meanwhile, I played here with ideas how to simulate and design precise 7xmem pinout; originally, I expected also there even 32bit data chips, but, in fact, as on 7xmod interface I firmly settled of A00-A23 and D00-D15 only, mandatory, as common "async-SRAM facade" for any CPU placed on 7xmod, I relaxed also for 7xmem to use/abuse any chips there with 16bit data only, so no problem to place there randomly cheap 32bit data and use only half of it, sure... but esp. bga variants are smaller AND, although its not so much retro, yet, when I was thinking about TESTING of these chips, I browsed through some MCUs with sdram interface (no intents to go over plain sdram, sure) and I found quite interesting beast ...
stm32h7r7l8h6(h) (originally it was even different, slower, but this one has also 2D graphics accelerator, so abusable also for testing fast TUI, aka 7xtgv, ... so, got 2 of these, as were also cheaper, surprisingly - its todays price of ZS18033 cca ...) - but this beast has internally up to 600MHz and has external FMC (sram/sdram bus) and also XSPI (settled only on "octo" serial memories, these are in tiny bga24 packages - simplicity is in low-pin-count, while speeds may reach 200MHz and in craziest cases also using DDR clock ... ufff, not expected, anyway, but anybody can experiment ....)
this thing is expected to "simulate" 7xsys-cpld-core, as fast as possible, having "free" interface to memory mapped chips and to some extent at some slow speed CAN be exposed also as 7xmod (not yet final pinout, apart from "async-sram bus facade", its evolving) ... I just designed 7xmem tiny modules around capabilities of THIS beast, period ... as around that MCU is in fact almost nothing required to boot it up ... only power, not even xosc/xtal ... it has quite small flash - 64kB but huge internal SRAM too 640kB or so .... so autonomous for simulations even without 7xmem at all .. but when attached to 7xmem ... it maps them freely into address space, even those xspi ... cool/new kids ... not retro, I know ... but, huge ... they are internally, psram/dram, in retro better rather for bursting, like sdrams, so, probably why not to expose them ALSO only as ""LBA-anywhere"" or "just in-memory block device/storage" supported by 7xsys DMA to/from SRAM and to/from 7xbus CF/SD, transparently, single approach, anywhere, simple block driver, nothing more ... just "solid state storage, ramdisks AND almost free "suspend to disk" ... something like this" ... but that's future, and simulations can be cheap using that MCU "beast" .... ya, hw datasheet is 300p and ref manual almost 4kp ...))) .... but those are exactly things where Lyra can help to read/find/compare ...
so, till now, I have prepared this, as its all around mostly BGA FlexZIF and some stable assembly form those 50x50mm max PCBs, while some can be only 2/4 layer and so cheap even up to 100x100 as 4 on panel ... or those memory modules as "1/2-height" or even "1/3-height ..." ... I want 7xmem to be open standard on which anybody can put anything he has, design its own modules, maybe it finally will have also some eeprom id, like sodimms ... it is just combo of fast sram and fast sdram/xspiocto "solid state ram-storage, integrated over transparent LBA-approach to physical media we have, so CF/SD ... "
!!! WARNING - crazy as hell !!! ))
even before that, during "torturing of design", I am trying to imagine what most crazy can be put on 7xmod then, having that "16MB physical async-sram facade" ... so, apart from over year old ideas about mars-curiosity-alike PPC750CXe (it has NO memory controller, just his sync 60x bus, but has internal 256k L2 cache, useable just as SRAM scratchpad too), I asked myself how the hell were interfaced say 486 or rather something first 3V3, so P55C (MMX - I liked that particular one, dreamed at that time about mmx will soon support some vsti/dxi audio effects and synthesizers ... which was not so quick, obviously ...) ... so was chatting with Lyra about P55C bus, specifically, as this is quite normal 3V3 IO, almost normal non-pipelined bus ... okay, then I searched for available processors on ebay, and they were quite pricey ... but then I realized I have randomly here few "tualatin celerons", few, unknown state, but ... got cheap MB to test them, will be here soon ... and I asked Lyra how much harder is P6 ......... she almost shoot me that it is drastically complex ))... as it needs northbridge, she screamed that it looks "impossible" ...sure, it has that AGTL logic, down to 1.5V or rather 1.25V, center referenced AND drastically more complex pipelined transactional bus (ya, its far more near to PPC750CXe,, only even harder.... where P55C looks still classic - at least vs pure async-SRAM) ... P6 or rather PPro has NO memory controller too, like PPC750 ... and his northbridge also knows ONLY SDRAMs ... so, ya ... IF and only IF somebody even more crazy will try to implement "tiny NB for async-SRAM facade" - we slightly chatted about it ... although SHE was constantly trying to scare me ... constantly ... - then, sometimes even PPro tualatin celeron may land there .... )))) you know, torturing design to imagine limits, mostly this was reason ... to have it open/standard enough, to imagine what all SUCH beast needs;
so, for now, only this ... it must settle in mind and ... will see;

This is just 7xsys slow-simulator concept ... finally some CPLD, usually MAX-II, but for 5V tolerance even MAX-I can be used ... as dual/linked tqfp144 or so)
Question is IF there will be ALWAYS good to have at least tiny cpld also oon 7xmod to "glue" that any-cpu to 7xmod, .... probably yes, and this can be primitive
from epm3064 to fpga cyclone-II (which probaby at that time supported that AGTL ... newer not too much, specifically, but possible taht AGTL was just marketing push, and all new fpgas will allow it too ... I long time argued wit Lyra, that by moving LVC logic precisely low, that center reference may be NOT necessary, but she was quite strict here, obviously, ... nobody sane did anything so much weird, probably .... (fact is it was quite hard to find PPro original bus descriptions, as intel doesn't repeat this in newer datasheets ... but it was cool/interesting read ... definitely ... and even WILL BE ))
stm32CubeMX config for the MCU pins ... 7xmem now, exact pinout not yet selected ... will see;
(here are proposed those 2x40 1-27mm connectors I have here ... but cheaper PCBs/boards can target that "DDR2-80+GND" ... but it needs some "case" to hold it flat by screw...)
(now, using gravity or just force for BGAs to sit in "micro-grids" ... for testing ... it may be some fingers gymnastics also, or some 3D printed clips ... evolving still ...)
P.