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PC-98 was a series of x86-based Japanese computers that offers Kana-Kanji support, which shared some hardware similarities and operation systems with IBM PC, and software was relatively easy to port between the two architectures. But the two architectures were never compatible.

What technical aspects in hardware and software make NEC PC-98 architecture incompatible with IBM PC architecture? (Not to be confused with the reason why NEC chose to differ.)

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As both share the same CPU architecture which has a dedicated set of support chips, both the IBM PC and NEC PC-98 platforms are built from same or similar building blocks, but the design how they actually are connected together and to which addresses is different.

At least it has the following differences

  • BIOS implementation is different
  • C-bus instead of ISA bus
  • support chips are at different IO addresses, even if most basic chips are of the same type
  • memory map after the 640kB base memory is different
  • Interrupt and DMA channel usage
  • Video interface done with NEC UPD7220 chips

This means that any program which uses the hardware directly instead of using DOS calls will not work on both architectures without porting over the code which accesses the platform specific hardware.

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    Does the use of C-bus result in changes which affect software directly?
    – Stephen Kitt
    Dec 14, 2022 at 15:34
  • 6
    @StephenKitt Depends what you mean by that. It is just a bus for add-on boards like ISA bus. I don't think the bus itself causes differences. But e.g. for a game that wants to access a sound card for playing FM music or PCM samples, you likely don't have a C-Bus versions of ISA sound cards, just a completely different set of sound cards with different chips.
    – Justme
    Dec 14, 2022 at 15:50

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