\ R0 := x; R1 := y
ADD       R2, R0, R0, LSL #2  \ R2 := R1 + (R1 << 2)   [== (5 * y)]
MOV       R2, R2, LSL #6      \ R2 := (R2 << 6)        [== (320 * y)]
ADD       R2, R2, R0          \ R2 := R2 + R0          [== (320 * y) + x)]
                              \ R2 := screen offset