(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).
Contains the first 16 KiB of the ROM.
Same as for MBC1, except that accessing banks 20h, 40h, and 60h is supported now.
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.
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.
Mostly the same as for MBC1, a value of 0Ah will enable reading and writing to external RAM - and to the RTC Registers! A value of 00h will disable either.
Same as for MBC1, except that the whole 7 bits of the RAM Bank Number are written directly to this address. As for the MBC1, writing a value of 00h, will select Bank 01h instead. All other values 01-7Fh select the corresponding ROM Banks.
As for the MBC1s RAM Banking Mode, writing a value in range for 00h-03h maps the corresponding external RAM Bank (if any) into memory at A000-BFFF. When writing a value of 08h-0Ch, this will map the corresponding RTC register into memory at A000-BFFF. That register could then be read/written by accessing any address in that area, typically that is done by using address A000.
When writing 00h, and then 01h 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 00h->01h procedure. This provides a way to read the RTC registers while the clock keeps ticking.
08h RTC S Seconds 0-59 (0-3Bh) 09h RTC M Minutes 0-59 (0-3Bh) 0Ah RTC H Hours 0-23 (0-17h) 0Bh RTC DL Lower 8 bits of Day Counter (0-FFh) 0Ch RTC DH Upper 1 bit of Day Counter, Carry Bit, Halt Flag Bit 0 Most significant bit of Day Counter (Bit 8) 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 total 9 bits of the Day Counter allow to count days in range from 0-511 (0-1FFh). 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 to count any number of days, making your program Year-10000-Proof, provided that the cartridge gets used at least every 511 days.
When accessing the RTC Registers it is recommended to execute a 4ms delay (4 Cycles in Normal Speed Mode) between the separate accesses.