Well, I've pulled my head out of the DevNotes and GttMFH2E so here goes!

. . . sometimes there are buffers so that the memory controller can detach the RAM portion of the data bus from the rest of the data bus. Buffers are used to split a bus into two, so that separate action can take place in the two segments, or so that stray signals on one segment won't interfere with activity on the other segment.
I would start out by tracing the data lines on the logic board. Get the pinout of the 68030 and an ohmmeter and trace the data signals to see if they lead directly to the SIMM pins, or if there are buffers in between. I would bet that there must be, at least on Bank A, because the video circuitry is supposed to be able to access Bank A while the CPU is accessing bank B and the only way that can happen is if there are some buffers to isolate the two chunks of data bus from each other.

So the data bus will be fairly simple, but that built-in video may be a problem. And you can probably get access to the data bus in the PDS slot.

Click to expand...

From reading the docs on the IIfx, IIci and the IIsi, it seems that the only buffering is on Bank A. Bank A is a four SIMM array on the IIfx and IIci, but a pathetic 1 MB of SMT RAM ICs on the lowly IIsi.

Bank A is buffered for access as video memory on the IIci and the IIsi for the rudimentary Vampire Video hardware configured as a Pseudo Slot at address 00.

The IIfx, lacking this circuitry or buffering, still shows "Mac II Video" at Pseudo Slot 00, which I find very interesting.

If I'm reading the information on RAM access times of the IIci and the IIsi correctly, the video buffering on bank A has absolutely no effect on access times if there is no monitor (probably detected by polling the sense lines during initial setup of the machines at boot) using the Vampire Video Circuitry.

Therefore I surmise that I can likely ignore the buffering of Bank A on the IIsi, so long as no monitor is hooked up to Video Out on the MoBo.

Desoldering (or cutting the chip select a/o power traces to) the I MB of Bank A Mobo Ram and installing the equivalent of a maxed Bank A like the four SIMM Slots available on the IIci ought to be a possibility.

I haven't figured out the logistics of the hack as yet, I think I'll wait for some comments before proceeding.

Comments? :?: PLEASE!!! )