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A Dangerous Discovery and HP's Swift Response
by Joe Horn & Bill Wickes.
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3 A Cautionary Tale for HP 48 Programmers 3
àÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄù
(Comp.sys.handhelds)
Item: 3374 by _joehorn at hpcvbbs.UUCP
Author: [Joseph K Horn]
Subj: HP 48 Faster PGDIR
Date: Wed Jun 05 1991
FPGDIR, a FAST PGDIR, by Joseph K Horn. Makes PGDIR obsolete.
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3[NO, NO, NO IT DOESN'T! 20/20 Hindsight. Read On... -jkh-]3
àÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄù
When you use PGDIR on a very large directory, it can take a long time to
finish. For example, I just used Donnelly's XTIME function to time PGDIR on a
big directory, and it took a painful 50 seconds!
Bill Wickes explains why in his excellent book, "HP 48 Insights":
"PGDIR removes a directory specified by name. It does
this by recursively executing CLVAR and PURGE recursively
on each subdirectory until the original directory is
empty. (This process can take a relatively long time if
the directory is large.)" [Part I, page 96]
Guess what! There's a better way! [No there isn't! Read on... -jkh-]
Those of you with HP-41CX's will remember the CLRALMS command that cleared all
the alarms out. If you had a lot of alarms, it took a long time to finish,
because it looped through the alarm buffer clearing each one individually. But
a much better way was to treat the entire buffer as one object, and clear it in
one fell swoop. This was possible with the buffer clearing functions available
in several plug-in ROMs. For example, with the Extended-IL ROM, just execute
10 CLRBUF, and instantly the entire alarm buffer gets cleared, no matter how
large it is.
It turns out that there is a similar trick for purging directories on the HP
48. Rather than painfully looping through them, clearing their contents, we
can treat the whole thing as a single object and PURGE it in one fell swoop.
Here's the trick. The HP 48 Owner's Manual says:
"Once a directory is empty, you can purge it like any
other variable -- put its name on the stack and execute
PURGE." [Volume 1, page 123]
But "empty" means "when VARS == { }". And VARS == { } when all the vars are
"hidden" behind a null-named object (see NULLNAME.DOC on Goodies Disk #1).
So the quick way to purge a directory is:
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3[DO NOT DO THE FOLLOWING! IT'S TOO DANGEROUS! Read on... -jkh-]3
àÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄù
(1) Put its name on the stack;
(2) Get into that directory: DUP EVAL;
(3) Unhide everything (just in case): '' PURGE;
(4) Hide all its vars: 0 '' STO;
(5) Go back to the parent directory: UPDIR;
(6) Purge the "empty" directory: PURGE.
This process is automated by the following replacement for PGDIR:
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3[DO NOT USE THIS PROGRAM! Read on... -jkh-]3
àÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄù
---------------------< FPGDIR >---------------------------------
%%HP:T(1);
\<<
IF DUP TYPE 6 ==
THEN
IF DUP VTYPE -1 ==
THEN DROP
ELSE
IF DUP VTYPE 15 ==
THEN DUP EVAL 0 # 15777h SYSEVAL DUP PURGE STO UPDIR PURGE
ELSE 515 DOERR
END
END
ELSE 514 DOERR
END
\>>
----------------------------------------------------------------
Except for its improved speed, its action is exactly like PGDIR. It even
generates the same error messages as PGDIR in case of bad arguments.
Nonexistent names simply get dropped (like PGDIR does).
To use: place name of directory on stack, and run it.
It does full argument type checking, so it should be idiot proof.
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3[Unless the idiot is the author! Read on... -jkh]3
àÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄù
I keep it assigned to my blue-shifted DEL key. That way it takes only three
keystrokes to delete a directory (tic, menu, FPGDIR).
It should be in "system RPL" ... but that's left as an exercise for the
reader. :-)
-- Joseph K Horn -- Peripheral Vision, Ltd. --
----------
Resp: 1 of 4 by billw at hpcvra.cv.hp.com.
Author: [William C Wickes]
Date: Thu Jun 06 1991
An extra benefit of Joseph's method is that it can not only blow away a
directory instantly, but it can also blow away everything in main RAM at no
extra charge.
> Except for its improved speed, its action is exactly like PGDIR.
Not quite. PGDIR is safe.
