(max 2MByte ROM and/or 32 KiB RAM)
This is the first MBC chip for the Game Boy. Any newer MBC chips work similarly, so it is relatively easy to upgrade a program from one MBC chip to another - or to make it compatible with several different types of MBCs.
Note that the memory in range 0000-7FFF is used both for reading from ROM and writing to the MBCs Control Registers.
This area normally contains the first 16 KiB (bank 00) of the cartridge ROM. Can contain banks $20/$40/$60 in mode 1 (see below), or banks $10/$20/$30 in mode 1 for a 1MB MBC1 multi-cart (see below).
This area may contain any of the further 16 KiB banks of the ROM. Cannot address any banks where the main ROM banking register would be $00, which usually means banks $00/$20/$40/$60. Instead, it automatically maps to 1 bank higher ($01/$21/$41/$61).
This area is used to address external RAM in the cartridge (if any). External RAM is often battery-backed, allowing for the storage of game data while the Game Boy is turned off, or if the cartridge is removed from the Game Boy. Available RAM sizes are: 2 KiB (at $A000-$A7FF), 8 KiB (at $A000-$BFFF) and 32 KiB (in form of four 8K banks at $A000-$BFFF).
Before external RAM can be read or written, it must be enabled by writing to this address space. It is recommended to disable external RAM after accessing it, in order to protect its contents from corruption during power down of the Game Boy. Usually the following values are used:
00h Disable RAM (default)
0Ah Enable RAM
Practically any value with 0Ah in the lower 4 bits enables RAM and any other value disables RAM. RAM is automatically disabled when the gameboy is powered off. It is unknown why Ah is the value used to enable RAM.
This 5-bit register (range $01-$1F) selects the ROM bank number. Higher
bits are discarded - $E1 (binary 11100001) would select bank $01.
If the ROM Bank Number is set to a higher value than the number of banks
in the cart, the bank number is masked to the required number of bits.
e.g. a 256 KiB cart only needs a 4-bit bank number to address all of its
16 banks, so this register is masked to 4 bits. The upper bit would be
ignored.
On larger carts which need a >5 bit bank number, the secondary banking
register at 4000-5FFF is used to supply an additional 2 bits for the
effective bank number:
Selected ROM Bank = (Secondary Bank << 5) + ROM Bank.
The ROM Bank Number defaults to 01 at power-on. When 00 is written, the MBC translates that to bank 01 also. Not being able to select bank 00 isn't normally a problem, because bank 00h is usually mapped to the 0000-3FFF range. But on large carts (using the secondary banking register to specify the upper ROM Bank bits), the same happens for banks $20, $40, and $60, as this register would need to be 00 for those addresses. Any attempt to address these ROM Banks will select Bank $21, $41 and $61 instead. The only way to access banks $20, $40 or $60 is to enter mode 1, which remaps the 0000-3FFF range. This has its own problems for game developers as that range contains interrupt handlers, so it's usually only used in multi-game compilation carts (see below).
This second 2-bit register can be used to select a RAM Bank in range from $00-$03 (32 KiB ram carts only), or to specify the upper two bits (bits 5-6) of the ROM Bank number (1 MiB ROM or larger carts only). If neither ROM nor RAM is large enough, setting this register does nothing.
In 1MB MBC1 multi-carts (see below), this 2-bit register is instead applied to bits 4-5 of the ROM bank number and the top bit of the main 5-bit main ROM banking register is ignored.
This 1-bit register selects between the two MBC1 banking modes, controlling the behaviour of the secondary 2-bit banking register (above). If the cart is not large enough to use the 2-bit register (<= 8 KiB RAM / <= 512 KiB ROM) this mode select has no observable effect. The program may freely switch between the two modes at any time.
00 = Simple ROM Banking Mode (default)
01 = RAM Banking Mode / Advanced ROM Banking Mode
In mode 0, the 2-bit secondary banking register can only affect the 4000-7FFF banked ROM area. If the cart is a "small ROM"/"large RAM" cart (< 1 MiB ROM, > 8 KiB RAM) then 4000-7FFF is unaffected by this register anyway, so the practical effect is that RAM banking is disabled and A000-BFFF is locked to only be able to access bank 0 of RAM, with the 2-bit secondary banking register entirely ignored.
In mode 1, the behaviour differs depending on whether the current cart is a "large RAM" cart (> 8 KiB RAM) or "large ROM" cart (1 MB or larger). For large RAM carts, switching to mode 1 enables RAM banking and (if RAM is enabled) immediately switches the A000-BFFF RAM area to the bank selected by the 2-bit secondary banking register.
For "large ROM" carts, mode 1 has the 4000-7FFF banked ROM area behave the same as mode 0, but additionally the "unbankable" "bank 0" area 0000-3FFF is now also affected by the 2-bit secondary banking register, meaning it can now be switched between banks $00, $20, $40 and $60. These banks are inaccessible in mode 0 - they cannot be mapped to the 4000-7FFF banked ROM area.
Also known as MBC1m, these carts have an alternative wiring, that ignores the top bit of the main ROM banking register (making it a 4-bit register) and applies the 2-bit register to bits 4-5 of the bank number (instead of the usual bits 5-6). This means that in mode 1 the 2-bit register selects banks $00, $10, $20, or $30, rather than the usual $00, $20, $40 or $60.
These carts make use of the fact that mode 1 remaps the 0000-3FFF area to switch games. The 2-bit register is used to select the game - switching the zero bank and the region of banks that the 4000-7FFF rom area can access to those for the selected game and then the game only changes the main ROM banking register. As far as the selected game knows, it's running from a 256 KiB cart!
These carts can normally be identified by having a Nintendo copyright header in bank $10. A badly dumped multi-cart ROM can be identified by having duplicate content in banks $10-$1F (dupe of $00-$0F) and banks $30-$3F (dupe of $20-$2F). There is a known bad dump of the Mortal Kombat I & II collection around.
An "MBC1M" compilation cart ROM can be converted into a regular MBC1 ROM by increasing the ROM size to 2MB and duplicating each sub-rom - 00-0Fh duplicated into $10-$1F, the original $10-$1F placed in $20-$2F and duplicated into $30-$3F and so on.