Peripheral 1 Register

Number	$05
Readable	Yes
Writable	Yes
Short Description	Sets joystick mode, video frequency and Scandoubler.

Bit	Function
7-6	Joystick 1 mode (LSB)
5-4	Joystick 2 mode (LSB)
3	Joystick 1 mode (MSB)
2	50/60 Hz mode (0 = 50Hz, 1 = 60Hz) (0 after a PoR or Hard-reset)
1	Joystick 2 mode (MSB)
0	Enable Scandoubler (1 = enabled) (1 after a PoR or Hard-reset)

Joystick modes are as follows:

Code	Effect
%000	Sinclair Joystick 2 (maps to keys 6,7,8,9,0)
%001	Kempston Joystick 1 (uses port Kempston Joystick ($xx1F / 31))
%010	Cursor Joystick (maps to keys 5,6,7,8,0)
%011	Sinclair Joystick 1 (maps to keys 1,2,3,4,5)
%100	Kempston Joystick 2 (uses port Kempston Joystick 2, Joystick I/O ($xx37 / 55))
%101	MD 1 (3 or 6 button joystick port Kempston Joystick ($xx1F / 31))
%110	MD 2 (3 or 6 button joystick port Kempston Joystick 2, Joystick I/O ($xx37 / 55))

Older version of this page warned about configuring at most one joystick as Kempston-1 and at most one as Cursor. The specnext.com refreshed documentation about ports does not warn about such limitation, but rather proceed with caution and avoid setting up both joysticks to the same type (unless somebody from core team clarifies this).

"MD" above refers to controllers compatible with SEGA Mega Drive system. The 6 (and 3) button variant is mapped onto Kempston port this way (mirroring XYZ buttons on the ABC buttons, so only 4 distinct buttons are supported):

Bit	Joystick action
7	"start" button
6	A or X button
5	C or Z button
4	B or Y button
3	Up
2	Down
1	Left
0	Right