Here is an experiment. I'd do it myself if I thought there were any chance I would get round to it before the Millennium...
Have a counting loop, say "lp: inc bc; jp lp" (note that this takes 20 cycles off-screen and 32 on-screen). Have an interrupt routine which says "pop hl; ei; ret". Now push a return address, push the address of the loop, set bc to 0, enable interrupts and halt. The first interrupt will cause a jump to the counting loop, and the next one will cause an exit to the return address. There you return the interrupts to normal and print out the value of bc. If the screen is off we expect an answer in the region of 5989. Someone else can calculate what we expect when the screen is on, but more importantly the answer gained in mode 1 will probably allow the contention behaviour to be inferred. Especially if you do the experiment again with a line interrupt set at the top of the screen (this will tell you if there is any contention in the top border area). imc
