Do you know at which rate the mouse is polled? I'd like to know it in order to improve the mouse emulation in MAME for the Geneve. Currently, in MAME the mouse is polled at every video interrupt (60 Hz), which I believe is not correct; the rate must be much higher.
In fact, the mouse emulation has some glitches - it only works as long as the video interrupt is enabled. This includes the left button, which is sensed by the 9901, not by the v9938. I noticed that in the real system there are only two lines per axis, so I'd suspect that they carry values like 0, +1, -1 (while in MAME, values in the 8-bit range are sensed). But this does not suffice when polled at 60 Hz, because the mouse would need at least 4 seconds to travel from left to right (256 columns, move up to 60 columns per second).
I could not find this (actually quite important) information in the v9938 specification document.