CD Backup RAM Cart

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Chilly Willy
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Post by Chilly Willy » Mon Mar 07, 2011 6:13 pm

sega16 wrote:
Chilly Willy wrote:There's not enough space in the CD memory map for that much ram. Actually, the save cart is in the MD space, but you know what I mean. I think 1MB is the most if you follow that "standard" design. If you simply take all the empty space, you can have at most 4MB of ram with the SCD.
Still 4mb is alot of ram (for a sega cd).Also I forgot to post that I was asking If it could just be used as regular ram and not just ram for game saves.I think a ram cartridge like that would have alot of uses for example if you want to play a movie you could have a 4mb buffer and 2mb decompressed of the movie could be stored while the other 2mb is being decompressed so in turn that would allow better compressed movies because it has more time to decompress it.
Yes, I'd love to have a 4MB ram cart for CD usage. Lack of ram is a big problem on the MD in general.

sega16
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Post by sega16 » Mon Mar 07, 2011 7:49 pm

Chilly Willy wrote: Yes, I'd love to have a 4MB ram cart for CD usage. Lack of ram is a big problem on the MD in general.
I agree with you with a ram cart you could do so much more than you can with just 64kb of ram.I don't want to derail this topic too much but has this already been asked?Is it even possible to make a ram cart.Anyway I think the best way to make a ram cart is to research ram and find a good one and figure out a "plan" to make it.

l_oliveira
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Post by l_oliveira » Tue Mar 08, 2011 4:25 pm

I think that the first step towards making an *WORK RAM* expansion is search the MD side of the CD-BIOS and see if it has mechanisms to detect and determine if any extra connected memory is or isn't backup memory. From that information we can derive an circuit for such a cartridge.

I believe that 1MB of ram is actually an ENORMOUS amount of memory for such a cart (considering that we have an CD-ROM drive and we can aways load stuff from it) also I suppose that 1MB was SEGA's own plan for the work ram memory expansion maximum size. Because of the way how the expansion connector works and how the address lines are connected I suspect that the maximum of work ram memory we can set-up and use (without memory banking systems) is 1MB. :roll:


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sega16
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Post by sega16 » Wed Mar 09, 2011 1:50 am

I think to make a ram cart you would want to figure out how the 32x and make something simular.. just a thought what if someone made a modern day sega genesis enhancer using something like a 166mhz intel celoron,I even seen them going for about $7 dollars (usa money)Maybe you could you the chip to access/help the ram and enhance the sega genesis.

Chilly Willy
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Post by Chilly Willy » Wed Mar 09, 2011 4:56 am

We don't want to stick a modern computer on the MD - that would defeat the whole purpose of working on the MD. We just want more ram. Period. Nothing else. Well, maybe a nice SD card interface to go with it.
:D

Tomy
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Re: CD Backup RAM Cart

Post by Tomy » Wed Mar 06, 2013 3:29 am

Charles MacDonald wrote: The memory map looks like this:

400000-4FFFFF : Cart ID; reads return 0x04 on D7-D0 only.
680000-6BFFFF : 1st 128Kx8 RAM chip; data on D7-D0 only.
6C0000-6FFFFF : 2nd 128Kx8 RAM chip; data on D7-D0 only. (not used)
700000-7FFFFF : Writes by /LWR latch D0; 0=RAM write enabled, 1=disabled

So the RAMs span 128K words (256K space) at D7-D0 only using /LWR for the write strobe.
From Charles's post, the maximum memory map on it is 256K Bytes.

If I want to make a 512K Bytes CD Backup RAM Cart, does memory map look like this :

400000-4FFFFF : Cart ID; reads return 0x04 on D7-D0 only.
600000-63FFFF : 1st 128Kx8 RAM chip; data on D7-D0 only.
640000-67FFFF : 2nd 128Kx8 RAM chip; data on D7-D0 only.
680000-6BFFFF : 3rd 128Kx8 RAM chip; data on D7-D0 only.
6C0000-6FFFFF : 4th 128Kx8 RAM chip; data on D7-D0 only.
700000-7FFFFF : Writes by /LWR latch D0; 0=RAM write enabled, 1=disabled

Can someone confirm this ?
Thank you very much.

