Memory card

SRAM memory card, showing the SRAM chip and voltage detection circuit. Picture by HPMAN

NEO-IC8, original SNK card. Picture by ArcadeTV

NEO-IC8, back of original SNK card. Picture by ArcadeTV

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. The header always occupies the first block. Eg. A 2KiB card can hold (2048/64)-1 = 31 blocks.

The data is split into 5 regions:

Header

Always 64 bytes.

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

List of saves of 32, 64, 96, 128, 160, 192, 224 or 256 entries. The size depends on the card's capacity.

Note that since the header occupies the first block, there will always be at least one unused directory entry.

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 used blocks of 64, 128, 192, or 256 entries. The size depends on the card's capacity.

• $02 is a block reserved by the system ROM
• $01 is a block used by a game
• $00 is a free block

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.