6502 tracer

elektor July august 1985 11.44114411 111441144111 A program that has been written into an assembler will rarely run error free on the first run. It often exhibits blurbs and other ramblings: in bad cases, there is a complete hang up and it is then necessary to start the computer afresh with a RESET. To find such faults in a relatively easy manner, the tracer described here will be found very useful. The circuit layout of the tracer is shown in figure 1. Gate N1is an address decoder, whose output in the address range $F000...$FFFF is logic 0. NAND gate N2 is fed with the SYNC signal from the computer and the 0 signal; it is disabled by either the address decoder, N1, or bistable FF2. The address decoder disables N2 when the EPROM is addressed from the CPU. This prevents the SYNC line of the 6502 processor generating an MI (maskable interrupt). If the pro- cessor passes through a machine program somewhere in the RAM, N2 generates an interrupt as soon as the processor reads an opcode...