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66D0: Advance game scroll and update bridge position
Used by the routines at play, main_loop and overview.
Called each frame to advance the vertical scroll position. Adds current speed (state_speed) to scroll offset (state_scroll_offset), updates the bridge's Y position, then resets speed to SPEED_NORMAL and updates control flags.
The control bits modified are: clears CONTROLS_BIT_SPEED_CHANGED, sets CONTROLS_BIT_SPEED_NOT_FAST. This triggers beep_engine_normal to play the normal speed engine sound.
advance_scroll 66D0 LD BC,($5F70) Add speed to scroll offset and store result.
66D4 LD H,$00
66D6 LD A,($5F64)
66D9 LD L,A
66DA ADD HL,BC
66DB LD ($5F70),HL Update bridge position; reset speed to SPEED_NORMAL.
66DE CALL update_bridge_scroll
66E1 LD A,$02
66E3 LD ($5F64),A
66E6 LD HL,$6BB0 Clear CONTROLS_BIT_SPEED_CHANGED, set CONTROLS_BIT_SPEED_NOT_FAST in state_controls.
66E9 RES 2,(HL)
66EB SET 1,(HL)
66ED RET
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