Dreamcast VMUs can store 128k in 200 "blocks". Third party memory cards increased the storage by having multiple virtual cards, each with 200 blocks.

With some hacking it is possible to free up some extra blocks on most memory cards, but there are reports that some games don't work with cards modified in that way.

Looking at the format of the VMU filesystem, it appears that there is support for more than 200 blocks. The number of blocks is stored as a 16 bit value, suggesting that at least 32767 blocks is possible. There may be other limitations, or compatibility issues as seen when modifying a card to unlock extra blocks.

Have any memory cards ever attempted to support more than 200 blocks without the virtual card trick?

  • 2
    Even if the data structures allow it, there may be other limitations in the code. And with simple calculation, adding 40 blocks is impossible. 240 blocks would require 15 blocks for directory entry, which adds up to 255 blocks, so you can't fit the FAT and ROOT blocks. So only up to 239 blocks should be possible.
    – Justme
    Commented Sep 26, 2022 at 13:11
  • That's what I'm asking, is there some other limit that prevents larger numbers of blocks working in practice. It seems like every memory card ever made stuck to the 200 block limit. The number 40 may be wrong, it could be less. Might remove it, I put it in because I wanted to indicate that I know about that hack.
    – user
    Commented Sep 26, 2022 at 13:20
  • The VMU specification specifies and defined the numbers how many blocks to allocate for each area. If you reinitialize/format a VMU with corrupted memory, it will likely be written/initialized with the default values by the DreamCast. A real VMU reserves the 28K area for System BIOS use and it is write protected except during formatting from DreamCast. There is also no way to signal any custom sizes, the memory is expected to be 2 banks of 64k. Why the data structure contains words is unknown, but as the VMU is really accessed in 128 byte chunks, it might have had support for 1024 chunks.
    – Justme
    Commented Sep 26, 2022 at 14:31
  • @Justme but you can make some of those reserved blocks usable using software that re-writes the "System BIOS" area, which in reality is a disk geometry and file allocation table. It simply adds those blocks to the list and most, but not all, games work with it. The VMU manager in the Dreamcast menu doesn't have any problem with it either. The only thing is if you re-format it with the Dreamcast VMU manager it sets it back to 200 blocks.
    – user
    Commented Sep 26, 2022 at 15:12
  • Yes, but I am talking about the 28k reserved area, not info sector and FAT. if VMU BIOS itself uses that 28k reserved area for own use, wouldn't it assume it's unused by Dreamcast for data storage? And what use would be a larger-than-standard file system, if it is not compatible with all programs, and reverts to standard size when reinitializing it in the original system, so why would anyone make a non-standard VMU? They might exist though, but who would have a custom initializer to format it to original non-standard size.
    – Justme
    Commented Sep 26, 2022 at 16:40


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