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600A: Scan Kempston joystick
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Reads Kempston joystick port $1F. Unlike keyboard, Kempston uses active-high bits (NZ = pressed).
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| scan_kempston | 600A | LD A,$FE | Scan RIGHT | |
| 600C | IN A,($1F) | |||
| 600E | LD ($5EF1),A | |||
| 6011 | BIT 0,A | |||
| 6013 | CALL NZ,handle_right | |||
| 6016 | LD A,($5EF1) | Scan LEFT | ||
| 6019 | BIT 1,A | |||
| 601B | CALL NZ,handle_left | |||
| 601E | LD A,($5EF1) | Scan DOWN | ||
| 6021 | BIT 2,A | |||
| 6023 | CALL NZ,handle_down | |||
| 6026 | LD A,($5EF1) | Scan UP | ||
| 6029 | BIT 3,A | |||
| 602B | CALL NZ,handle_up | |||
| 602E | LD A,($5EF1) | Scan FIRE | ||
| 6031 | BIT 4,A | |||
| 6033 | CALL NZ,handle_fire | |||
| 6036 | JP main_loop | |||
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