Memory card
The NeoGeo home console and some MVS cabs can use JEIDA V3 memory cards to store game saves and/or high scores, allowing players to save their progress and continue on any system running the same game.
Even if the memory card address space can be freely used, games should only perform memory card operations through the CARD system ROM call to avoid data corruption.
Supported card types
The SP-S2 system ROM (and probably all the others) can handle 8 memory card sizes from 2KiB up to 16KiB, in 2KiB steps:
- 2KiB
- 4KiB
- 6KiB
- 8KiB
- 10KiB
- 14KiB
- 16KiB
And 4 different card types:
- 8 bits wide
- 16 bits wide
- 16 bits wide "doubled"
- "SNK ROM" cards
For 8-bit cards (all official SNK cards ?) only the lower byte is used, but accesses must always be done in words. This is because memory card accesses are only enabled for word or upper byte operations (/UDS 68k signal).
Original SNK card
The original SNK-branded memory card was a battery-backed 8-bit wide 2KiB card using a LH5116 SRAM chip and voltage switching circuitry.
Since it uses a battery, it may be prone to data loss if the voltage drops below 2V. Unlike the MVS backup RAM battery, the memory card one isn't rechargeable.
There are at least 2 known versions of the original SNK Memory Card, each with its individual design and circuit-board:
The first version has the C10075-X2-2 while the later one hase the EZ866 board.
CD systems
A "virtual" memory card is embedded in CD systems as a 8KiB battery-backed SRAM chip.
Data format
All cards use the same format regardless of capacity, only regions sizes differ. Since the JEIDA data bus is 16-bits wide, make sure to double the address for 8-bit cards if you choose to access their memory directly.
The data is referred to as "blocks", one block being 64 bytes. Whatever the game save size is, it is rounded up to a multiple of 64 bytes.
The data is split into 5 regions:
Header
Always occupies the first block.
| Address | Name | Size | Description |
| $0~$9 | 10 bytes | Zeros or "SNK ROM " (development unit ?)
For 16 bits "doubled" cards, $6 is set to $0001 | |
| $A | CARD_SIZE | word | Address space used by the memory card. Eg. A 2KiB 8-bit card is $1000. |
| $C | byte | 0 | |
| $D | CARD_FAT_1_CHKSUM | byte | Checksum of FAT 1 |
| $E | CARD_FAT_2_CHKSUM | byte | Checksum of FAT 2 |
| $F | CARD_USERNAME_AVAILABLE | byte | 0 : Username unavailable, !0 : Username available |
| $10 | CARD_USERNAME | 16 bytes | Card holder's username |
| $20 | CARD_MAGIC | 16 bytes | $4E $XX $45 $XX $4F $XX $2D $XX $47 $XX $45 $XX $4F $XX $80 $XX ("NEO-GEO", $80) |
| $30 | CARD_REGION | byte | Region of the system that formated the card, 0 = Japan, 1 = USA, 2 = Europe |
Directory
Always starts at the 2nd block.
List of available saves. Can store from 32 to 256 entries. The size depends on the card's capacity.
Note that since the header and directory occupy at least 1 + 2 blocks for the smallest 2KiB cards, there will always be at least 3 unused directory entries.
Each entry is 4 bytes, with the following format:
| Address | Size | Description |
| $0 | byte | Save sub-number (CARD_SUB), 0 to 15. $FF if entry is free. |
| $1 | word | Game's NGH number |
| $3 | byte | FAT entry index (see below) |
FAT 1
List of block indices entries used to reconstruct linear save data from possibly fragmented blocks. 64 to 256 entries. The size depends on the card's capacity.
Each entry is a byte representing a block.
- $00 is a free block
- $01 is the last block used by a save
- $02 is a block reserved by the system ROM (at least 1 header + 2 directory + 2 FAT = 5 used at beginning)
- Any other value indicates the next block in the chain
For example a directory entry has a start block of 5. If the FAT entry for block 5 is $01, then the save is only a single block in size. If the FAT entries are $06 $09 $01, then the save is 3 blocks in size: $05, $06 and $09.
FAT 2
FAT 2 is simply a mirror of FAT 1.
If any of the FAT tables checksum is bad, the correct table is written over the other one. If both checksums are bad, the system ROM displays an error.
Game Data
Many games use only one block, but some may use more.
The first 20 bytes contain the data title (usually the game and stage name).
Registers
- Write is enabled if REG_CRDUNLOCK1 and REG_CRDUNLOCK2 are written to
- The WP (Write Protect) pin is read through bit 6 of REG_STATUS_B
- CD1 (Card Detect 1) is read through bit 4 of REG_STATUS_B
- CD2 (Card Detect 2) is read through bit 5 of REG_STATUS_B
- Both must be 0 if the card is inserted correctly
- Attribute memory is selected if REG_CRDREGSEL is written to, never used ?
Pinout
See Memory card pinout.