I designed an internal 512K Ram upgrade for the Sinclair QL computer and a system Rom selection circuit.
I used a 512k x 8bit static ram 684000 IC and an AM29F040B 4Mbit (512kb) flash chip for the roms.
I use 2 toggle switches for rom set selection.
By removing the original QL rom chips I was able to house the 684000 ram chip into IC33 socket and the AM29F040B into the IC34 socket of course after bending the pins that do not match and connecting them using wires to the correct signals.
I programmed the JS rom from 0000 to BFFF, the TK2 from C000 to FFFF, Minerva from 10000 to 1BFFF, Tk2 again from 1C000 to 1FFFF and so on with the greek rom+tk2 as the third set and finally the GST 68K/OS.
This is the schematic:
