Multicart Controller




The multicard controller (MC) is inserted between the VP cartridge port and the game ROM/EPROM/FLASH storage chip. It generates the address for the storage chip based on the VP address and the current mode settings.
In general, the MC operates in either of two modes:
After reset, it starts in menu mode and traverses to application mode once under software control. Switching back to menu mode is only possible with reset or power cycle.

Menu Mode

The first 2k of the external storage memory are available as plain program memory in menu mode. I.e. this range is mapped as a standard 2k game cartridge with exclusion of A10, BS0 and BS1. To allow for 256 menu entries, there is a bank switching mechanism active that banks the last page of the first 1k
--> programm address range from 0700h to 07ffh

Whenever this range is accessed, a programmable bank offset is added to the external memory address:

External memory       8048 program memory
+-------+             +-------+
| 0700h | ebank 0     | 0700h |
| 07ffh | --------+-> | 07ffh |
+-------+         |   +-------+
| 0800h | ebank 1 |
| 08ffh | --------+
+-------+         |
| 0900h | ebank 2 |
| 09ffh | --------+
+-------+

  [...]

In menu mode, there are several registers defined and active that serve the purpose of configuring the application mode by means of software.

CART_2K_OFFSET  (@ 080h)
Specifies the offset for external storage memory access in application mode in units or 2k blocks. All 8 bits are used, thus the max offset is
255 * 2k = 510k

CART_SIZE_MASK  (@ 081h)
Bit 7: 0 = skip A10, 1 = use A10
Selects the usage of A10 in application mode.
Setting this to 0 skips A10, resulting in a memory layout compatible with standard 2k, 4k and 8k cartridge.
Setting this to 1 uses A10 for cartridges with 12k or 16k size.
Bit 1..0: Size mask
Selects the size mask in application mode.
These bits mask the BS0 (bit 0) and BS1 (bit 1) to generate the proper addresses for 2k, 4k and >= 8k cartridges.
  Cart  Bits
  Size   1 0
 ------------
    2k   0 0
    4k   0 1
  >=8k   1 1

CART_EBANK (@ 082h)
Specifies the offset for program memory range 0700h to 07ffh in menu mode in units of a page. Only the lower 4 bits are used, thus the max offset is
15 * 256 = 3840

CART_MODE (@0ffh)
When written with 0a5h, the mode is switched from menu to application. A reset on mc_res_n_o output is generated and the settings in CART_2K_OFFSET and CART_SIZE_MASK become effective.

Application Mode

All registers of menu mode are disabled, thus there's no means for the game software to change the settings done during menu mode.