Slot check security

Warning screen as shown by Sengoku 3 when booting on MVS hardware with BIOS_MVS_FLAG set to 0 (AES mode).

Slot checking is a cartridge system security measure put in place by some games produced after 2000 due to the emergence of bootleg MVS boards. It displays a warning screen and locks the game up if some checks fail.

Someone who would want to bypass this check will typically search for the warning text string in the P ROM and references to it in the code. Such text can't be found because the checking routine uses a XORed version of the same text instead, certainly to confuse hackers.

Genuine MVS board checks

• The calendar data zone in the BIOS RAM is cleared, then READ_CALENDAR is called. If

BIOS_MONTH ( $10FDD3) is greater than 13 (should be 12 ? Is that a bug ?), the check fails. This causes bootleg boards with no RTC to trigger the security.

• If

BIOS_MVS_FLAG ( $10FD82) is zero (BIOS in AES mode), and bit 7 of REG_STATUS_B is set (MVS hardware), the check fails. This causes bootleg boards using a copy of the AES BIOS or not handling REG_STATUS_B reads to trigger the security.

Depending on the result of those checks, the value $8B55 (pass) or $9DBD (fail) is written to VRAM at $7FFF. This furthermore verifies that the board has full VRAM.

That VRAM value is then read back and added to $74AB (=$10000 if checks passed) to set $10FCEF (fail) or not.

A later version of this procedure also times the Z80 reply to command $01. $10FCEE = $FF if Z80 was too slow.

Todo: Add more details.

Use by games

As seen in MAME's source ([neogeo.c]), a few games are known to do this check:

• Nightmare in the Dark
• The King of Fighters 2000
• Sengoku 3: MVS SLOT CHECK Ver2.30 00/04/25
• Matrimelee
• Metal Slug 5

Sengoku 3: Checks M1 ROM too, displays "M1-ROM ERROR." if Z80 doesn't reply to command 1 in $7FFF iterations loop.