Configuration cartridge

(Pointed out by ElBarto)

MVS boards are able to auto-configure theirselves by detecting and communicating with a special cartridge holding its own uPD4990, which may have been used during production to avoid tedious manual configuration through the regular MVS settings menu.

The MVS BIOS (SP-E at least), checks for the "NEO-CLN" string in the 68k program header in place of the usual "NEO-GEO" and makes various read/writes to the cartridge in the $200000~$2FFFFF range.

$280051 is used for writes:

Writes are 4 bits and sent serialy LSB first. STB is strobed when the 4 bits are written.

Bit 7 of $220001 is used for reads. Each new bit is clocked in using CLK.

Reading sequence

 * Send 3, wait (uPD4990 "Time Read")
 * Send 1 (uPD4990 "Register Shift")
 * Read and set BIOS_SECOND
 * Read and set BIOS_MINUTE
 * Read and set BIOS_HOUR
 * Read and set BIOS_DAY
 * Read and set BIOS_WEEKDAY, convert to BCD and set BIOS_MONTH
 * Read and set BIOS_YEAR
 * Read 4 bits
 * Copy RAM data to the board's uPD4990.

This sequence also matches the uPD4990's 52bit shift register format.

(What is that N,$00,O,$00,G,$00,O,$00 check after "NEO-CLN" ? 8bit bus ?)