Charles MacDonald
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Re: CD Backup RAM Cart

Post by Charles MacDonald » Wed Mar 06, 2013 3:56 am

Can someone confirm this ?
Thank you very much.
This seems correct, but somebody should verify that the Sega CD BIOS can really detect and use more memory. I don't know myself, but there's a chance it only ever checks 680000+ and never looks at 600000+.

I think you could test this by modifying a Genesis emulator to have a 512K save RAM cartridge with an ID of 0x05 instead of 0x04, and see if the BIOS allows that much data to be saved and loaded.

Tomy
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Post by Tomy » Wed Mar 06, 2013 5:19 am

Charles, Thanks for your reply.

TmEE co.(TM) told me that only MCD/SEGACD version 1 can not use 512K Bytes RAM Card. Other MCD/SEGACD should be no problem.

Charles MacDonald
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Post by Charles MacDonald » Wed Mar 06, 2013 6:21 am

Tomy wrote:Charles, Thanks for your reply.

TmEE co.(TM) told me that only MCD/SEGACD version 1 can not use 512K Bytes RAM Card. Other MCD/SEGACD should be no problem.
Oh great, that's good to know. And I suppose if you make such a card you can test the prototype out first on a version 2 machine to be sure. :)

I wonder if 512K is the limit, or if the version 2 BIOS could even do 1MB at 500000+.

l_oliveira
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Post by l_oliveira » Wed Apr 24, 2013 1:14 am

The only MEGA-CD rom which can't use 8192 blocks (512KB) backup ram cart is:
SEGA MEGA DRIVE
(C)SEGA 1991.NOV
MEGA-CD BOOT ROM
12/17-1991
12:00 1.00p
MEGA-CD BOOT ROM
Basically the very first release of the BIOS(9111) for Japan.

The 9112 1.00 version (I tested ASIA version which bears "PAL COMPATIBLE MEGA-CD" into the regional lockout screen) works fine with this cart and is what I used to make the screenshots back in 2011. I assume the Japanese 9112 BIOS is identical to the ASIA version regarding the backup RAM function...

Eke
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Post by Eke » Wed Apr 24, 2013 9:11 pm

I think you could test this by modifying a Genesis emulator to have a 512K save RAM cartridge with an ID of 0x05 instead of 0x04, and see if the BIOS allows that much data to be saved and loaded.
I emulate 512KB cartridge RAM by default in Genesis Plus GX (but returns 0x06 as RAM ID, not 0x05) and it works with every OS ROM I tried, even the one mentioned above by l_oliveira, although I cannot confirm if emulation is 100% exact :?:

also works with older version

Code: Select all

SEGA MEGA DRIVE 
(C)SEGA 1991.NOV
MEGA-CD BOOT ROM
12/02-1991     
15:20      1.00l
MEGA-CD BOOT ROM
Image

l_oliveira
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Post by l_oliveira » Wed Apr 24, 2013 9:34 pm

Eke wrote:
I think you could test this by modifying a Genesis emulator to have a 512K save RAM cartridge with an ID of 0x05 instead of 0x04, and see if the BIOS allows that much data to be saved and loaded.
I emulate 512KB cartridge RAM by default in Genesis Plus GX (but returns 0x06 as RAM ID, not 0x05) and it works with every OS ROM I tried, even the one mentioned above by l_oliveira, although I cannot confirm if emulation is 100% exact :?:

also works with older version

Code: Select all

SEGA MEGA DRIVE 
(C)SEGA 1991.NOV
MEGA-CD BOOT ROM
12/02-1991     
15:20      1.00l
MEGA-CD BOOT ROM
Image
Very interesting reply. That could mean a bug on my design then ! :)

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db-electronics
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Post by db-electronics » Fri Mar 14, 2014 7:11 pm

Charles MacDonald wrote: Oh great, that's good to know. And I suppose if you make such a card you can test the prototype out first on a version 2 machine to be sure. :)

I wonder if 512K is the limit, or if the version 2 BIOS could even do 1MB at 500000+.
I can confirm a 512KB Backup RAM Cart works on:

Sega CD Model 1 Bios 1.10
Sega CD Model 2 Bios 2.00
Mega CD Model 1 (Bios unsure, tested by others)
Mega CD Model 2 (Bios unsure, tested by others)
Sega CDX
Sega CDX + 32X
Sega Multimega
JVC X'eye

I haven't tested any cart IDs higher than 0x06.
What does db stand for? Well that's an excellent question...
http://www.db-electronics.ca

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