Forums

Many moons ago I promised that I would post something like this here but I never got around to. Someone at other forum PM'd me with this request so I done this circuit for him.

Now, going down to business, the basic ROM mapper need to have:

Bank switch latch, which holds the active page of the rom is shown at the banked region of the processor address space.

Memory map control, which makes sure the memory segments which aren't mapped to bank switching stay at the expected pages (0 and 1 are supposed to show up from 0x0000 up to 0x7FFF).

And finally the register array address decoder which generates the four necessary addresses for making a full mapper (over engineering but heck I need that much to generate just one strobe, why not generate all four at once?).

Obviously, this works like a 315-5208 but is the starting point for drawing more complete mapper chips, like the 315-5235.

Ideas of any kind are welcome. If mistakes are found, please do mention too.

The original design I built in 2000 was too confusing because I used an array of 74LS74s to achieve a fixed power on state so I just decided to start from scratch with a new design.

Also, instead of choosing chips from a shopping list, I tend to pick whatever chips I have on my drawer and plug them on to make a circuit.

So the circuit might look slightly kludgey.

MASTER_SYSTEM_MAPPER.JPG (124.78 KB)

It's worth mentioning that while the boot ROMs use frame 2 for checksumming, they can use a more advanced mapper for the built-in game. Sonic, for example, pages in slot 1 too. So your diagram comment is a bit confusing.

Probably 95%+ of commercial games only use slot 2 (or are 32KB anyway), and homebrew if probably even higher, so this is very widely compatible.

Maxim wrote

It's worth mentioning that while the boot ROMs use frame 2 for checksumming, they can use a more advanced mapper for the built-in game. Sonic, for example, pages in slot 1 too. So your diagram comment is a bit confusing.

Probably 95%+ of commercial games only use slot 2 (or are 32KB anyway), and homebrew if probably even higher, so this is very widely compatible.

I just added the second latch register, for 0x4000-0x7FFF for you to see how badly the mapper increases in wiring complexity. No wonder they went through the trouble of making a simpler one (315-5208) for the 1 megabit games...

MASTER_SYSTEM_MAPPER_2.JPG (103.87 KB)

If it's being integrated into a mask ROM, wouldn't it be fairly insignificant?

Maxim wrote

If it's being integrated into a mask ROM, wouldn't it be fairly insignificant?

Yes, but at the time they were making those 315-5208s they didn't have integrated mappers on maskroms yet I believe.

Author	Message
l_oliveira Joined: 29 Jan 2014 Posts: 95 Location: Brasilia, Brazil	Discrete SMS mapper designs. Posted: Thu Nov 12, 2015 5:45 pm
	Many moons ago I promised that I would post something like this here but I never got around to. Someone at other forum PM'd me with this request so I done this circuit for him. Now, going down to business, the basic ROM mapper need to have: Bank switch latch, which holds the active page of the rom is shown at the banked region of the processor address space. Memory map control, which makes sure the memory segments which aren't mapped to bank switching stay at the expected pages (0 and 1 are supposed to show up from 0x0000 up to 0x7FFF). And finally the register array address decoder which generates the four necessary addresses for making a full mapper (over engineering but heck I need that much to generate just one strobe, why not generate all four at once?). Obviously, this works like a 315-5208 but is the starting point for drawing more complete mapper chips, like the 315-5235. Ideas of any kind are welcome. If mistakes are found, please do mention too. The original design I built in 2000 was too confusing because I used an array of 74LS74s to achieve a fixed power on state so I just decided to start from scratch with a new design. Also, instead of choosing chips from a shopping list, I tend to pick whatever chips I have on my drawer and plug them on to make a circuit. So the circuit might look slightly kludgey. MASTER_SYSTEM_MAPPER.JPG (124.78 KB)

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14745 Location: London	Posted: Thu Nov 12, 2015 8:09 pm
	It's worth mentioning that while the boot ROMs use frame 2 for checksumming, they can use a more advanced mapper for the built-in game. Sonic, for example, pages in slot 1 too. So your diagram comment is a bit confusing. Probably 95%+ of commercial games only use slot 2 (or are 32KB anyway), and homebrew if probably even higher, so this is very widely compatible.

l_oliveira Joined: 29 Jan 2014 Posts: 95 Location: Brasilia, Brazil	Posted: Thu Nov 12, 2015 9:19 pm
	Maxim wrote It's worth mentioning that while the boot ROMs use frame 2 for checksumming, they can use a more advanced mapper for the built-in game. Sonic, for example, pages in slot 1 too. So your diagram comment is a bit confusing. Probably 95%+ of commercial games only use slot 2 (or are 32KB anyway), and homebrew if probably even higher, so this is very widely compatible. I just added the second latch register, for 0x4000-0x7FFF for you to see how badly the mapper increases in wiring complexity. No wonder they went through the trouble of making a simpler one (315-5208) for the 1 megabit games... MASTER_SYSTEM_MAPPER_2.JPG (103.87 KB)

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14745 Location: London	Posted: Thu Nov 12, 2015 9:35 pm
	If it's being integrated into a mask ROM, wouldn't it be fairly insignificant?

l_oliveira Joined: 29 Jan 2014 Posts: 95 Location: Brasilia, Brazil	Posted: Thu Nov 12, 2015 9:39 pm
	Maxim wrote If it's being integrated into a mask ROM, wouldn't it be fairly insignificant? Yes, but at the time they were making those 315-5208s they didn't have integrated mappers on maskroms yet I believe.

Forums

View topic - Discrete SMS mapper designs.