WTD: NEC V20 Hardware Manual, IBM PC schematics

From: Fred Cisin <cisin_at_xenosoft.com>
Date: Tue Mar 11 13:06:00 2003

On Tue, 11 Mar 2003, Hans Franke wrote:
> > I might be able to use the 8088 docs. The V20 is *supposed* to be a drop-in
> > replacement, but IIRC it needed some (minor) BIOS alterations to get it to
> > work properly in PCs when the 8088 was swapped with a V20.
> Not that I know off. The V20 did behave exactly like an 8088.
> At some point I did use them on every PC and XT I could. The
> additional speed was quite cheap. The only time I couldn't
> use a V30 was in the Siemens PC-MX, a small 8086 Unix box.

There ARE some differences. It'll drop in as a replacement on a
5150. But on a Gavilan, which uses an 8088, there are modifications
required. Uncle Roger has my old Gavilan files, which include a Gavilan
document detailing the modifications needed.


Besides, it wasn't a completely Bug-For-Bug replacement. NEC failed to
implement some of the quirks of the 8088. For example, if an interrupt
occurred during the execution of an instruction with a double prefix, the
NEC would continue, but the intel would drop one of the prefixes when
resuming.
REP MOVSB DS:[SI] DS:[DI]
will resume with a V20, but with an 8088 will only do one more rep after
an interrupt.
Received on Tue Mar 11 2003 - 13:06:00 GMT

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