Thanks, but I scrapped all my code and am starting from the ground up using your timer routine as the core
I know the timerTick is being called, because if I put, for example, a mode 8 screen trap in there then the screen constantly resets to mode 8
BUT when I try to increment a counter instead, the counter never increments (I can't use the example in your original timer code as none of the items are defined and I don't know what A3 is mean't to be set to either)
Am going to take a break from QL Assembler for now
Many thanks