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 introduced by some games produced after 2000 due to the emergence of bootleg MVS boards, which 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 in the P ROM and references to it in the code. Such text can't be found because the checking routine uses a NOT'ed version of the same text instead, probably to confuse hackers.

The checking code first sets one out of two values in VRAM at the unused address $7FFF depending on two checks:

• The calendar data location in the BIOS RAM ($10FDD2 and up) is cleared, then BIOSF_READ_CALENDAR is called. If BIOS_MONTH($10FDD3) is greater than 13, the check fails.
• If BIOS_MVS_FLAG($10FD82) is zero (AES mode), and bit 7 of REG_STATUS_B($380000) is set (MVS hardware), the check fails.

That VRAM value is then read back and compared to set $10FCEF or not. The last check is done with the FIX_CLEAR bios call, which sets $10FCEF and makes the game pass the test.

Use by games

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

• Nightmare in the Dark (nitd)
• King Of Fighters 2000 (kof2000)
• Sengoku 3 (sengoku3): MVS SLOT CHECK Ver2.30 00/04/25
• Matrimelee (matrim)
• Metal Slug 5 (mslug5)

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