; PURSUE -- point the engine at the nearest asteroid and burn.
; Crude: it makes no attempt to match velocity, so it will fly straight past.
;
        .equ    P_THROTTLE      0x20
        .equ    P_AZIMUTH       0x21
        .equ    P_PITCH         0x22
        .equ    P_SELECT        0x30
        .equ    P_NEAREST       0x31
        .equ    P_RELX          0x32
        .equ    P_RELY          0x33
        .equ    P_RELZ          0x34
        .equ    P_MATHX         0x80
        .equ    P_MATHY         0x81
        .equ    P_ATAN2         0x83
        .equ    P_HYPOT         0x84

        in      r1, P_NEAREST           ; id of the nearest asteroid
        out     P_SELECT, r1            ; track it
        ldi     r7, 1000
        out     P_THROTTLE, r7          ; full power

aim:    in      r1, P_RELX              ; where is it?
        in      r2, P_RELY
        in      r3, P_RELZ
        out     P_MATHX, r1
        out     P_MATHY, r2
        in      r4, P_ATAN2             ; azimuth = atan2(dy, dx)
        out     P_AZIMUTH, r4
        in      r5, P_HYPOT             ; horizontal range = hypot(dx, dy)
        out     P_MATHX, r5
        out     P_MATHY, r3
        in      r6, P_ATAN2             ; elevation = atan2(dz, range)
        out     P_PITCH, r6
        yield
        jmp     aim