Leigh Klotz was the man who ported MIT Logo to the Apple ][ and then the Commodore 64. According to him “The Commodore 64 CPU 6510 has a bidirectional parallel port at location 0 and 1, taking up 2 of the 256 "page zero" locations, which are the only ones you can indirect through. When I ported MIT Logo from the Apple II, there were lots of places that dereferenced nil without checking, and those caused crashes. Commodore gave me a chip they fabbed in qty 12 yield that brought out the I/D decide status as a pin, and we used a Nicolet-Paratronica logic analyzer to feed the address and data bus to a Pet running a BASIC disassembler. I could then set a breakpoint in-circuit to see the 256 instructions prior to or after the errant memory access, so I could go put on guard code...”
I got it from Jamie Zawinski's weblog. Be forewarned. Following the link from Hacker News will lead to an unsavory image. Just copy & paste it, instead.
https://www.jwz.org/blog/2018/11/weird-machines/#comment-192...