bus/megadrive/titan.cpp, bus/megadrive/rom.cpp: Add notes for mapper chip

[cam900]

In Mega drive/Genesis Super Street Fighter 2 Cartridge, Sega 315-5709/315-5779 controls bankswitching and determines upper half of ROM space (Backup RAM or ROM).

Titan Overdrive 2 requires compatible mapper hardware as above, but ROM space is forced to ROM?

Reference (for mapper chip): https://segaretro.org/images/7/78/IC_BD_16M_42PIN_x_4_EPROM_32X_RD_User%27s_Manual.pdf

[angelosa]

Phantasy Star IV and Sonic 3 & Knuckles (thru lock-on?) actually depends on the backup RAM to be view selectable (those uses different mappers).
Super Street Fighter 2 shouldn't have any SRAM, and it's the only official release known to need the ROM banks. The 32x development cartridge refers to a different configuration, very unlikely that is implemented here.
For homebrews the conventional form is to use SEGA SSF in the header at $100, cfr. plutiedev reference, and the official documentation actually states that bank 0 is mappable and SRAM overlay is on a different bit, something I haven't touched because Titan mapper is a one-shot PoC until I have the chance to rewrite this.

Further reading:
TASEmulators/BizHawk#695

[cam900]

"No official licensed software used these features" in here means "No official licensed software used SRAM overlay and write protect register in 315-5709/315-5779 mapper chip".
These registers are used in official licensed software without 315-5709/315-5779 mapper chip.

[angelosa]

Again: what's your rationale/reference that makes you think Super Street Fighter 2 uses a 32x dev cart with those numbers?

[cam900]

Reference is for mapper chip, and i think dev cart mentioned at reference and actual MD Super Street Fighter 2 cart PCB has same mapper chip. There's another dev cart has same mapper chip as reference/MD Super Street Fighter 2 cart PCB: https://segaretro.org/images/a/a0/IC_BD_4M_32PIN_x_8_EPROM_32X_RD_User%27s_Manual.pdf

[angelosa]

PCB picture, post it.

[cam900]

https://gendev.spritesmind.net/mirrors/www.nepereny.com/misc2/games/genesis_super_street_fighter_2.htm here is PCB for MD Super Street Fighter 2; it has 315-5779, but compatible as 315-5709.
Some PCB has 315-5709 instead 315-5779.
see also: megadriv.xml mentioned 315-5709 IC in Super Street Fighter 2.

PCB with 315-5709: https://storage.googleapis.com/images.pricecharting.com/6b41c02b48a6a12967bf17268e0b259a0dc1c94097afd863c1c6ebee0d85bec5/1600.jpg

[angelosa]

That doesn't mean the mapper has an internal SRAM / has the SRAM view select hooked up, does it?

[cam900]

Yeah: SRAM is external, and just overlays upper half of 68K cartridge space if access enabled.

[cam900]

Yeah: SRAM is external, and just overlays upper half of 68K cartridge space if access enabled.

If SRAM space overlay in 0x200000 onward is enabled when SRAM is not connected, then memory space area in 0x200000 onward will be just empty or open-bus.