Adventures in PC Emulation

A V20 CPU, courtesy of Konstantin Lanzet ( CC ) The NEC V20 was a 16-bit CPU released in 1984. It is pin-compatible with the Intel 8088 , and clones the 8088's instruction set. It also includes newer instructions that had been introduced two years prior by the Intel 80186 and 80286 , but does not include the latter's protected-mode features. Besides the 186 instruction set, NEC also added new opcodes and instruction prefixes of their own, enabling performance enhancements for software that could detect or require a V20. NEC also incorporated various other improvements Intel had made in the 80186, namely hardware support for address calculation and for division and multiplication. The former was a big benefit overall, as it could decrease the execution time of any instruction utilizing a memory address operand.  The V20 was one of the first 3rd-party, drop-in CPU replacements for the PC. An owner of an original IBM 5150 could pull the 8088 out of their motherboard's CPU so...

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. Getting this right is crucial to whether the demo effect produces this: Artwork by VileR Or this: 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 ou...