Alien 8: Routine at 51540

51540: Read Kempston Joystick

Used by the routine at ReadControls.

Reads the controls from a Kempston joystick.

Output
C	The controls

Read the port and map each bit to a control.

ReadKempstonJoystick

51540

IN A,(31)

Read port 31.

51542

LD C,0

Set C to 0.

51544

BIT 0,A

Set bit 1 of C if bit 0 of A is set.

51546

JR Z,ReadKempstonJoystick_TestLeft

51548

SET 1,C

ReadKempstonJoystick_TestLeft

51550

BIT 1,A

Set bit 0 of C if bit 1 of A is set.

51552

JR Z,ReadKempstonJoystick_TestDown

51554

SET 0,C

ReadKempstonJoystick_TestDown

51556

BIT 2,A

Set bit 4 of C if bit 2 of A is set.

51558

JR Z,ReadKempstonJoystick_TestUp

51560

SET 4,C

ReadKempstonJoystick_TestUp

51562

BIT 3,A

Set bit 2 of C if bit 3 of A is set.

51564

JR Z,ReadKempstonJoystick_TestFire

51566

SET 2,C

ReadKempstonJoystick_TestFire

51568

BIT 4,A

Set bit 3 of C if bit 4 of A is set.

51570

JR Z,ReadKempstonJoystick_Finish

51572

SET 3,C

ReadKempstonJoystick_Finish

51574

JP ReadControls_Finish

Jump to ReadControls_Finish.

View the equivalent code in;