Memory Bank Controller (MBC) 3 for GameBoy Information
by bRILLO hEAD, 1-Jan-98
The MBC3 is a memory controller for some GameBoy carts
that supports
ROMs up to 16megabits (2megabytes) and RAM up to 256kbits
(32kbytes). It has a built-in clock counter that requires
an external 32.768KHz crystal for operation.
Reg 0 - $0000-$1fff - RAM/clock write protect
$0A - Enable
$00 - Disable
Reg 1 - $2000-$3fff - ROM Bank Select
$00-$7F - Rom bank #
Reg 2 - $4000-$5fff - RAM Bank/Clock Reg Select
$00-$03 - RAM bank #
$08-$0c - Clock register #
Reg 3 - $6000-$7fff - Latch clock counter data
Writing $00 and then $01 latches clock data.
Another write $00 and write $01 is required
to latch data again.
Reg SEC - $08 - Seconds counter
Reg MIN - $09 - Minutes counter
Reg HRS - $0A - Hours counter
Reg DAYL- $0B - Lower 8 bits of day counter
Reg DAYH- $0C - bit 0 = Upper 1 bit of day counter
bit 6 = start(0)/stop(1) clock counter
bit 7 = Day counter carry bit
Notes:
Bit 7 of Reg DAYH once set stays set until a 0 is written.
To access the clock counter the ram bank must first be turned
on by writing $0A to Reg 0.
To read the clock counter value, set Reg 3 to $01. This latches
the values of all registers so they won't change on you while your
trying to read them. However, this does not prevent the internal
counters from keeping correct time. If reg 3 is already set to
$01 then write $00 and then $01.
For example, to read Reg SEC write $08 to Reg 2. The value of
Reg SEC can then be read from any address in $A000-$BFFF range.
Use a similar process for writing to a register.
Due to slow MBC3/clock interfacing, 4 machine cycles (16 clock cycles)
are required between register accesses.
