Have you seen Hoglet's 6502 protocol decoder? With little more needed than the databus, it can decode the full 6502 register state and program execution. So a cheap 16 channel logic analyser will do.
It also handles 65816, and he's done versions for Z80 and 6809 too. And 6309. Even found a previously unknown extra register in that!
Can install on a Raspberry Pi if your OS is too slow to keep up.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko