REXX Tips & Tricks, Version 2.80


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Note about VME


"Virtual Mode Extensions is a hardware enhancement Intel added to their 
CPUs beginning with the DX2 models. All Pentium processors also come with 
this feature. VME is intended to speed up a number of privileged functions 
(CPU instructions) for applications running in virtual mode sessions (for 
example, DOS and WinOS2 sessions). 
DOS (and Windows) apps running in an OS/2 Virtual DOS Machine (VDM) 
believe they "own" the hardware. On occasion these apps will try to do 
something to the (virtual) hardware that isn't permitted while a protected 
mode operating system such as OS/2 is in control of the real hardware 
because permitting such "privileged" operations would negatively impact 
overall system integrity and reliability. 
On a non-VME processor attempting such privileged operations would result 
in a (hardware generated) program interrupt, and hence an operating system 
(in this case, OS/2) interrupt handler to get control. Control would 
eventually pass to a routine that would simulate the attempted function. 
On a VME processor the function simulation would be carried out by 
microcode on the processor itself. This is both transparent (to the 
operating system and app) and faster, BUT only if it works<g>. 
Unfortunately there are apparently some conditions that can arise where 
this (usually) transparent handling of privileged operations (in effect, a 
hardware implemented microcode "assist") can leave the system in an 
inconsistent operating state. In this case the operating system will force 
a TRAP condition to protect itself (and YOUR data). 
Adding the "VME=NO" line to "CONFIG.SYS" forces all VME "assisted" CPU 
instructions to be reflected to, and handled by, the operating system; in 
other words, the way it would have worked had the VME feature not have 
been present. 
"VME=NO" is not a panacea fix for all your DOS/Windows woes. Indeed the 
most common symptom of VME related problems is frequent "Trap D" and "Trap 
E" errors in DOS and WinOS2 sessions. Therefore, I doubt it will correct 
all the problems you've listed, but it MAY provide some relief. Can't hurt 
to try. 
Ron Higgin OS/2 Advisor! (see EMail Addresses)"   

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