=> R0 = joystick number and reason code
bits 0-7 joy stick number (0 = first)
bits 8-15 reason code
bits 16-31 reserved (must be zero)
<= Dependant upon reason code
This SWI reads the current state of the joystick, assuming the Joystick module is loaded and hardware present.
See PRM 4-208, PRM 5a-638
Joystick_Read reason codes
Code /Meaning/
0 Return 8 bit state of digital or analogue joystick
1 Return 16 bit state of analogue joystick
Joystick_Read 8 bit state
=> R0 = joystick number and reason code
bits 0-7 joy stick number
bits 8-15 0 (reason code)
bits 16-31 reserved
<= R0 = 8 bit state of joystick
Applications which are only concerned with state (up, down, left, right) should not merely test bytes for postive, negative or zero. At rest should span a range such as -32 to 32 since analogue joysticks cannot be relied upon to produce 0 at rest.
This reason code is available from RISC OS 3 onwards.
See also: Note about analogue joysticks
PRM 5a-640
8 bit joystick state
Represented in the form &RRSSHHVV.
Byte /Value/
V Signed Y value in the range -127 to 127 (hence 0 = centre)
For a single switch joystick, -64 = Down, 0 = Rest and 64 = Up.
H Signed X value in the range -127 to 127 (hence 0 = centre)
For a single switch joystick, -64 = Left, 0 = Rest and 64 = Right.
S Switches (eg fire buttons), unimplemented switches return 0
R reserved
Joystick_Read 16 bit state
=> R0 = joystick number and reason code
bits 0-7 joy stick number
bits 8-15 1 (reason code)
bits 16-31 reserved
<= R0 = 16 bit joystick position
R1 = joystick switch state
Applications which are only concerned with state (up, down, left, right) should not merely test bytes for postive, negative or zero. At rest should span a range such as 24576 (&6000) to 40960 (&A000) since analogue joysticks cannot be relied upon to produce a given value at rest.
This reason code is available from RISC OS 3.60 onwards.
See also: Note about analogue joysticks
PRM 5a-641
16 bit joystick position
Bits /Value/
0-15 unsigned Y value in the range 0 (down) to 65535 (up)
15-31 unsigned X value in the range 0 (left) to 65535 (right)
16 bit switch state
Bits /Value/
0-7 switches (eg fire buttons) starting in bit 0; unimplemented switches return 0
8-31 reserved
Note about analogue joysticks
For analogue joysticks, Joystick_Read reads the last conversion made, but does not force a conversion itself. Conversions are not started until the first call of Joystick_Read, hence the first call to Joystick_Read results in X = 0, Y = 0 with no closed switches since there is no completed conversion to read.
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