Bus sniffing the IBM 5150

Writing a cycle-accurate emulator for a computer system is more than just understanding all the CPU instruction timings. A computer is a complete system with peripherals, interrupts, IO bus signals, and DMA. All this comes with an array of different timings and quirks.

When software like Area 5150 is written that requires perfect cycle timing, it can be a challenge to provide the level of accuracy needed for the software to function. Area 5150 in particular requires precise coordination with the CGA's CRTC chip and timer interrupts to begin the end credits demo effect at precisely the right time.

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It would be very handy then if we could somehow peek into the operation of the system while it was running and understand how all these parts interact. As it turns out, we can! This process is typically referred to as bus sniffing', and there's a lot of a technical information out there on the topic in general. Sniffing can be done on everything from ethernet networks to vending machines, and you can even bus sniff your car. This article will specifically discuss sniffing the IBM PC 5150.

A very in-depth and technical article, and one that can easily lead to another weekend project.