.

How does the Astro Warrior / Pit Pot combo cartridge work? Both were separate games prior to the western release, right?

Yes, but they are effectively combined at a source level, sharing the interrupt handlers for example.

sverx wrote

Also it would work with one 256 KiB and two 128 KiB games I think.

Yes, I think so, also one 384KB and one 128KB should work if one or other hosts the menu.

Maxim wrote

Yes, but they are effectively combined at a source level, sharing the interrupt handlers for example.

So there's really no known official release using that mapper feature - I wonder then how somebody (and who were them?) found out how it worked...

I think Charles MacDonald found them by desoldering (or otherwise ROM-replacing) and testing all the registers to see what happens. There’s also some hints at extra registers in the official Sega docs, but I don’t think it mentions this feature.

Maxim wrote

There’s also some hints at extra registers in the official Sega docs, but I don’t think it mentions this feature.

Where exactly?

I was misremembering https://www.smspower.org/Development/GGOfficialDocs#BIfTheCapacityIs2MBytesOrMor... which mentions the bank shift as well as a write protect bit and a work RAM selection bit in the register at $fffc.

This also refers to ROMs below 2MB (16Mbits) only having a single paging slot, and warns that the first and last bytes of save RAM may be corrupted at power on; this seems to not match reality.

Author	Message
willbritton Joined: 06 Mar 2022 Posts: 672 Location: London, UK	Purpose of ROM bank shifting - multicart support? Posted: Tue Apr 16, 2024 12:52 pm
	I've often pondered what the purpose of ROM bank shifting is. The linked wiki page says "No known software uses this feature". Recently it popped into my head that perhaps it was designed to facilitate multicarts. The shift, perhaps better described as a rotate, has the effect of re-mapping a 128KB chunk of ROM into one of four "super-banks". This should mean that one could put four standalone 128KB games on a cartridge and switch between them without needing for them to be aware of what's happening from the perspective of memory addressing. The only thing that would be needed is some way to facilitate the switching, so potentially the first 128KB game could contain an initial switching menu. This could probably be achieved with a pretty minimal hack to that original ROM, but maybe there's an even more elegant way to do it. Anyone else had any other ideas about the purpose of ROM bank shifting, would love to hear them!

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14749 Location: London	Posted: Tue Apr 16, 2024 1:04 pm
	I assumed it was to facilitate multicarts. It doesn’t seem useful for dev work, and the only true multicart Sega made was an old fashioned power-cycle-to-increment hack just like pirates.

sverx Joined: 05 Sep 2013 Posts: 3829 Location: Stockholm, Sweden	Posted: Tue Apr 16, 2024 3:48 pm
	I do agree with Maxim - I suspect they were planning to create n-in-1 games cartridge using a single 512 KiB ROM chip with this integrated mapper. Maybe a 3-in-1 with a nice menu at boot, or a 4-in-1 hacking the menu into some free space in the 'first' game ROM. Also it would work with one 256 KiB and two 128 KiB games I think.

Kagesan Joined: 01 Feb 2014 Posts: 880	Posted: Tue Apr 16, 2024 4:17 pm
Kagesan Joined: 01 Feb 2014 Posts: 880	How does the Astro Warrior / Pit Pot combo cartridge work? Both were separate games prior to the western release, right?

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14749 Location: London	Posted: Tue Apr 16, 2024 5:28 pm
	Yes, but they are effectively combined at a source level, sharing the interrupt handlers for example.

willbritton Joined: 06 Mar 2022 Posts: 672 Location: London, UK	Posted: Tue Apr 16, 2024 5:46 pm
	sverx wrote Also it would work with one 256 KiB and two 128 KiB games I think. Yes, I think so, also one 384KB and one 128KB should work if one or other hosts the menu.

sverx Joined: 05 Sep 2013 Posts: 3829 Location: Stockholm, Sweden	Posted: Wed Apr 17, 2024 12:26 pm
	Maxim wrote Yes, but they are effectively combined at a source level, sharing the interrupt handlers for example. So there's really no known official release using that mapper feature - I wonder then how somebody (and who were them?) found out how it worked...

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14749 Location: London	Posted: Wed Apr 17, 2024 1:28 pm
	I think Charles MacDonald found them by desoldering (or otherwise ROM-replacing) and testing all the registers to see what happens. There’s also some hints at extra registers in the official Sega docs, but I don’t think it mentions this feature.

asynchronous Joined: 14 Aug 2000 Posts: 742 Location: Adelaide, Australia	Purpose of ROM bank shifting - multicart support? Posted: Thu Apr 18, 2024 11:29 am
	Maxim wrote There’s also some hints at extra registers in the official Sega docs, but I don’t think it mentions this feature. Where exactly?

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14749 Location: London	Posted: Thu Apr 18, 2024 11:43 am
	I was misremembering https://www.smspower.org/Development/GGOfficialDocs#BIfTheCapacityIs2MBytesOrMor... which mentions the bank shift as well as a write protect bit and a work RAM selection bit in the register at $fffc. This also refers to ROMs below 2MB (16Mbits) only having a single paging slot, and warns that the first and last bytes of save RAM may be corrupted at power on; this seems to not match reality.

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