The vLA81 has a few ‘hidden’ features which can only be fully unlocked with a 32K memory upgrade. If this is present then, as well as the standard 16K memory map, there are three other modes available.
They are all documented in the user manual but in summary they are:
RAM from 0x2000 (8192 dec) to 0x9FFF (40959 dec) – useful for HRG (high resolution graphics)
RAM from 0x4000 (16384 dec) to 0xBFFF (49151 dec) – the standard 32K expansion
RAM from 0x0000 to 0x7FFF (32767 dec) – a special mode which maps the ROM space to RAM
There have been several designs available to upgrade the ZX81 to 32K and I’ve always had the intention to provide a simple, inexpensive, design in kit form or fully built. I finally got around to it and here’s the result:
The design incorporates a thin PCB ‘shim’ between the 32K ram chip and the socket. The PCB does the simple job of rerouting a few pins to the right place and has no active components. Most of the ram chip’s existing pins are left intact. Four address wires are required to be connected to the ZX81 PCB as well as one short jumper under the board to connect A13 to the vLA81.
Available soon at all good vLA81 stockists 🙂