ESGG External RAM Disk

[jltursan]

After building the Dreamdisk, the main issue I've found is the presence of this board in my Sorcerer:

ESGG EXTRAMDISK.jpg (204.03 KiB) Viewed 92670 times

It originally had a 2732 EPROM and a flat cable that goes to socket 1E, replacing the first Sorcerer ROM. The second ROM, 2E, was also replaced in the same motherboard. Seems that the contents of these EPROMs was a combo of "ESAG monitor 1.3B" + standard Exidy monitor 1.0. It booted with the 1.3B version and all worked fine.

I've located a complete documentation of the board; but it's in dutch and I cannot easily translate the text . You can found the manual here, it has a truckload of docs, the RAMDISK is the one titled ESGG RAM DISK uitbreiding

Now the funny part, the PCB seems to have some kind of "port selector" set to 7Fh and looking at the manual and schematics seems that it's used to something related to addressing. The manual also mentions some kind of RAM replacing with 256Kb ICs...or at least I think so.

And yep, my motherboard has all RAM replaced:

768KB_or_not.jpg (161.78 KiB) Viewed 92670 times

I've always thought that it's only a relatively common mod to use 256Kbx1 ICs instead the original 4116 ones; but no, seems that there're some wires described in the manual and given that it's a RAMDISK extension, maybe it could be some kind of paged RAM expansión used as a RAMDISK??. Does the ESAG monitor adds some commands to make it work?

So if anyone knows anything about this expansion, decipher the schematics or at least can read the manual, please, let me know...

[Abzman]

That is very interesting, I'll let you know if I come up with anything

[Abzman]

Do you have exact images of the ROMs in the system? I'd be interested in looking at them

[jltursan]

Thanks!. Sure, I've dumped the EPROMs:

EPROMS monitor 1.3.zip

(4.89 KiB) Downloaded 441 times

Remember that they're dual EPROMs. 1E has first halves of 1.3b and 1.0, 2E has the 2nd halves. If you want to disassemble the 1.3b monitor you'll need first to divide each binary file and then concat both parts of the 1.3b monitor.

[Abzman]

can I also get a picture of the back side of that PCB and any of the wire bodges on the sorcerer motherboard? I'm trying to be 100% sure what they're doing there.

[RedskullDC]

Hi All,

Judging by the schematic, it appears that one of 16 banks of 16KB can be selected for each of the normal 16KB areas == 768KB total (?)
Select by writing D3-D0 at port 7Fh (jumper selectable), which controls the top 2 address bits of the DRAMs.
Always resets to 00 for A7,A8.

Presumably, the modified monitor takes care of maintaining the stack during context swaps?

Will take a look at the monitor rom a bit later on.

Cheers,
Leslie

[RedskullDC]

Just having a look at the ESGG Monitor.

Most obvious difference is that the 'MO' routine is now smart enough to detect overlaps,
and uses LDDR/LDIR as appropriate.

Routine which copies the GFX chars to the FC00h area is quite ingenious. Instead of just a block move, it builds the characters on the fly from a bit-packed table. Saves lots of space.

Still working out what is in that saved space....

Haven't found any code to control the RAMDISK as yet

[jltursan]

That's really interesting!

Peeking the manual, although my lack of dutch knowledge, I've seen a tool is mentioned, EXTRAMDSK that obviously is a resident program that load and works as a driver to use the extra RAM as a real formatted disk. This probably means that all or most of the code logic related to the expansion is into this tool (I need to find it, must be preserved somewhere...).

I'm still worried about how this RAM is initialized, maybe ESGG monitor does some OUT right after booting to enable a valid RAM bank and SCUAMON, obviously, does not?. Using SCUAMON I'll try to check if RAM is present in each of the three 16KB RAM banks (0000, 4000 and 8000)

Btw, here're the pics of the back of the PCB and the motherboard wiring:

EXTRAMDSK_back.jpg (144.39 KiB) Viewed 92630 times

EXTRAMDSK_wiring.jpg (239.9 KiB) Viewed 92630 times

[Abzman]

what sort of hacks are going on to get the 28 pin 2764 chips into the 24 pin spots on both the motherboard and the ram board? I don't think it'll end up being super important, I'm just interested now.

[Abzman]

I think I understand the jumpering that makes it fit 2764 chips, but what's going on with A11 to switch between the monitors? Is it just hooked to a toggle switch?

Once we figure out how to use this I'm tempted to make one. I already laid out a replica in KiCAD