In my excellent book I explained what PGDIR does, not WHY it does it. Guess
what! There isn't a better way! What? you mean there's a reason that PGDIR
goes through all that agony? Well, yes: directories are not composite
objects, and thus a directory in temporary memory, such as might be created by
PURGE, must not contain referenced objects. Garbage collection is doomed by
such references, so PGDIR's labor is to remove the directory without leaving
any references to any of its innards. PURGE doesn't bother with such
niceties, so it's not allowed to handle directories. Fooling it with
null-named variables is courting disaster. FPGDIR may work safely most of the
time if you execute a system halt (ON-C) first, but even that's not 100%
bulletproof.
In case the above is too subtle for anyone, just take it that using FPGDIR
(i.e. creating a null-named variable, then purging a directory) is NOT a good
thing to do.
Bill Wickes
HP Corvallis
----------
Resp: 2 of 4 by _joehorn at hpcvbbs.UUCP
Author: [Joseph K Horn]
Date: Thu Jun 06 1991
Bill Wickes writes that Joe Horn's FPGDIR is too dangerous to use.
Please post an example of how to lose memory with its use. I've tried,
and haven't been able to. Thanx!
-- Joe Horn --
----------
Resp: 3 of 4 by billw at hpcvra.cv.hp.com.
Author: [William C Wickes]
Date: Fri Jun 07 1991
Joe Horn requests:
> Please post an example of how to lose memory with its use. I've tried,
> and haven't been able to.
Using FPGDIR gives the 48 a chance to execute code at a random address; what
will actually happen is hard to predict--most of the time you just get a
system halt, but you can get Memory Clear (it's just like executing SYSEVAL
with random addresses). The easiest way to see the danger is to recall some
objects from a directory you are going to purge, then execute FPGDIR.
Everything looks fine, but now execute MEM and see your recalled objects turn
to Externals. Try executing one, if you're brave. In a running program, of
course, this can be disastrous--one example is to insert a MEM into FPGDIR
after the directory PURGE, then use FPGDIR to purge the subdirectory that
contains FPGDIR. I tried this on my HP48 (with everything archived, of
course), and got a system halt. Another way is to use FPGDIR to purge a
directory containing a suspended program, then execute MEM and CONT. (The use
of MEM in these examples forces a garbage collection, which otherwise might
happen most anytime, like a ticking time bomb.)
> Thanx!
Yer welcome!
Bill Wickes
HP Corvallis
----------
Resp: 4 of 4 by _joehorn at hpvvbbs.UUCP
Author: [Joseph K Horn]
Date: Sat Jun 08 1991
Bill Wickes writes that running my FPGDIR after recalling objects from the
target directory to the stack can cause problems.
Wow! That's for sure! I just tried it, and the very first attempt tanked
everything in the machine.
Rats. The moral here is twofold:
(1) The obvious: don't use FPGDIR unless you KNOW FOR SURE that there are no
references to objects that will be left floating in limbo.
(2) Less obvious: Be Careful When You Hide Objects In Directories! Many
articles and programs about "hiding" things have floated down this bitstream,
and not once did anybody mention that hiding things in directories can send
the 48 out to lunch. But if ALL the items in a directory are hidden, it's an
accident waiting to happen. Because then simply PURGEing the directory (if
there are any memory references to its contents) will cause problems at the
next garbage collection.
The first moral is worded with "unless" because the situation is exactly the
same for the new 256K and 512K bankswitched cards now available for the 48. I
just played with a 512K card tonight at the 48 Club meeting in L.A., and it
has a BANKN function that allows you to bank switch WITHOUT PERFORMING A
SYSTEM HALT. The owner's manual clearly states that doing this while there
are any memory references to objects in the bank being switched out will "at
best cause unpredictable results." The reason this is so is the same as the
reason that FPGDIR is dangerous. If Tripod can sell an item with
functionality that's dangerous to use and assumes that the user takes proper
precautions, then I hope I can give away FPGDIR for free with the same danger
and assumptions.
What scares me, however, is the second moral. People are hiding things
gleefully, without any idea that Memory Clear awaits them if they don't know
the pitfalls of PURGE on directories whose contents are all hidden.
EduCALC Goodies Disk #5 will, of course, have all this documented, hopefully
to make up for the blissful endorsement of using null-named objects that has
been on previous Goodies Disks. [You're reading it! -jkh-]
Thanx, Bill, for warning us of the dangers here, before my "wonderful new
discovery" (*sigh*) proliferated too far.
-- a crestfallen jkh --
úÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
3[If nothing else, this should teach the reader the value of participating3
3in the activity on the Hewlett Packard Bulletin Board System! -jkh-] 3
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