Memory Bank Controller (MBC) 3 for GameBoy Information

Game Boy MBC 3

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.