Servicing Personal Computers, 3rd Edition

120 Servicing Personal Computers

The result of a forced NOP instruction is that the CPU successively increments the program

counter and continuously cycles through its complete address range. Hence a binary count

sequence is performed with the waveform on each address line appearing as a square wave.

The frequency of the square wave on address line 0 (A0) will be twice that appearing on

address line 1 (ΑΊ) and so on, as depicted in Figure 3.21. This can be a most useful test since

the repetitive waveforms generated on each address line are ideal for display on a

conventional oscilloscope. It is simply a matter of connecting the oscilloscop