Max. 1Mbyte PRG ROM.
Max. 1Mbyte CHR ROM.
This is one heck of a complicated cart board. This board is found on the Maxi-15 multicart
released by AVE. It was meant to be sold to rental outlets, and when run, it includes an
amusing text message saying you could call a 1-800 number to get your own if you so desired.
In the ROM, there are more than 15 games listed. Supposidly, AVE wanted to release a
Maxi-30, and it would've most likely used the same PCB. This board handles multiple
ROM sizes of the games it runs, which is why it is so complicated. There are 4 spots
on the board for ROMs. The board can support four 512K ROMs or two 1Mbyte ROMs.
Interestingly, they screwed up on the circuitry for the ROM size control. whoops. They
used an OR gate instead of an AND gate like they should've used. And they got the inverter
on the wrong input to it. Oh well. It was never used so they didn't have to worry about it.
Address decoding is done using the 7430 and the two 74138's. 8 register ranges are decoded,
but only 4 are used.
Main banking/control register is accessed at FF80-FF9F. This register cannot be written to
again if bits 0-5 are anything but 000000b! Only a reset will clear this condition. The
cart detects reset by watching A0. If it stops toggling, this register is cleared.
B: Select a 64K bank of PRG and CHR. Note: on the Maxi-15, selecting anything with bit
4 = 1 results in open bus.
U: Select a 32K bank of PRG and CHR when mode bit S is set.
Z: ROM size control
0 - 512K ROMs installed
1 - 1Mbyte ROMs installed
M: Mirroring control.
0 - Vertical
1 - Horizontal
S: Mode control.
0 - Bit U has no effect when this bit is clear.
1 - Bit U of this register will select a 32K bank of both PRG and CHR.
The auxilliary banking register is accessed at FFE8 through FFF7. This register is reset
at the same time the main banking register is reset. It is only 4 bits in size and controls
up to 64K of PRG and CHR.
C - Select an 8K CHR bank.
P - Select a 32K PRG bank.
Note: when bit S (above) is set, then the P bit and upper C bit have no effect (since the
main register is now controlling them).
There is another register at FFC0-FFDF, but it is related to the lockout defeater and is not
required to run the game.