(Keep in mind my knowledge on CPU memory addressing is very rudimentary)
Let's start with the Genesis rom. Pictured is Earthworm Jim 2:
Ok, everyone remember all those 64k RAM 6502 CPU based computers like the Atari 800, Apple II, C64? The CPU had 16 address lines which allowed it to access 64k of data.
Every bit (lead) of address line you add DOUBLES the amount of accessible memory. In the above photo we see that EWJ 2 uses 21 address lines which makes it a 2 meg cartridge. Here's the math, watch how it doubles:
16 lines=64k (like an old Apple II)
17 lines=128k
18 lines=258k
19 lines=512k
20 lines=1024k
21 lines=2048k
The cartridge slot has 23 address lines so it could access.... that's right 8 megs! (or so I think...) As far as I know the 68000 CPU also has 23 address lines, so that's the max the Genesis can access (as opposed to some systems such as the Atari 2600 which had more access lines on the CPU than the cartridge slot, thus limiting its cartridge size to 4k rather than 8k)
Ok, now here's a pinout of the ROM. This matches both Vectorman and EWJ 2 (about the only two newer Genesis games I didn't mind ripping apart)
It has 42 pins total and 21 address lines, meaning this is a 2 meg rom. If the ROM is only 1 meg it omits an address line.
So here's what we know:
1) A Genesis ROM needs only 42 pins to be a 2 meg cartridge
2) A Radica ROM has 44 pins.
2 more pins... why, that makes 23! So, IN THEORY the Radica could have the same memory access as a Genesis.
The next step is a little tricker... mapping the lines on a Radica ROM spot. Once this is done, however, hooking it to cartridge connector should be a snap.
Let's WHUP this thing, guys!
