The vertical sync interrupt generated in vc_svc() was too slow for Ultrix. The driver enables interrupts and then calls DELAY(20000), which I guess is 20ms. This translates to about 10000 simulated instructions, but tmxr_poll works out at ~75000 instructions. I've added a new unit to simulate just the vsync interrupts activating every 8000 instructions.
This approach will completely disable the ability to idle while the QVSS device is enabled.
Prior fix for cursor tracking allowed this to now be observed.
The description of the prior change should have been:
The status bits need to be set in the upper byte of the DUART receiver buffer. This is not mentioned in the DEC manual but I found a Phillips datasheet for the 2681 chip, which has a small footnote about it.
