To interpret the commands being sent in the serial data, it used a primitive central processing unit (CPU) built from small-scale-integration integrated circuits. It examined the data while the display hardware was inactive between raster scan lines, and then triggered the display hardware to take over at the appropriate time. The display system returned control to the CPU when it completed drawing the line. The CPU was so basic that addition and subtraction could only be done by repeatedly incrementing or decrementing two registers. Moreover, the time taken by such a loop had to be nearly constant, or text lower on the screen would be displayed in the wrong place during that refresh.