[comp.lang.smalltalk] Digitalk Smalltalk on the AT&T PC 6300 Plus

bwk@mitre-bedford.ARPA (Barry W. Kort) (09/14/88)

I have Digitalk Smalltalk V/286.  It runs fine on a Zenith 248 AT
(IBM clone).  But I cannot run it on my AT&T PC 6300 Plus.  My
Plus has 1 MB of RAM on the motherboard, and another 4MB of extended
memory. I have tried all combinations of command-line parameter
settings, but nothing seems to work.  Can anyone offer some insight
into the problem or suggest a fix?  Many thanks.

--Barry Kort

jr@amanue.UUCP (Jim Rosenberg) (09/18/88)

In article <39875@linus.UUCP> bwk@mbunix (Barry Kort) writes:
>I have Digitalk Smalltalk V/286.  It runs fine on a Zenith 248 AT
>(IBM clone).  But I cannot run it on my AT&T PC 6300 Plus.

I've only used Smalltalk V/286 lightly, so I'm only guessing.  ST/V 286 uses
protected mode, which means it has to be switching in and out of protected
mode constantly.  The designers of the 286 thought they were building the
Great Shell in the Sky's gift to multitasking, so they made it impossible to
switch from protected mode to real mode.  The reasoning was quite sound,
actually:  if you can switch under program control from protected mode to real
mode this could be a security hole in a multiuser operating system.  (So why
didn't they just make it privileged??)  The details on how one actually
achieves a switch from protected mode to real mode are almost too gross to
even contemplate.  The CPU farkles some bits in the CMOS and actually *RESETS*
the CPU chip!!  At some point this involves sending data to the *KEYBOARD*.
(Come on now, could I have made this up???)  When the CPU is reset (can you
spell context switch take forever?) it sees from the CMOS setting, Oh, this
isn't a real reset.  The whole thing does work, but just barely.

The problem comes if the interface between the CPU and keyboard is not
completely 100% AT compatible.  Although I'm only guessing, I would bet this
is the problem.  The 6300+ is a strange beast, and was never designed as an AT
clone.  Do you have any other software that does work that you know is
switching between protected mode and real mode?  You're also guaranteed to
have trouble if the 6300+ does not have CMOS that it addresses the same way as
a genuine AT.  Methinks you may be completely out of luck.

(Apologies that this message has nothing to do with Smalltalk; follow-ups
should probably be directed to comp.sys.ibm.pc.)
-- 
 Jim Rosenberg
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