[SiriusHardware]

One good reason to stick with your TTL decoder and avoid PROMs (apart from the difficulty they present for most people programming them) is that they are quite thirsty devices, easily over 100mA drawn by one chip. I don't think we need more excess current drain than we already have. So yes, if we must use a one chip address decoder then a GAL would presumably be better in that respect. I have wondered, without looking into it, whether an appropriately programmed GAL with its power pins in the right place could be used as a low power, lower cost substitute for the 74S157s. To hold the OS I mean, not as an address decoder.

I will try your 'simple' memory upgrade, but I'll do it in a phased way, first of all just with an edge connector and DIN connector joined one to one by 'double sided' stripboard and I'll make the minor mods required for basic connection to the VDU on that. Unlike Tim I'll probably use the 8154 to control the VDU initially because all the required connections are already in place. I'll get that working first, with the VDU scanning existing RAM.

Then on the copper underside of that interconnect board I will fix a smaller square of stripboard with its blank side 'up' against the underside of the interconnect PCB and its copper side 'down', facing the table, and on that smaller square I will build your RAM upgrade 'ugly' style by folding the pins of the ICs flat out sideways along the copper tracks and soldering them to the tracks - then I'll use wire-wrap type wire to pick up the data, address and control lines (NRDS, NWDS, etc) from the tracks on the underside of the interconnect PCB.

Hopefully, that will all 'Just Work'.