The 64K ROM has the top 2 bits controlled by the DIP switch to
select 1 of 4 different software versions. The two bits were also
connected to the HSYNC pld to configure the hardware at the same
time.
Each image occupies 16K. The images are selected by DIP
switches.:
| A15,14: | Selection: |
| 00 | ZX80 |
| 01 | ZX81 |
| 10 | Lambda ZX81 |
| 11 | Lambda standard |
The 'Lambda' modes were for a (near) clone of the ZX81 which had slightly different hardware timing and had two ROMs available. One was functionally identical to the ZX81 and the other was similar but didn't have single key entry etc. and wasn't code compatible.