Big RAM on FME-7: I have a test ROM

Discuss hardware-related topics, such as development cartridges, CopyNES, PowerPak, EPROMs, or whatever.

Moderator: Moderators

User avatar
aquasnake
Posts: 515
Joined: Fri Sep 13, 2019 11:22 pm

Re: Big RAM on FME-7: I have a test ROM

Post by aquasnake »

krzysiobal wrote: Thu Jun 10, 2021 5:03 am
aquasnake wrote: Thu Jun 10, 2021 1:46 am **Although FME-7 has the bank switching capability of PRG RAM, no game relies on it.
Why you and author of the wiki's article consider it as something special? What would you expect on the PRG lines without that "special feature" when accesing $6000 when RAM is enabled? Zeroes? Ones? Tri-state? Then it would be surprisingly special.

The mux inside FME that chooses between one of four PRG registers takes into account just CPU-A14, CPU-A13 and ROMSEL and nothing more. It does not care about the RAM/ROM bit because that would be waste of resources.

Then maybe we should also add a note that the E bit is just a general purpose output latch, that can be alternatively used as PRG-RAM-A13, PRG-ROM-A19 or something other that you wouldn't even dream of. Just no games take care of that.

MMC3, for example, outputs ones on PRG lines when accessing $6000-$7fff not because it does not have PRG banking feature, but because ROM is only enabled at $8000-$ffff, and so the mux only takes into account CPU-A14, CPU-A13 and the PRG mode bit), so it has no physical ability to distinguish $6000 from $e000..
Due to the delay behind m2 on romsel, there is a bus conflict at the first 33ns of M2 positive pulse in accessing $8000 - $ffff and $0 - $7fff. The write operation at the address of $e000 - $ffff will gate the wram of $6000 - $7fff a short while, and then access the real address of $e000 - $ffff.

The write register of fme-7 is not in the range of $e000 - $ffff. Therefore, even if the CS of wram has no hardware delay, it will not be written unexpectedly.

In addition, The active high CS1 of SRAM is controlled by E-bit. Because it can control two CS logics, the protection of SRAM will be better.
tepples
Posts: 22708
Joined: Sun Sep 19, 2004 11:12 pm
Location: NE Indiana, USA (NTSC)
Contact:

Re: Big RAM on FME-7: I have a test ROM

Post by tepples »

One game using the "5B" variant of this mapper (Gimmick!) adds an AY-3-8910 clone at $C000-$FFFF, with the address port nominally at $C000 and the data port at $E000. But then unlike Batman: Return of the Joker/Dynamite Batman, Gimmick! doesn't have RAM at $6000-$7FFF.
Post Reply