Nimbus joystick fix

[RobinSergeant]

This pull request fixes RM Nimbus joystick emulation by using the correct port address for directional data and setting the correct bits when that address is read. Currently the mouse data port (0xa4) is used for all joystick data, but only the buttons are actually read from this port. The joystick port (0xa0) is used for the directional data as described in chapter 2 of of the service manual (https://www.thenimbus.co.uk/upgrades-and-maintenance/service-manual).

This can be demonstrated/tested by running the following RM Basic program which does not work without my change:

10 SET MODE 40
20 SET JOYSTICK
30 REPEAT
40   ASK JOYSTICK X%, Y%, Button%
50   POINTS X%, Y%
60 UNTIL Button% = 1

I have also removed the "Input Port 0 Device" config port as this is no longer necessary and prevented the mouse buttons from working properly when set to the wrong value (the original code read joystick data when this was to "mouse", and mouse data when it was set to "joystick"). The mouse and joystick can now co-exist quite happily with joystick buttons doubling as mouse buttons and vice versa.

The wrong bits were also being set for each direction and I have of course corrected these to match those given in the service manual. The above RM Basic program proves that theses are now correct.

The original machine also allowed switching between two joysticks by writing to ports 0xa0 (to select joystick 0) and 0xa2 (to select joystick 1), but I have not implemented this as it seems unnecessary - I have not found any software that makes use of it and I don't think it's possible to test using RM Basic.

Please let me know if you have any questions!

[cuavas]

This seems like a very odd design decision, and at first glance it would seem to require more logic than just mapping one direction to each bit. Do you know why it has this quirk of always setting the left bit for horizontal directions and always setting the down bit for vertical directions?

[RobinSergeant]

I'm guessing it's related to the physical hardware design, but it's actually stated in the service manual. They have a table saying something like "right = left + right" :-) Maybe it's really a bit to say the X axis is effected and then a bit for positive/negative.

It threw me at first because without setting the extra bits only left and down worked. Then I noticed the cryptic table in the service manual.

All 8 directions work with my RM basic test app so fairly confident it's correct now!