MegaSD vs. Nextor - cayce-msx/msxpp-quick-ref GitHub Wiki

A few notes about using MegaSD vs. Nextor:

• the terms 'ESE-MegaSD', 'MegaSD' and 'MegaSDHC' seem interchangeable
• NEXTOR.SYS can be used as drop-in replacement for MSXDOS2.SYS - even without the Nextor kernel, i.e., with MegaSD.
• (ESE-)MegaSD is based on ESE-MegaSCSI but supports only a part of the original ESE-MegaSCSI commands, which causes these (and maybe other) limitations:
  • ESE-RAMDISK is not implemented - ESET.COM [A]uto setup (A: ESE-RAMDISK) will fail; drive A: will be unavailable
  • ESET.COM cannot handle MegaSDHC kernel partitions
  • ESET.COM expert options [M]emory, [D]OS, [V]FD, [P]rimary (DOS) partition and [E]xt(ended) partition don't function - which means only [C]hoose kernel, [S]CSI and [K]ey settings work
• ESET.COM identifies MegaSD as kernel 0, #8B M2 MEGA-SCSI v2.15 at initiator ID7
• The ESE MEGA-SCSI tools (see OCM-SDBIOS Pack) only work with MegaSD; not with Nextor:
  • MGLOAD.COM, ESET.COM, EP.COM
• Nextor is not MegaSCSI compatible - thus does not support these three tools. It does have these (and other) advantages compared to MegaSD:
  • faster FAT16 support
  • compatible with 4096KiB RAM on Turbo-R
• Partitions & maximum addressable μSD card size:
  • MegaSD: one FAT16 of 4GiB, plus 254 FAT12 (32MiB) partitions - about 12GiB total
  • Nextor: 255 FAT16 (4GiB) partitions - about 1TiB total. SDXC supports that amount of storage - but it's unknown whether OCM-PLD supports SDXC. SDHC goes up to 32 GiB.
• The MegaSD kernel can boot with one (A:) or two (A: and B:) drives.
  • one is the default
  • drive B: can be added dynamically using ESET.com
  • or it can be hardwired into SD-BIOS using Disk-ROM choice "MegaSDHC FAT16X Double SD-slot"
    • for OCM/MSX++, that has nothing to do with a 'Double SD-slot', as in: 2 SD-card slots, but rather with two MSX drives enabled at boot - A: and B:. Even in this mode, just a single SD-card partition is mapped to drive A:.
    • However, it seems that MegaSD, the 'father' of MegaSDHC (original site, HTTP-only) - which is a cartridge by ESE Artists' Factory from 2007 for use on a real MSX - does support two SD-cards! The source code for MegaSD, which itself is an adaptation of ESE Artists' Factory's Mega-SCSI from 1995, seems to have been lost, so support for two physical SD cards is still there.
    • OCM-PLD only supports MegaSD ID 0, i.e., one μSD card.
  • Either way (dynamic or hardwired), drive B: is inaccessible unless either:
    • EP.COM is used to map a floppy disk image into it.
    • An SD-card FAT12 partition is mapped onto drive B: using MSDPAR.
• removal / swapping a μSD-card:
  • using MegaSD: unknown what will happen when swapping to another medium. You can insert a card after booting, which will be properly recognized. Re-inserting the same card after removal also works; disk access while the card is removed results in the well known "Not ready reading drive A:/Abort or Retry (A/R)? ". After re-inserting, MegaSD will continue without complaints.
  • using Nextor: supported, although the documentation does not explicitly state what happens when multiple partitions are mapped and the card is removed/swapped.
    • When you're NOT going to remove the card, then, using...:
      • DOS: it's recommended to lock the card using LOCK.COM to speed up disk access: LOCK A: ON
      • BASIC: it's also possible to map a partition and lock the card at the same time (CALL MAPDRVL("A:", <partition 0-255>, 0))