traps defined by ROM 2

0       memory overflow
1       stack overflow
2       undefined opcode
3       divide by zero
4       keyboard scancode handler(places scancode at 0x102a)
5 - 15  reserved
16      command goes in d0(0=read, 1=write). disk goes in d1, number of bytes
        in d2, position to start at in d4, pointer to data in a0, error code
        in d3. 2 means the disk doesn't exist, 1 means read/write truncated,
        3 means invalid command.
17      draw an image(position goes in d6,d7; size goes in d4,d5; pointer to image
        goes in a0)
18      print a character to the screen(character goes in d0)
19      print a null-terminated string to the screen(address is popped off stack)
20      gets last scancode and places it in d0
