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The PC-9821 is the first computer in the PC-9821 series.
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A stock PC-9821 builds on ideas seen in the PC-9800 series, or more specifically ideas in use in late 1992, primarily the 80386 variants which prior to the PC-9821's introduction, stood as NEC's high-end consumer computers. At this point it becomes difficult to define "standards" for NEC computers as much of the components were interchangable, though most PC-9821 software was designed with this setup in mind - the inclusion of two 3½-inch floppy disk drives and a CD-ROM drive (features subsequently removed or replaced in future "budget" models).
It's as if I don't know what I'm typing, and that NEC Retro could really use some more volunteers with a more concrete understanding of what's going on.
Two variants of the original PC-9821 were released concurrently, the S1 and S2, the latter of which which adds a 40MB harddrive.