ocr: byte 0 one-second interrupt VIAOS CA2 control line 4 vertical retrace interrupt VIAOS CA1 control line 8 shift-register interrupt VIAOS shift register 12 not used 16 not used 20 T2 timer: Disk Driver VIAOS timer 2 24 T1 timer: Sound Driver VIAOS timer 1 28 not used Figure 5DLevel-1 Secondary Interrupt Vector Table