MBC3
(max 2MByte ROM and/or 32KByte RAM and Timer)
Beside for the ability to access up to 2MB ROM (128 banks), and 32KB RAM (4 banks), the MBC3 also includes a built-in Real Time Clock (RTC). The RTC requires an external 32.768 kHz Quartz Oscillator, and an external battery (if it should continue to tick when the Game Boy is turned off).
Memory
0000-3FFF - ROM Bank 00 (Read Only)
Contains the first 16 KiB of the ROM.
4000-7FFF - ROM Bank 01-7F (Read Only)
Same as for MBC1, except that accessing banks $20, $40, and $60 is supported now.
A000-BFFF - RAM Bank 00-07 or RTC register (Read/Write)
Depending on the current Bank Number/RTC Register selection (see below), this memory space is used to access an 8 KiB external RAM Bank, or a single RTC Register.
Registers
A000-BFFF - RTC Register 08-0C (Read/Write)
Depending on the current Bank Number/RTC Register selection (see below), this memory space is used to access an 8KByte external RAM Bank, or a single RTC Register. The mapped RTC register can be read/written by accessing any address in that area, typically using address A000.
0000-1FFF - RAM and Timer Enable (Write Only)
Mostly the same as for MBC1, a value of $0A will enable reading and writing to external RAM - and to the RTC Registers! A value of $00 will disable either.
2000-3FFF - ROM Bank Number (Write Only)
Same as for MBC1, except that the whole 7 bits of the ROM Bank Number are written directly to this address. As for the MBC1, writing a value of $00 will select Bank $01 instead. All other values $01-$7F select the corresponding ROM Banks.
4000-5FFF - RAM Bank Number - or - RTC Register Select (Write Only)
Controls what is mapped into memory at A000-BFFF.
| Value | Selection |
|---|---|
| $00-$07 | The corresponding RAM Bank. |
| $08-$0C | The corresponding RTC Register (see below). |
6000-7FFF - Latch Clock Data (Write Only)
When writing $00, and then $01 to this register, the current time becomes latched into the RTC registers. The latched data will not change until it becomes latched again, by repeating the write $00->$01 procedure. This provides a way to read the RTC registers while the clock keeps ticking.
Clock Counter Registers
| Register | Name | Description | Range |
|---|---|---|---|
| $08 | RTC S | Seconds | 0-59 ($00-$3B) |
| $09 | RTC M | Minutes | 0-59 ($00-$3B) |
| $0A | RTC H | Hours | 0-23 ($00-$17) |
| $0B | RTC DL | Lower 8 bits of Day Counter | ($00-$FF) |
| $0C | RTC DH | Upper 1 bit of Day Counter, Carry Bit, Halt Flag. Bit 0: Most significant bit (Bit 8) of Day Counter Bit 6: Halt (0=Active, 1=Stop Timer) Bit 7: Day Counter Carry Bit (1=Counter Overflow) |
The Halt Flag is supposed to be set before writing to the RTC Registers.
The Day Counter
The total 9 bits of the Day Counter allow counting days in range from 0-511 ($000-$1FF). The Day Counter Carry Bit becomes set when this value overflows. In that case the Carry Bit remains set until the program does reset it. Note that you can store an offset to the Day Counter in battery RAM. For example, every time you read a non-zero Day Counter, add this Counter to the offset in RAM, and reset the Counter to zero. This method allows counting any number of days, making your program Year-10000-Proof, provided that the cartridge gets used at least every 511 days.
Delays
When accessing the RTC Registers, it is recommended to wait 4 µs (4 M-cycles in Single Speed Mode) between any separate accesses.