buffer[2048] screen @ 1024
byte joy2 @ $dc00

pointer ptr @ 254
word pos
word delta

//
// The constraints on these 2 vectors are kind-of sort-of big fibs.
// They're only written this way so they can be compatible with our
// routine.  In fact, CINV is an interrupt routine where it doesn't
// really matter what you trash anyway, because all registers were
/// saved by the caller (the KERNAL) and will be restored by the end
// of the code of the saved origin cinv routine that we goto.
//
// I wonder if this could be arranged somehow to be less fibby, in
// a future version of SixtyPical.
//

vector cinv
  inputs joy2, pos
  outputs delta, pos, screen
  trashes a, x, y, c, z, n, v, ptr
  @ 788

vector save_cinv
  inputs joy2, pos
  outputs delta, pos, screen
  trashes a, x, y, c, z, n, v, ptr

routine read_stick
  inputs joy2
  outputs delta
  trashes a, x, z, n
{
    ld x, joy2
    ld a, x
    and a, 1                   // up
    if z {
        copy $ffd8, delta      // -40
    } else {
        ld a, x
        and a, 2               // down
        if z {
            copy word 40, delta
        } else {
            ld a, x
            and a, 4               // left
            if z {
                copy $ffff, delta  // -1
            } else {
                ld a, x
                and a, 8           // right
                if z {
                    copy word 1, delta
                } else {
                    copy word 0, delta
                }
            }
        }
    }
}

routine our_cinv
  inputs joy2, pos
  outputs delta, pos, screen
  trashes a, x, y, c, z, n, v, ptr
{
    call read_stick

    st off, c
    add pos, delta

    copy ^screen, ptr
    st off, c
    add ptr, pos

    ld y, 0
    copy 81, [ptr] + y

    goto save_cinv
}

routine main
  inputs cinv
  outputs cinv, save_cinv, pos
  trashes a, n, z
{
    copy word 0, pos
    with interrupts off {
        copy cinv, save_cinv
        copy our_cinv, cinv
    }
}