Path: utzoo!utgpu!water!watmath!221.162.fido!Geoffrey_Welsh
From: 221.162.fido!Geoffrey_Welsh@watmath.waterloo.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: Nec v20 and simcga/hgcibm and accelerator boards and ...
Message-ID: <17145@watmath.waterloo.edu>
Date: 27 Feb 88 12:59:27 GMT
Sender: ugate@watmath.waterloo.edu
Lines: 27


   The posting asked for a semi-informed reply, and I am definitely 
qualified to provide that! (B^>)
 
   The only compatibility problems in the NEC V20 are caused by its 
built-in Z80 emulation, which is controlled by a bit in the CPU's FLAGS
register. Some software recklessly loads the FLAGS register disregarding
Intel's warning about reserved bits. The problem is that, if the bit that
NEC has appropriated to enable Z80 emulation is loaded to the wrong value (a
50% chance if you don't pay attention to 8086 programming manuals), the V20
drops into Z80 emulation.
 
   Naturally, a Z80 won't make any sense whatsoever of the 8086 instruction 
set. Murphy's law states that one of the 8086 op codes the V20-cum-Z80 will
try to execute will be interpreted as a Z80 "halt & catch fire" instruction (
hence the crash?)
 
   As for accelerator cards, if it works with an 8088 (or requires 
that you remove the 8088 from its socket and place a cable header in its
place), it SHOULD work with a V20 (but I have to warn you: I have NOT tried
this myself!).
 
   Geoff ( watmath!fido!221.171!izot )
   AKA "Incompetent Hacker [maxint]"

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