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Re: [ECS] screen saver / execute Ingo Pakleppa Wed, 10 Jun 1998 10:05:01 -0400

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You fell into one of Microsoft's traps. When they say Windows 95 is a
multithreaded 32-bit OS, they aren't outright lying, but they are
fudging the truth a lot; it's really neither 32-bit nor fully
multithreaded.

Your problem is in the fact that at heart, Windows 95 is still a
single-threaded 16-bit OS in a multithreaded 32-bit disguise (in fact,
it really is just Win 3.x with an enhanced Win32s and a new user
interface). In particular, all code that draws to the screen has to use
GDI, which is still almost completely a 16-bit component - and not
reentrant. To "solve" this problem, Microsoft implemented a kludge known
as the Win16Mutex - every time you (or the screen saver) makes a GDI
call, or for that matter any call into any 16-bit code, the Win16Mutex
will be set, and all other programs in 95 will be prohibited from
calling 16-bit code until the Win16Mutex is free again. The same
Win16Mutex will also be set as long as a 16-bit executable program
executes. That is, if you run, say, a 16-bit version of Excel in the
background and let it do a lengthy recalculation, none of your 32-bit
applications will be able to draw anything to the screen and appear
frozen (even though as long as the 32-bit application stays in 32-bit
code to, say, make computations, it will continue to run - it just can't
display anything).

Now obviously screen savers do a lot of drawing onto the screen - it's
their whole purpose in life -, so they essentially keep the Win16Mutex
locked every time they make a GDI call, which is almost permanently. And
if you happen to have a 16-bit screen saver, it is even worse because it
will never exit the Win16Mutex at all.

Thus, the W95 program will continue to run exactly until the point when
it needs to draw or call any other 16-bit function, and then be stopped
dead in it's tracks by the Win16Mutex until the screen saver no longer
needs to draw to the screen - i.e., until you hit a key.

The only workaround is not to use a screen saver, or to give up on 95
and move to NT. NT truly is a 32-bit operating system and won't suffer
from this problem.

Ingo

Mark Gilmore wrote:

> I have found that when my screen saver has cleared the screen, that
> any W95 pgms that ECSW attempts to execute will pause until a
> key is pressed. Any clues on this one ?
> BTW, to have ECSW execute a W95 pgm AND wait for its completion,
> use "START /WAIT {pgm} {args...}". This is only required for
> W95 pgms (not DOS pgms).
> --
> Mark Gilmore
> Omnipotence
> http://ourworld.compuserve.com/homepages/mark_gilmore_2

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