To pass the time and maybe have some fun along the way I've started to experiment with different Sega Dreamcast mods.
I figure the long term goal is to be able to play some Sega Naomi games using a Sega Dreamcast. It *isn't* about hacking Naomi games to work on a Sega Dreamcast, but hacking Sega Dreamcast games to use the Sega Naomi's higher quality textures.
More textures = more memory and in the case of more memory, it can also mean more pins used on 86 pin TSOP-II on the System RAM and the 50 Pin for the Video and sound RAM.
I have found replacement memory up to 32MB in size which at first glances appears compatible. The Dreamcast has 2 System RAM chips, so that's upto 64MB of installable memory. It may sound a lot, in fact it is a lot compared to the Standard 16MB, but the Naomi only has 32MB (2x 16MB?) of system RAM. The Hikaru and Naomi 2 both have 64MB of system RAM, but then again they have dual SH4 CPU's too so I'm guessing it is 2x (2x 16MB) ie the same size and chips asthe Naomi per CPU, but at is has more CPU's it has more RAM microchips.
AFAIK The Sega Dreamcast Memory Map Unit is usually disabled with support for upto 64MB RAM into several key areas mapped out already, just less is installed at the factory. The MMU can also be enabled so the system can look for the extra memory all by itself, but then I guess if you get too much RAM in there CPU or bus speed bottleneck issues may crop up.
Anyhow I digress. The replacement larger 86 pin TSOP-II ram for all intents and purposes looks and works the same as lesser RAM, but with a catch. There is an extra address line on pin 21 called A11. Now the system could work fine without it, the only way to know for sure is to just buy the stuff and test it, but I'm skint at the minute, I'll need to rebuy equipment to replace the stuff I broke ages ago and I have other priorities for my money at present but someone else may find the information helpful.
With the help of the Tec Toy VA0 CPU and DRAM schematic posted by l_oliviera, various microchip PDF's, a VA1 motherboard, a screwdriver, snippers and time and effort I managed to strip my Sega Dreamcast motherboard, albeit crudely, and map the traces for the most relevant pins from the system RAM to the SH4 CPU.
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There is no connection from pin 21 on the DRAM to any of the three spare address pins on the SH4 CPU (Y5, W6 and Y14 circled in white) and if I had a Naomi or Hikaru it would really help finding some clues, BUT the hope is should a connection be made the new, larger memory should take to the SH4 like a duck to water. The only way to know for sure is to test it and see what happens, the problem is finding a trace.
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With the map I made of the top side of the motherboard, I was able to make out some landmarks (which was a pain at first due to mirror image and other complications!) eventually finding the spots where the relevant SH4 CPU pins lie. As you can see, Y5 and W6 are probably a no go due to obstructions, but Y14 could prove useable if drilled through the back. Yes, I could have saved hours mapping if I just drilled through the top, then looked at the bottom and known for sure, but I couldn't find my small drill bits so <img alt="" class="mceSmilieSprite mceSmilie7" src="styles/default/xenforo/clear.png" title="Stick Out Tongue "/>
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Alternatively I think someone who is sneaky could perhaps slide a wire through the top side straight to the SH4 pins Y5 or Y14 as there is sufficient clearance, but it will be difficult to check for correct contact or shorts.
Anyhow it may be some time before I get rolling, by the time I'm in a position to do so, I'll probably have forgot all about it, given up, or been distracted by other things so hopefully someone else will either pick up where I left off and experiment some more, or provide help and advice to help keep the ball rolling! I think I'll end up buying an SPCO switch and wiring pins 21 to Y5 on one end, Y14 on the other and of course off in the middle so I can experiment and see exactly what happens with the larger RAM in either state.
Also, it looks like the Sega Dreamcast can be Jtagged using the ARM7/AICA in Serial Wire Debug mode.
I think I found the relevant contacts (with the aid of the VA0 schematics once again) on the back of the board, but again I have no way to test this until I rebuy some equipment -_-. The reason I started snooping for this was because I saw a G2_RQDEV# pin on the ext port schematic whilst lookjing to map it to a PCMCIA port. A little snooping around and most of the G2 external bus for the modem etc went to the Sound chip. After some more snooping I noticed the chip has an AICA TRST, AICA TDI, AICA TCK <i>(SWCLK!)</i> and AICA TMS <i>(SWDIO!)</i> , then the alarm bells were ringing <img alt="" class="mceSmilieSprite mceSmilie7" src="styles/default/xenforo/clear.png" title="Stick Out Tongue "/>
<a class="LbTrigger" data-href="misc/lightbox" href="https://assemblergames.com/attachments/sega-dreamcast-arm7-aica-serial-wire-debug-jtag-jpg.4444/" target="_blank"><img alt="Sega Dreamcast ARM7 AICA Serial Wire Debug JTAG.jpg" class="bbCodeImage LbImage" src="data/attachments/2/2422-f03e7d35275f8437cecf788a7ebce672.jpg"/>
I'm pretty sure I found the correct pads to JTAG, but it's late and I'm tired so I may have drawn them in the wrong order due to one of the resistors being labelled out of the expected sequence, but you should have no problem finding a source for ground or current for your 20 pin connector yourself. It is definitely these 4 connections though! The AICA TDO leads to IC401 pin 46 but I'm off to bed and I'll follow the end route when I wake up, but a full 5 pin JTAG Sega Dreamcast would rock hehe.
In the meantime, I have been struggling to find a link to linux DC that actually works. Many of the old project pages are full of dead links or years out of date. It would be useful to load it simply because it may be possible to view the memory availability states with or without the MMU enabled, or perhaps someone more famiilar with homebrew tools etc could write a short selfbootable app to report on the memory size and used space etc?