ZX81ers don’t despair! This next trick was originally published in Syntax Quarterly, Summer 1983. It will allow you to use the 32-48K RAM area for machine code, if you have the Memotech or JLO 64K boards (and maybe some others, but not the Byte-Back UM).
It is necessary to add a little extra circuitry to separate out the video system from the dynamic RAM system, as Sinclair never dreamed of a ZX81 with more than 16K. Very simply, you cut the M1 not trace between the Z80 (pin 27) and the ULA (pin 10). Connect pin 2 to the Z80 side and the output on pin 8 to the ULA side. Again, use an ohm meter to chase down the A14, A15, Vcc (5 volts) and ground lines. They can all be picked up at or near the edge connector (except pin 8), but you may find some more convenient places. The diagrams show some very good places to pick up these traces. Be sure to remove all ICs (if possible) before doing any soldering.
Any relocatable programs will run here and not be affected by NEW or reset. ZXLR-8, Delphic toolkit, compiled programs with the Bob Berch Compiler, Hot Z and an assortment of other utilities will work up there and NOT interfere with any 16K program on the market.
